Identification of Supernova Remnants in the Sculptor Group Galaxies ...

THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 108 : 261È277, 1997 January ( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.

IDENTIFICATION OF SUPERNOVA REMNANTS IN THE SCULPTOR GROUP GALAXIES NGC 300 AND NGC 7793 WILLIAM P. BLAIR1 Department of Physics and Astronomy, Johns Hopkins University, 34th and Charles Streets, Baltimore, MD 21218 ; wpb=pha.jhu.edu

AND KNOX S. LONG1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 ; long=stsci.edu Received 1996 May 28 ; accepted 1996 July 19

ABSTRACT We have obtained CCD imagery of eight Ðelds in the Sculptor Group Sc spiral galaxy NGC 300 and two Ðelds covering the Sculptor Sd spiral galaxy NGC 7793, using interference Ðlters to isolate emission from Ha ] [N II], [S II], and a continuum band near 6100 Ó. Using these images, we have identiÐed isolated nebulae that are bright in [S II] relative to Ha, indicating probable shock heating, and that have morphologies consistent with identiÐcation as supernova remnants (SNRs). Our lists of candidates contain 28 objects each in NGC 300 and NGC 7793. We then obtained optical (4800È6900 Ó) long-slit CCD spectra of many of these nebulae, as well as many adjacent photoionized regions along the slits for comparison. The spectra conÐrm that the SNR candidates have [S II] : Ha ratios in excess of 0.4, thus satisfying the usual optical criterion for being supernova remnants. Approximately 80% of the objects observed spectroscopically show additional emission lines that support their identiÐcation as supernova remnants. These are the Ðrst large samples of supernova remnants identiÐed in galaxies outside the Local Group. In general, many properties of these SNR samples parallel the more extensive sample identiÐed in M33. However, the dividing line between the [S II] : Ha ratios appropriate for photoionized and shock-heated nebulae appears to be much less distinct in these galaxies than for Local Group galaxies, causing confusion and incompleteness in these samples. This confusion e†ect is most prevalent in NGC 7793, where the observed [S II] : Ha ratio in the most di†use photoionized gas rises well above the value of 0.4 normally used to discriminate SNRs from H II regions. Subject headings : galaxies : clusters : individual (Sculptor) È galaxies : individual (NGC 300, NGC 7793) È galaxies : ISM È supernova remnants 1.

INTRODUCTION

tion, abundance studies based on SNRs can provide an important check on H II region abundance analyses and abundance-gradient studies (see Blair, Kirshner, & Chevalier 1982 ; Dopita, DÏOdorico, & Benvenuti 1980 ; Blair & Kirshner 1985). Most SNRs in external galaxies have been identiÐed as a result of Ha and [S II] interference-Ðlter imaging surveys. SNRs are normally identiÐed as nebulae that have elevated [S II] : Ha ratios (º0.4) compared with H II regions (typically ¹0.2). The physical basis for this discriminant is well understood. Most of the optical emission from an SNR is due to radiative shocks occurring in relatively dense material ; that is, a short impulse heats and ionizes the gas, which then recombines over a period of time. As a result, SNRs have the spectrum of a recombining plasma with strong forbidden-line emission arising from a wide range of ionization states, including S`. In H II regions, however, the ionization state of the plasma is maintained in an ongoing manner by light from hot stars, and most of the sulfur is in the form of S``. Hence the optical [S II] lines are quite weak. Mathewson & Clarke (1973) Ðrst used this criterion, in conjunction with radio spectral index information, to identify SNRs in the Large Magellanic Cloud (LMC). Subsequently, DÏOdorico, Dopita, & Benvenuti (1980), Blair, Kirshner, & Chevalier (1981), and others applied the optical criterion alone to Local Group and other nearby galaxies, using interference Ðlters and image-tube photography. The advent of more sensitive and sophisticated electronic imaging detectors has permitted great advances in this Ðeld by allowing much better subtraction of galaxy background

There are many reasons for wanting to Ðnd and study samples of supernova remnants (SNRs) in other galaxies. Surveys of SNRs in external galaxies are more likely to provide unbiased samples than Galactic studies. Galactic SNRs are found almost exclusively in the plane of the Milky Way and are so heavily obscured that many can only be studied at radio wavelengths. The distances to individual Galactic SNRs are often poorly constrained, making even such basic information as the linear diameters of the SNRs highly uncertain. These problems make systematic studies of Galactic SNRs and their evolution very difficult. SNRs in another galaxy, on the other hand, can be treated as if they were all at the same distance. Hence their relative sizes, galactocentric distances (GCDs), luminosities, and galactic environments can be assessed directly to study SNR evolution. This is especially true for relatively face-on systems, where absorption e†ects are minimized (Long et al. 1990 ; Gordon et al. 1996b). SNRs are also important to study because they are powerful mechanisms for energizing and chemically enriching the interstellar medium (ISM) of a galaxy. The rate and distribution of supernovae and, consequently, SNRs determines what fraction of the ISM in a galaxy is heated to coronal temperatures and whether a galaxy has a hot halo (Lehnert & Heckman 1996, and references therein). In addi1 Guest Observer, Las Campanas Observatory, operated by the Carnegie Institution of Washington.

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contamination and by pushing to much fainter surface brightness levels. In the Magellanic Clouds, optical imaging in conjunction with X-ray and/or radio data has proved most e†ective (Mathewson et al. 1983, 1985), although the optical criterion alone is often sufficient (Winkler & LeWinter 1992). At greater distances, such as in M33, the optical [S II] : Ha criterion has been the most sensitive means of detecting candidate SNRs (Long et al. 1990, hereafter LBKW). Subsequent spectroscopy of these candidates (Smith et al. 1993) has demonstrated the e†ectiveness of the technique for Ðnding SNRs and has provided information about individual objects. In M31, which is at approximately the same distance as M33 but is not as nearly face-on, Williams, Schmitt, & Winkler (1995) have recently reported 105 SNR candidates found by using the optical criterion alone. As X-ray and radio techniques have become more sensitive, combining these with the optical criterion has permitted the identiÐcation of additional objects missed in the optical surveys alone (Gordon et al. 1993, 1996a ; Long et al. 1996). At present, the SNR candidate list in M33 stands at 98, many of which have been conÐrmed spectroscopically (Gordon et al. 1996b). With these successes in hand, we have begun to push the optical technique beyond the Local Group of galaxies. Surveys of more distant galaxies are bound to have more serious confusion e†ects since the spatial scale degrades with distance. Also, as distance increases, our ability to measure SNR diameters, especially for the smaller objects, will degrade as well (at least with typical ground-based spatial resolution). However, it is important to sample more galaxies and begin to understand the systematics of SNR populations as a function of galaxy type. Furthermore, some functions of these surveys, such as using the SNRs to study abundance gradients relative to H II regions, do not depend on accurate diameters or sample completeness. In this paper, we report optical CCD/interference Ðlter surveys of two nearly face-on spiral galaxies in the loose galaxy association usually referred to as the ““ Sculptor Group,ÏÏ in the southern hemisphere. These galaxies are some of the nearest to the Local Group. NGC 300 is an Sc(s) II.8 galaxy (Sandage & Tammann 1987) with an inclination of 42¡.3 (Puche, Carignan, & Bosma 1990). It does not have well-deÐned spiral arms, although the spiral pattern is clearly evident (see Sandage & Bedke 1988), and numerous giant H II regions and evidence of ongoing star formation are present. Recent distance estimates vary from 1.5 to 2 Mpc (Graham 1984 ; Richer, Pritchet, & Crabtree 1985 ; Carignan 1985 ; Deharveng et al. 1988 ; Puche et al. 1990). We adopt the distance derived most recently from the Cepheid variable analysis of Freedman et al. (1992), 2.1 ^ 0.1 Mpc, which is in good agreement with the distance from planetary nebulae (So†ner et al. 1996). Thus a spatial scale of 10.2 pc arcsec~1 will be assumed. Using numbers from de Vaucouleurs et al. (1991) and correcting to the assumed distance, NGC 300 has global properties similar to but smaller than those of M33 [D(0) \ 13.6 kpc, M \ [18.08 for NGC 300 ; B for M33, assuming a disD(0) \ 18.1 kpc, M \ [18.87 B tance of 840 kpc for M33 ; Freedman, Wilson, & Madore 1991]. Morphologically, the two galaxies are also very similar in appearance. Thus, NGC 300 will provide a very good comparison with the work in the Local Group. NGC 7793 is a reasonably face-on (i \ 53¡.7) Sd(s) IV galaxy (Sandage & Tammann 1987) whose overall spiral

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pattern is nearly lost in a general confusion of H II regions and star formation. Distance estimates range from 2.5 to 3.4 Mpc (de Vaucouleurs et al. 1991 ; Carignan 1985). Recent estimates have tended toward the upper end of this range, and we adopt 3.38 Mpc for use in this paper, along with other geometric and photometric information from Carignan & Puche (1990). For comparison, NGC 7793 has D(0) \ 9.4 kpc and M \ [18.28. At this distance, 1A correB sponds to 16.5 pc, so only relatively evolved SNRs will be resolvable from the ground. Because of their proximity and moderate inclination angles, these galaxies provide an excellent opportunity for optical SNR surveys. DÏOdorico et al. (1980, hereafter DDB80) carried out the only prior search for SNRs in NGC 300 ; they used Ha and [S II] photographic plates of NGC 300 obtained with the UK Schmidt to identify seven candidate SNRs. Only two of the objectsÈDDB 2 and DDB 5Èhave been conÐrmed based on optical spectroscopy (DÏOdorico & Dopita 1983). No prior searches for SNRs have been made in NGC 7793. In this paper, we describe our e†orts to systematically search for SNRs in these two galaxies. In ° 2, we describe our interference Ðlter CCD imaging and the search technique used to identify candidate SNRs, followed by a description of the follow-up spectroscopy of many of the candidates in each galaxy. In ° 3, we compare the results of our imaging and spectroscopy of the SNR candidates in each galaxy, compare various systematic di†erences between the SNR and H II region samples in each galaxy, and discuss incompleteness e†ects. 2.

OBSERVATIONS AND REDUCTIONS

2.1. CCD Imaging and SNR Search All the observations for this project were obtained at the Cassegrain focus of the 2.5 m du Pont Telescope at Las Campanas Observatory. For the imaging observations, we used a focal-plane reducer called the CHUEI and a Texas Instruments 800 ] 800 CCD. The CHUEI is a copy of a focal-plane reducer known as the PFUEI, which was described in detail by Gunn & Westphal (1981). On the du Pont Telescope and using this instrumental conÐguration, individual pixels subtend 0A. 41 and an entire Ðeld is [email protected] on a side. Interference Ðlters were inserted into the f/7.5 beam of the telescope prior to the reimaging optics. The interference Ðlters and their characteristics are summarized in Table 1. They include Ha and [S II] Ðlters, to permit identiÐcation of nebulae with large [S II] : Ha line ratios, and a red (D6100 Ó) continuum band, which was required to remove galaxy background light and to exclude objects with large continuum contributions (such as compact H II regions excited by a cluster or association). The Ha Ðlter was sufficiently broad that it also passed considerable emission from [N II] jj6548, 6583. This causes the derived Ha Ñuxes that we estimate from our images to be somewhat overestimated TABLE 1 INTERFERENCE FILTER CHARACTERISTICS Filter

j c (Ó)

*j FWHM (Ó)

T peak (%)

[S II] . . . . . . . . . . . . . Ha ] [N II] . . . . . . Continuum . . . . . .

6737 6563 6100

57 52 150

51 62 65

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SUPERNOVA REMNANTS IN NGC 300 AND NGC 7793

263

TABLE 2 LAS CAMPANAS OBSERVATORY 2.5 METER CCD IMAGING OBSERVATION LOG Field ID

R.A. (2000)

decl. (2000)

Filter

Exposure (s)

FWHM Seeing (arcsec)

Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum

1500 3000 1500 1500 4500 1500 1500 3000 1500 1500 3000 1500 1500 3000 1500 1500 3000 1500 1500 3000 1500 1500 3000 1500

1.2 1.2 1.1 1.2 1.2 1.3 1.2 1.4 1.0 1.2 1.1 1.3 1.5 1.6 1.4 1.0 1.0 1.5 1.1 1.0 1.1 1.1 1.1 1.1

1500 6000 1500 1500 6000 1500

1.2 1.3 1.3 1.2 1.3 1.1

Date (UT) NGC 300 Imaging

C .........

00 54 48.54

[37 35 53.1

1987 Nov 13

D .........

00 54 23.03

[37 35 50.6

1987 Nov 14

E..........

00 55 38.77

[37 40 47.9

1987 Nov 15

F..........

00 55 13.46

[37 40 51.9

1987 Nov 12

G .........

00 54 47.36

[37 40 50.4

1987 Nov 12

H .........

00 54 22.84

[37 40 50.7

1987 Nov 14

I ..........

00 54 47.44

[37 45 51.0

1987 Nov 15

J ..........

00 55 14.00

[37 45 50.3

1987 Nov 13

NGC 7793 Imaging West . . . . . .

23 57 38.01

[32 35 24.9

1987 Nov 16

East . . . . . .

23 58 01.93

[32 35 23.8

1987 Nov 17

Ha ] [N II] [S II] 6100 Ó continuum Ha ] [N II] [S II] 6100 Ó continuum

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds.

(and hence the [S II] : Ha ratios to be somewhat too low). Previous observations of H II regions (Pagel et al. 1979 ; Webster & Smith 1983) indicate that this should not cause a serious problem for the SNR surveys. The imaging observations were obtained in 1987 November, as summarized in Table 2. Conditions were excellent throughout the observing run. Seeing was quite stable, and stars typically have point-spread functions of 1A. 0È1A. 3 (FWHM). A total of eight Ðelds were observed in NGC 300, spaced on 5@ centers and covering most of the visible portion of the galaxy. Only two Ðelds (with generous overlap) were required to cover NGC 7793. Exposure times were at least twice as long in [S II] as in Ha, to permit comparable signal-to-noise ratios in objects having [S II] : Ha ratios of D0.5. For NGC 7793, the available time allowed us to experiment with deeper [S II] exposures, as shown in Table 2. Standard stars from the list of Stone & Baldwin (1983) were observed each night so that the resulting data could be placed on an absolute Ñux scale. The images were reduced and processed using standard techniques within IRAF.2 Reduction procedures included subtraction of CCD bias levels and Ñat-Ðelding, alignment and summation of individual frames (for the [S II] 2 IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation.

exposures), and the creation of aligned mosaics of all the Ðelds in each galaxy. For some purposes it was more convenient to work with the individual frames (or subframes of these), but the mosaics are useful for display purposes and also provide a framework within which to measure the coordinates of the objects. Figure 1 (Plates 2È3) shows the resulting Ha montage for NGC 300, where the galaxy background has not been subtracted. Figure 2 (Plates 4È8) shows the NGC 7793 mosaics in several formats. Figures 2a, 2b, and 2c show the 6100 Ó continuum, Ha, and [S II] data for the galaxy while Figures 2d and 2e show the Ha and [S II] mosaics after globally scaling and subtracting the continuum exposure. (Note that some regions of the [S II] are slightly oversubtracted in this global subtraction.) Analysis of the count rates in the standard-star exposures plus knowledge of the Ðlter characteristics allowed us to integrate the expected stellar Ñux and determine the conversion of count rate into Ñux for each of the Ðlters. The images were searched for isolated nebulae with large [S II] : Ha ratios and little or no continuum contamination. The technique used to e†ect this search was Ðrst to break the images up into a set of 200 ] 200 pixel subÐelds and then to subtract a locally scaled version of the continuum subÐeld from the Ha and [S II] images (see LBKW ; Gordon et al. 1996b). This allowed the continuum subtraction to be optimized locally for each of the subÐelds. We then took all the images for a particular subÐeld (which we call tiles) and created a single two-dimensional mosaic ; the various tiles in

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Vol. 108

below. (This has been particularly true since most H II region studies have concentrated on high surface brightness giant H II regions, which tend to have lower [S II] : Ha ratios than lower surface brightness regions.) Because of the probable (but uncertain) contamination of the Ha data by [N II], and because the intent of this exercise was to Ðnd candidate SNRs to be followed up with spectroscopy, we chose a somewhat lower threshold of 0.35 for this ratio in our search of the images. A few objects with even lower ratios, but with values elevated relative to other local nebulae in the tiles, were also carried forward in the candidate list for later inspection. In most cases, the apparent [S II] : Ha ratio was determined from the ratio of a Ñux measurement of the entire SNR candidate ; in some cases, in which the background was more complex or the average surface brightness was low, the ratio was obtained from the brightest portion of the nebula as observed in [S II]. The faintest objects that were identiÐed as SNR candidates had Ha surface brightnesses larger than D10 counts pixel~1 above background, which corresponds to a surface brightness of D1 ] 10~16 ergs cm~2 s~1 arcsec~2. This surface brightness limit may be dependent on the background brightness, which is a function of radial position in each galaxy. A total of 28 nebulae from the NGC 300 data set and 28 from the NGC 7793 images were identiÐed as candidate SNRs by use of this technique. In Tables 3A and 3B (for NGC 300 and NGC 7793, respectively), we list each candidate, its galactocentric distance (GCD) in kiloparsecs, its

this mosaic contained the raw continuum image, the continuum-subtracted Ha image, the continuum-subtracted [S II] image, an [S II] : Ha ratio image, and an Ha image with regions of high apparent [S II] : Ha ratios blocked out. (This latter tile attracts oneÏs eye speciÐcally to regions of potential interest.) An example of such mosaics can be seen in LBKW and Gordon et al. (1996b). Our special imagedisplay software permits the user to extract the value at the same pixel position in all the tiles simultaneously. It was therefore straightforward to measure the Ñux (and apparent ratio of [S II] to Ha) in a nebula or in a background region outside a nebula to establish the ““ true ÏÏ [S II] : Ha ratio for a region. It was also possible to visually inspect the nebulae as they were selected to ensure that they were discrete emission regions and not simply di†use emission from each galaxy. (The importance of this morphological criterion will be discussed below.) The software also allows the user to deÐne rectangular regions in which the source strength can be measured (with or without annular background subtraction). In regions confused by nearby H II emission, separate representative background regions were identiÐed and used for subtraction. The canonical value of 0.4 in the ratio of [S II] to Ha has worked well in the Milky Way and in Local Group galaxies to separate shock-heated nebulae from photoionized gas. However, in the majority of cases, the exact value of this cuto† has been unimportant because there has been a gap in the distribution, with most SNRs showing ratios in excess of 0.5 and photoionized regions showing ratios of D0.2 or

TABLE 3A SNR CANDIDATES IN NGC 300 Object

GCDa (kpc)

N300-S1 . . . . . . . . . . . . . . . . . . . . N300-S2 . . . . . . . . . . . . . . . . . . . . N300-S3 . . . . . . . . . . . . . . . . . . . . N300-S4 . . . . . . . . . . . . . . . . . . . . N300-S5 . . . . . . . . . . . . . . . . . . . . N300-S6 . . . . . . . . . . . . . . . . . . . . N300-S7 . . . . . . . . . . . . . . . . . . . . N300-S8 . . . . . . . . . . . . . . . . . . . . N300-S9 . . . . . . . . . . . . . . . . . . . . N300-S10 (\DDB 2) . . . . . . N300-S11 . . . . . . . . . . . . . . . . . . . N300-S12 . . . . . . . . . . . . . . . . . . . N300-S13 . . . . . . . . . . . . . . . . . . . N300-S14 . . . . . . . . . . . . . . . . . . . N300-S15 . . . . . . . . . . . . . . . . . . . N300-S16 . . . . . . . . . . . . . . . . . . . N300-S17 . . . . . . . . . . . . . . . . . . . N300-S18 . . . . . . . . . . . . . . . . . . . N300-S19 . . . . . . . . . . . . . . . . . . . N300-S20 . . . . . . . . . . . . . . . . . . . N300-S21 . . . . . . . . . . . . . . . . . . . N300-S22 . . . . . . . . . . . . . . . . . . . N300-S23 . . . . . . . . . . . . . . . . . . . N300-S24 . . . . . . . . . . . . . . . . . . . N300-S25 . . . . . . . . . . . . . . . . . . . N300-S26 (\DDB 5) . . . . . . N300-S27 . . . . . . . . . . . . . . . . . . . N300-S28 . . . . . . . . . . . . . . . . . . .

4.76 3.87 3.22 2.81 3.61 3.11 2.48 1.96 1.63 1.53 2.42 2.61 1.21 0.78 1.79 0.39 2.37 1.83 1.49 4.28 2.40 1.82 2.45 2.05 2.14 3.72 3.86 5.07

R.A. (2000) 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 54 55 55 55 55 55 55 55 55 55 55 55

19.21 21.85 28.86 30.62 30.99 31.91 33.17 38.17 40.20 40.87 42.54 43.86 46.60 47.15 53.32 54.46 56.68 01.39 05.41 05.68 07.15 07.50 09.10 09.48 10.68 15.46 17.54 33.76

decl. (2000) [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37 [37

37 40 41 40 37 38 40 41 41 40 43 43 39 41 38 40 43 39 41 46 39 40 39 40 41 44 44 43

23.96 27.11 51.32 53.75 33.96 25.68 16.90 14.88 02.12 48.73 14.16 39.08 44.32 07.63 48.24 35.46 57.70 18.17 21.04 13.35 15.17 43.18 32.61 46.21 27.13 39.31 36.65 13.33

Diameterb (pc)

SHaTc

Image [S II] : Ha ] [N II]

55 200 48 141 106 43 43 46 92 16 43 52 48 54 33 52 60 30 38 66 45 71 38 30 67 33 65 59

12 42 26 12 26 59 41 35 65 429 28 16 41 33 45 21 22 57 209 12 12 12 37 34 37 469 38 109

0.44 0.49 0.40 0.71 0.46 0.60 0.57 0.61 0.44 0.67 0.66 0.52 0.59 0.63 0.65 0.70 0.69 0.53 0.53 0.75 0.59 0.27 0.46 0.80 0.64 0.57 0.70 0.61

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Galactocentric distance of each object, after deprojecting onto the plane of the galaxy. (See text for assumed parameters.) b Estimated diameter of each object, assuming D \ 2.1 Mpc for NGC 300. c Average Ha surface brightness for each object as measured from the images, in units of 10~17 ergs cm~2 s~1 arcsec~2. A Ñat correction of 25% has been applied to correct (to Ðrst order) for [N II] contamination of the Ha images.

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265

TABLE 3B SNR CANDIDATES IN NGC 7793 Object

GCDa (kpc)

R.A. (2000)

N7793-S1 . . . . . . . . . . . N7793-S2 . . . . . . . . . . . N7793-S3 . . . . . . . . . . . N7793-S4 . . . . . . . . . . . N7793-S5 . . . . . . . . . . . N7793-S6 . . . . . . . . . . . N7793-S7 . . . . . . . . . . . N7793-S8 . . . . . . . . . . . N7793-S9 . . . . . . . . . . . N7793-S10 . . . . . . . . . . N7793-S11 . . . . . . . . . . N7793-S12 . . . . . . . . . . N7793-S13 . . . . . . . . . . N7793-S14 . . . . . . . . . . N7793-S15 . . . . . . . . . . N7793-S16 . . . . . . . . . . N7793-S17 . . . . . . . . . . N7793-S18 . . . . . . . . . . N7793-S19 . . . . . . . . . . N7793-S20 . . . . . . . . . . N7793-S21 . . . . . . . . . . N7793-S22 . . . . . . . . . . N7793-S23 . . . . . . . . . . N7793-S24 . . . . . . . . . . N7793-S25 . . . . . . . . . . N7793-S26 . . . . . . . . . . N7793-S26 ext . . . . . . N7793-S27 . . . . . . . . . . N7793-S28 . . . . . . . . . .

2.53 3.83 2.39 2.22 3.55 1.30 2.58 4.08 3.89 1.03 2.53 0.53 1.75 2.81 2.74 1.21 3.08 2.09 3.19 3.10 1.70 1.92 2.67 2.15 2.42 4.48 4.50 3.78 3.64

23 57 41.06 23 57 41.13 23 57 41.57 23 57 42.93 23 57 43.45 23 57 46.31 23 57 46.42 23 57 47.32 23 57 47.36 23 57 48.09 23 57 48.11 23 57 49.57 23 57 51.02 23 57 52.45 23 57 54.12 23 57 54.33 23 57 54.90 23 57 55.24 23 57 55.56 23 57 56.00 23 57 56.30 23 57 58.22 23 57 58.63 23 57 58.95 23 57 59.59 23 57 59.94 23 57 59 44 23 58 06.30 23 58 06.35

decl. (2000) [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32 [32

34 33 35 35 37 35 36 37 37 35 36 35 36 33 34 35 33 34 37 37 36 35 36 36 36 33 33 35 35

39.67 21.87 39.27 52.96 03.36 33.26 40.76 41.66 35.26 03.76 56.16 25.06 32.56 54.55 04.65 13.45 55.75 35.15 21.05 18.85 11.05 23.95 41.55 06.75 18.95 26.45 16.45 12.84 38.24

Diameterb (pc)

SHaTc

Image [S II] : Ha ] [N II]

49 101 71 37 60 155 94 46 155 33 44 27 69 60 70 30 104 87 192 27 47 77 27 50 169 197 260 94 50

33 34 35 119 62 8 77 45 16 170 159 469 53 12 109 56 68 119 28 459 249 77 42 249 21 130 50 22 25

0.58 0.37 0.51 0.37 0.37 0.63 0.43 0.60 0.37 0.58 0.53 0.70 0.73 0.60 0.36 0.60 0.35 0.35 0.36 0.53 0.35 0.26 0.58 0.45 0.35 0.38 0.46 0.50 0.46

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Galactocentric distance of each object, after deprojecting onto the plane of the galaxy. (See text for assumed parameters.) b Estimated diameter of each object, assuming D \ 3.38 Mpc for NGC 7793. c Average Ha surface brightness for each object as measured from the images, in units of 10~17 ergs cm~2 s~1 arcsec~2. A Ñat correction of 25% has been applied to correct (to Ðrst order) for [N II] contamination of the Ha images.

position (J2000), its mean diameter (in parsecs at the assumed distances), the Ha surface brightness, and the measured [S II] : Ha ratio associated with each nebula, as determined from our CCD images. The positions listed in Tables 3A and 3B are from astrometric plate solutions for our CCD frames based on stars in the Hubble Space T elescope Guide Star Catalog and are accurate to D2A. The GCDs are calculated from the observed positions relative to the nucleus of each galaxy and by deprojecting the plane of the galaxy to ““ face-on,ÏÏ assuming the inclination angles and distances given above. The surface brightnesses listed in Tables 3A and 3B have had a Ñat 25% correction applied to account roughly for the contamination by [N II]. The positions of the SNR candidates are indicated by circles in Figure 1a (for NGC 300) and Figure 2c (for NGC 7793) and are cross-referenced to Tables 3A and 3B. Figures 3aÈ3z (Plates 9È17) show 150 pixel (61A. 5) regions of the 6100 Ó continuum data, the continuum-subtracted [S II] data, and the continuum-subtracted Ha data centered on each of the 28 candidates in NGC 300. Figure 4aÈ4z (Plates 18È26) show the same for the 28 NGC 7793 candidates, except that the Ðeld size is 100 pixels (41A). In some instances in which the alignment is o† by a fraction of a pixel, residuals of bright stars can be seen in the continuumsubtracted panels. However, in general, the galaxy background subtracted very well, even in the inner regions of the galaxies. These Ðgures show the morphology of each

remnant candidate and its relation to other nearby emissions. In many cases, the candidates are too small or too faint to show much structure. In some cases, however, one can clearly see limb-brightened shells (or partial shells) or loop structures reminiscent of Galactic SNRs such as the Cygnus Loop or Vela SNRs. There are a few instances in which we have marked adjacent nebulae as separate candidates, although they may be physically related (e.g., N7793S8 and -S9 ; N7793-S19 and -S20). In other cases, the morphology and relation to nearby emission makes it difficult to decide exactly which portion of the observed emission is the SNR candidate (e.g., N300-S5, N300-S17, N7793-S17). We note that a number of candidates have been located even in fairly confused regions (e.g., N300-S9 and -S10, N300-S19, N300-S25, N7793-S4, N7793-S6, N7793-S12, N7793-S16, N7793-S21). Finally, there are a few instances of fairly large shells that have stars or star clusters seen (in projection at least) within their interiors (cf. N300-S2, N300-S4, N7793-S23, N7793-S25, N7793-S27). Some of these may involve stellar windÈdriven shocks instead of or in conjunction with true SNRs. 2.2. L ong-Slit CCD Spectroscopy In order to verify that the candidate SNRs have large [S II] : Ha ratios and to search for additional indicators that they are in fact SNRs, we carried out spectroscopic obser-

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vations of nebulae in these galaxies on 1988 August 16È20 and 1989 August 23È27. The spectroscopic data were obtained with the Modular Spectrograph on the 2.5 m du Pont Telescope at Las Campanas. This spectrograph, which was built by P. Schechter, is a long-slit moderate-resolution CCD spectrograph. For these observations, we used a 600 line mm~1 grating blazed at 7000 Ó, an f/1.2 camera, and a slit width of 2A. 5. With this setup, the slit length is D6@ and a spatial pixel subtends 0A. 8. Spectra covered the wavelength range 4650È7100 Ó with a resolution of D5 Ó near Ha, degrading to D9 Ó near Hb. In all, 35 separate long-slit spectra were obtained. Typical exposure times were 3600 s, although exposures ranged from 1500 to 4800 s for certain positions. The nebulae we wished to observe were not visible in the TV guider, and the galaxy Ðelds do not contain many bright stars. Hence essentially all the observations were made by blind o†sets of up to 5@ from the few reasonably bright stars in each of the original CCD frames, using a special o†set pointer built by R. Brucatto. In most cases, we adjusted the position angle of the aperture in a direction consistent with (but not necessarily equal to) the parallactic angle at the time of the observation (see Filippenko 1982). This was done mainly to place more than one SNR candidate (or other nearby emission region) along the apertureÏs length whenever possible while minimizing the e†ects of di†erential refraction. Since most of the emission regions are extended along the dimension of the slit width, any residual e†ects from di†erential refraction should be negligible. Also, while di†erential refraction might a†ect ratios of red to blue lines (such as Ha : Hb, which is a reddening diagnostic), the primary ratios of interest to us are between lines only in the red (or blue) portions of the data. As might be expected, we were more successful at sampling multiple SNR candidates within a single long-slit position in NGC 7793, where the typical separation of objects is much less. In all, acceptable spectra were obtained for 21 SNR candidates in NGC 300 and 24 candidates in NGC 7793.

Vol. 108

The spectra were reduced in IRAF by using the LONGSLIT reduction package and Ñux-calibrated using observations of Stone & Baldwin (1983) standard stars. However, since conditions were not always photometric for the spectra and because of residual uncertainties regarding the exact aperture placements, the absolute Ñuxes from the imaging are probably more accurate estimates of total Ñuxes from individual objects (with the caveat that [N II] contaminates the Ha images). One-dimensional spectra were extracted for each of the SNR candidates, with backgrounds taken as close to the nebulae as possible, using the APEXTRACT package in IRAF. Line Ñuxes were extracted using IRAFÏs SPLOT task. Results of these measurements for SNR candidates in NGC 300 and NGC 7793 are shown in Tables 4A and 5A, respectively. For some of the objects with [O I] values listed in the tables, the reality of the listed feature is questionable and, at some level, subjective. These are highlighted with colons after the table entry. However, the relative intensities of [O I], even when it is clearly present, are substantially more uncertain than for other lines. Examples of the SNR spectra are shown in Figures 5a (for NGC 300) and 5c (for NGC 7793) ; these spectra are representative of some of the brightest and faintest SNRs observed in each galaxy, to provide an indication of the range in data quality. A number of H II regions lie incidentally along the various slit positions in both galaxies, and spectra were extracted from these objects for comparison. Tables 4B and 5B list the intensities of the strongest lines in the H II region spectra for each galaxy. (Note that many of these spectra also show weaker lines such as He I jj5876, 6678.) The H II regions for NGC 300 are cross-referenced to the identiÐcations from Deharveng et al. (1988) whenever possible, although in some cases our spectra may correspond to an edge or outer portion of a much brighter Deharveng object. The regions corresponding to the extracted spectra are indicated in Figure 1b (for NGC 300) and Figure 2b (for NGC 7793) and cross-referenced to Tables 4B and 5B.

TABLE 4A SPECTRA OF SNR CANDIDATES IN NGC 300 Object

GCD (kpc)

Hb j4861

[O III] Total

[O I] j6300

Ha j6563

[N II] Total

[S II] j6716

[S II] j6731

F(Ha)a

SHaTb

N300-S1 . . . . . . . N300-S2 . . . . . . . N300-S4 . . . . . . . N300-S5 . . . . . . . N300-S6 . . . . . . . N300-S7 . . . . . . . N300-S8 . . . . . . . N300-S9 . . . . . . . N300-S10 . . . . . . N300-S13 . . . . . . N300-S14 . . . . . . N300-S15 . . . . . . N300-S18 . . . . . . N300-S19 . . . . . . N300-S20 . . . . . . N300-S22 . . . . . . N300-S24 . . . . . . N300-S25 . . . . . . N300-S26 . . . . . . N300-S27 . . . . . . N300-S28 . . . . . .

4.76 3.87 2.81 3.61 3.11 2.48 1.96 1.63 3.72 1.21 0.78 1.79 1.83 1.49 4.28 1.82 2.05 2.14 3.72 3.86 5.07

105 111 86 105 103 61 110 107 96 59 81 77 99 100 100 100 85 98 94 107 96

\50 245 172 35 378 485 \30 35 255 271 254 190 286 215 \100 \100 118 51 134 77 41

130 26 84 \30 67 \40 \50 16 23 36 : 79 68 53 : 50 85 \80 77 56 120 145 36

300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300

53 98 97 72 105 102 84 102 106 177 165 122 159 128 106 67 96 67 118 88 63

126 136 169 124 165 122 106 117 78 181 212 131 116 157 188 100 133 145 148 188 127

78 94 127 81 122 93 76 83 63 114 137 90 98 113 111 72 116 96 161 126 88

2.1 12 3.6 14 1.7 2.3 3.0 7.8 12 2.1 1.9 2.6 2.4 9.0 0.9 1.4 2.4 4.8 1.4 4.1 14

12 29 8.6 30 12 10 17 49 75 9.5 8.6 12 11 50 6.4 5.4 13 27 40 19 78

a Ha Ñuxes in units of 10~15 ergs cm~2 s~1. b Average Ha surface brightnesses, in units of 10~17 ergs cm~2 s~1 arcsec~2.

No. 1, 1997


267

TABLE 4B SPECTRA OF H II REGIONS IN NGC 300 Object

D88a ID

GCD (kpc)

Hb j4861

[O III] Total

Ha j6563

[N II] Total

[S II] j6716

[S II] j6731

F(Ha)b

SHaTc

N300-H1 . . . . . . . N300-H2 . . . . . . . N300-H3 . . . . . . . N300-H4 . . . . . . . N300-H5 . . . . . . . N300-H6 . . . . . . . N300-H7 . . . . . . . N300-H8 . . . . . . . N300-H9 . . . . . . . N300-H10 . . . . . . N300-H11 . . . . . . N300-H12 . . . . . . N300-H13 . . . . . . N300-H14 . . . . . . N300-H15 . . . . . . N300-H16 . . . . . . N300-H17 . . . . . . N300-H18 . . . . . . N300-H19 . . . . . . N300-H20 . . . . . . N300-H21 . . . . . . N300-H22 . . . . . .

... ... 24 30 32 ... 37(s) 37 39 45 52 56 98 100 ... ... 119a 118a ... 129 ... 137a

5.25 3.19 3.24 3.46 3.13 2.48 2.41 2.44 2.08 1.64 1.47 1.21 0.65 0.42 0.58 0.98 2.11 1.77 2.32 2.52 3.57 2.42

120 96 93 79 80 74 93 80 79 78 111 105 82 102 64 106 84 95 82 85 92 82

\30 1075 158 \40 19 : 96 200 39 36 182 \30 37 : 25 : 41 173 291 263 72 45 145 214 28 :

300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300

48 29 56 89 85 78 43 65 86 106 61 98 78 84 129 56 47 90 106 64 49 68

52 33 32 80 55 44 41 48 39 60 60 89 36 55 66 32 36 62 69 24 21 56

35 25 22 71 46 34 32 42 35 50 39 46 22 39 39 22 28 42 54 20 19 47

6.6 8.3 9.3 1.7 6.5 30 15 7.3 14 3.7 3.2 5.7 7.8 75 5.7 52 110 19 10 120 110 2.9

1.3 5.9 6.6 0.77 3.0 14 7.0 3.3 7.8 1.7 1.5 2.6 3.0 42 2.6 29 26 8.6 4.5 55 50 1.3

a Cross-references to Deharveng et al. 1988 H II region catalog ; see also So†ner et al. 1996, Tables 3È5. b Measured Ha Ñuxes in units of 10~15 ergs cm~2 s~1. c Measured average Ha surface brightness over the long-slit spectral extraction region, in units of 10~16 ergs cm~2 s~1 arcsec~2.

Example H II region spectra are shown in Figures 5b and 5d, once again spanning the range from bright to faint nebulae in each galaxy. Since our H II region data were gathered largely serendipitously, our sample is not as heavily biased toward bright H II regions as typical H II region studies (cf. Pagel et al. 1979 ; Webster & Smith 1983).

From inspection of these long-slit spectra in twodimensional format, it was clear that there was a very substantial di†use component of emission present in NGC 7793, especially in the inner portions of the galactic disk, and that this di†use gas had a considerable [S II] : Ha ratio. This di†use component is quite apparent in Figure 2e and is

TABLE 5A SPECTRA OF SNR CANDIDATES IN NGC 7793 Object

GCD (kpc)

Hb j4861

[O III] Total

[O I] j6300

Ha j6563

[N II] Total

[S II] j6716

[S II] j6731

F(Ha)a

SHaTb

N7793-S1 . . . . . . . . N7793-S2 . . . . . . . . N7793-S3 . . . . . . . . N7793-S6 . . . . . . . . N7793-S7 . . . . . . . . N7793-S8 . . . . . . . . N7793-S9 . . . . . . . . N7793-S10 . . . . . . . N7793-S11 . . . . . . . N7793-S12 . . . . . . . N7793-S13 . . . . . . . N7793-S16 . . . . . . . N7793-S17 . . . . . . . N7793-S18 . . . . . . . N7793-S19 . . . . . . . N7793-S20 . . . . . . . N7793-S21 . . . . . . . N7793-S22 . . . . . . . N7793-S23 . . . . . . . N7793-S24 . . . . . . . N7793-S25 . . . . . . . N7793-S26 . . . . . . . N7793-S26a . . . . . . N7793-S26b . . . . . . N7793-S26c . . . . . . N7793-S27 . . . . . . . N7793-S28 . . . . . . .

2.53 3.83 2.39 1.30 2.58 4.08 3.89 1.03 2.53 0.53 1.75 1.21 3.08 2.09 3.19 3.10 1.70 1.92 2.67 2.15 2.42 4.48 4.50 4.50 4.50 3.78 3.64

\100 104 \80 100 : 97 51 \80 142 111 83 122 \100 84 87 82 90 85 100 102 88 82 94 92 60 \60 99 96

\100 221 \80 753 125 97 \80 541 178 97 1309 \100 231 56 76 132 265 56 92 261 203 190 468 223 \100 173 90

\100 30 : 60 140 27 : 93 : 74 : 110 74 46 166 \120 29 : 19 : \20 23 28 24 15 : 25 98 : 73 40 36 60 : 42 : 72 :

300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300

109 64 86 335 115 67 58 207 113 176 270 229 89 117 84 83 150 106 88 128 64 70 81 76 79 88 122

117 77 186 306 114 138 79 263 155 113 250 285 76 96 80 97 116 85 74 104 55 107 97 90 81 138 156

92 57 138 232 80 105 61 201 109 91 217 177 52 75 48 70 85 58 57 80 45 84 69 66 70 99 106

10 44 14 8.0 78 31 18 13 51 150 9.1 6.7 68 120 38 120 66 84 39 140 19 360 120 47 14 20 9.1

4.5 20 10 5.7 35 22 10 9.3 51 110 9.1 4.8 48 55 27 88 47 38 28 64 8.6 200 68 34 9.6 9.1 6.5


268

BLAIR & LONG

Vol. 108

FIG. 5a

FIG. 5b

FIG. 5c

FIG. 5d

FIG. 5.ÈExample SNR and H II region spectra for NGC 300 and NGC 7793, showing the range in data quality from the faintest to the brightest objects observed of each type. (a) NGC 300 SNRs ; (b) NGC 300 H II regions ; (c) NGC 7793 SNRs ; (d) NGC 7793 H II regions. Note the strong [S II] : Ha in the SNR candidates in comparison with the H II regions ; however, the faint H II regions show somewhat elevated values of this ratio in comparison with the brighter H II regions.

also the subject of a recent study by Ferguson et al. (1996). We have extracted a number of sample spectra of this component from the regions labeled in Figure 2e, again where they lay along our long-slit positions. The measured line intensities are listed in Table 5C. These spectra were extracted from relatively large slit sections, and so the GCDs shown are only representative. The sky background

was removed using regions of the slit as far from the inner galaxy as possible, so the galaxy background continuum is not well subtracted in many of these spectra. While this could a†ect our measurements of these faint emission lines in principle, we believe that the data are of sufficient quality to indicate that the trend toward very high [S II] : Ha (and elevated [N II] : Ha ratio) in this di†use component is real.

No. 1, 1997


269

TABLE 5B SPECTRA OF H II REGIONS IN NGC 7793 Object

GCD (kpc)

Hb j4861

[O III] Total

He I j5876

Ha j6563

[N II] Total

[S II] j6716

[S II] j6731

F(Ha)a

SHaTb

N7793-H1 . . . . . . . N7793-H2 . . . . . . . N7793-H3 . . . . . . . N7793-H4 . . . . . . . N7793-H5 . . . . . . . N7793-H6 . . . . . . . N7793-H7 . . . . . . . N7793-H8 . . . . . . . N7793-H9 . . . . . . . N7793-H10 . . . . . . N7793-H11 . . . . . . N7793-H12 . . . . . . N7793-H13 . . . . . . N7793-H14 . . . . . . N7793-H15 . . . . . . N7793-H16 . . . . . . N7793-H17 . . . . . . N7793-H18 . . . . . . N7793-H19 . . . . . . N7793-H20 . . . . . . N7793-H21 . . . . . . N7793-H22 . . . . . .

3.82 3.77 2.58 3.22 2.07 1.67 1.14 1.45 0.93 0.34 3.36 0.79 1.62 2.84 1.15 1.49 2.64 1.79 2.11 3.47 2.87 4.01

102 94 60 80 91 43 81 : 46 71 59 124 68 78 73 68 82 86 97 66 81 85 96

59 203 58 111 41 73 \90 88 41 51 179 91 62 167 \30 144 107 125 34 115 49 : 399

\20 10.2 6.6 : 9.0 12.0 10.2 \20 10.8 \20 \20 12.0 \20 \15 \20 \20 15 15 11.5 \15 9.5 \20 9.6

300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300 300

75 54 73 60 93 62 46 74 98 101 68 100 97 94 112 59 93 68 76 57 67 40

62 36 42 30 65 25 48 38 76 36 44 49 37 62 73 28 46 21 50 34 45 28

48 27 30 22 44 19 27.8 31 50 29 30 32 32 45 51 18 37 15 35 25 30 20

5.5 53 29 66 17 46 2.9 42 41 15 13 15 29 10 4.0 18 17 63 9.5 25 51 31

2.5 24 13 30 7.8 21 2.1 19 9.4 7.0 5.7 6.8 13 4.7 1.8 8.2 7.5 45 4.3 14 2.3 14


We were unable to extract a similar data set from our NGC 300 spectra. The larger angular size of NGC 300 made it difficult to extract clean ““ sky ÏÏ regions in our longslit data. However, it is clear that the di†use component in NGC 300, if it exists, is much fainter than that in NGC 7793. 3.

RESULTS AND DISCUSSION

3.1. Comparison of Imaging and Spectral Results Before proceeding with the discussion of the Ðndings of our survey, we perform some checks for systematic e†ects between our imaging and spectral results. Figures 6a and 6b show a comparison of Ha surface brightnesses for each of the SNR candidates as obtained from our images and our spectral data, for NGC 300 and NGC 7793, respectively. As can be seen, the image surface brightnesses are systematically higher than the spectroscopic values. This can be attributed to several causes. Since the bandpass for imagery TABLE 5C SPECTRA OF DIFFUSE REGIONS IN NGC 7793 Object

GCD (kpc)

[N II] : Ha

[S II] : Ha

F(Ha)a

SHaTb

N7793-D1 . . . . . . . N7793-D2 . . . . . . . N7793-D3 . . . . . . . N7793-D4 . . . . . . . N7793-D5 . . . . . . . N7793-D6 . . . . . . . N7793-D7 . . . . . . . N7793-D8 . . . . . . . N7793-D9 . . . . . . . N7793-D10 . . . . . . N7793-D11 . . . . . .

0.86 0.84 0.36 2.33 1.57 1.98 2.49 1.73 1.51 1.86 1.21

0.49 0.54 0.82 0.51 0.47 0.70 0.47 0.41 0.48 0.31 0.52

0.73 0.57 0.68 0.58 0.54 1.25 0.56 0.59 0.60 0.57 0.77

4.6 7.9 3.9 2.1 6.3 1.2 6.2 3.9 4.6 14.8 13.6

11 19 18 9.5 7.6 4.1 15 13 11 24 17


also included emission from the [N II] lines, this would cause the image data to be an overestimate of the ““ Ha ÏÏ Ñux. In SNRs, the [N II] lines are often stronger than in H II regions, which could exacerbate this e†ect (cf. Fig. 3 of Smith et al. 1993). Figures 7a and 7b, which include data for both H II regions (open circles) and SNRs ( Ðlled circles), show that this e†ect is present in the current sample as well.3 Comparison with Tables 4A and 5A indicates that this e†ect could typically cause a 25%È50% overestimate in the Ha Ñuxes from imaging. Second, the weather conditions were photometric for the imaging observations, while conditions were variable for the spectra, with cirrus present at times. This would lower some of the spectral Ñuxes. Finally, since the slit placements for the spectra were done by blind o†set, it is possible that some of the SNR candidates were not entirely within the slit, which would systematically lower their derived surface brightnesses from the spectra. The observed e†ect in many cases is larger than expected for [N II] contamination of Ha in the images alone, and therefore we regard the Ñuxes derived from the images as more uniform than the Ñuxes from the spectra. Figures 8a and 8b show comparisons between image and spectral data, this time for the derived [S II] : Ha ratios. Here a systematic e†ect is seen whereby the ratios from spectral data are higher than those derived from the images (in many cases, signiÐcantly higher). Recalling again that the image data included [N II] along with Ha, the reason for the trend is obvious. However, the magnitude of the e†ect is larger 3 The crosses in Fig. 7a and subsequent Ðgures for NGC 300 are bright NGC 300 H II region data from Pagel et al. (1979). Note that these bright H II regions have systematically smaller [S II] : Ha ratios than our H II region sample. Interestingly, the single high [S II] : Ha Pagel et al. point (their H II region No. 4 \ Deharveng No. 159) was found by us to be an SNR (N300-S26), including strong [O I] emission ! Since Pagel et al. only used six H II regions (and this was one of only two at reasonably large GCD), their abundance-gradient results cannot help but be a†ected by this accidental occurrence.

270

BLAIR & LONG

FIG. 6a

Vol. 108

FIG. 6b

FIG. 6.ÈComparison of Ha surface brightnesses as estimated from our images and spectra : (a) NGC 300 ; (b) NGC 7793. The values from our imaging are systematically higher, for reasons discussed in the text.

3.2. Results of the SNR Survey Inspection of Tables 4A and 5A reveals that all the SNR candidates observed spectroscopically in both galaxies have

conÐrmed [S II] : Ha ratios that exceed 0.4, with the single exception of N7793-S25. (This is one of the objects mentioned earlier as having a star cluster projected near its center.) Even the candidate objects with imaging ratios below 0.4 have spectroscopic ratios well in excess of the normal threshold to be considered SNRs. Surprisingly, this also includes the SNR ““ candidates ÏÏ with substantially lower values of [S II] : Ha that were carried along (cf. N300S22 and N7793-S22), although their ratios are at least toward the lower end of the values observed for other SNR candidates. Over 80% of the objects also show evidence of emission from [O I] jj6300, 6363, which is often taken as secondary evidence for shock heating ; [O I] is exceedingly

FIG. 7a

FIG. 7b

than we expected, approaching or exceeding a factor of 2 in the ratio in many cases. One of the implications of this e†ect, since the image ratio was used to deÐne the sample in the Ðrst place, is that we have probably missed a signiÐcant number of candidate SNRs. In the comparisons that follow, we will use the Ha surface brightnesses from our imagery for SNR candidates, but we will use spectroscopically determined values of the [S II] : Ha ratio.

FIG. 7.ÈObserved [S II] : Ha ratios plotted vs. [N II] : Ha ratios for all the objects in Tables 4 and 5, for (a) NGC 300 and (b) NGC 7793. In this and the following Ðgures, H II regions are indicated by open circles and SNRs are indicated by Ðlled circles. The crosses in (a) indicate bright H II regions from Pagel et al. (1979) for comparison. (The highest cross in [a] was identiÐed in our survey to be an SNR.) Note that the SNRs tend toward both higher [S II] : Ha and [N II] : Ha ratios but that there is no clear dividing line in [S II] : Ha ratio between SNRs and H II regions. This latter point is di†erent than seen in other SNR samples.

No. 1, 1997


FIG. 8a

271

FIG. 8b

FIG. 8.ÈComparison of the [S II] : Ha ratios as derived from our imagery and spectroscopy for SNRs in (a) NGC 300 and (b) NGC 7793. Note the tendency for higher ratios from our spectra, due to [N II] contamination of the Ha images.

weak in H II region spectra because nearly all the oxygen is ionized (see Evans & Dopita 1985). (Note that our tables only list the measurement of the stronger [O I] line at 6300 Ó.) The [O I] lines are very strong in the night sky, however, and the redshifts of both galaxies are small (145 km s~1 for NGC 300 and 215 km s~1 for NGC 7793 ; see KraanKorteweg & Tammann 1979). Although our long-slit spectra allow good subtraction in most cases, any [O I] from the faintest candidates is lost in the noise or else the line is not present in the spectrum. There are four objects in the NGC 7793 sample with [S II] : Ha ratios in the 0.33È0.45 range that do not clearly show [O I] emission (N7793-S17, -S19, -S23, and -S25). Because the H II region sample in NGC 7793 appears to have a high mean value of [S II] : Ha compared with other galaxies (see discussion below), the status of these nebulae is unclear. With these possible exceptions (and with the probable exception of N7793-S25), we conclude that the objects we have observed spectroscopically are indeed SNRs. Because the spectroscopic [S II] : Ha ratios seem to be systematically higher than the imaging ratios, we believe it likely that the remaining SNR candidates that have not been observed spectroscopically are also SNRs. Of the seven objects that DDB 80 identiÐed as SNR candidates in NGC 300, Ðve are contained within our survey region. Of these, only twoÈDDB 2 and DDB 5Èwere identiÐed as SNRs in our survey. These are the same two nebulae DÏOdorico & Dopita (1983) conÐrmed as SNRs previously with spectroscopy. Three other DDB80 candidatesÈDDB 1, DDB 4, and DDB 7Èare present in our images, but they are dominated by continuum emission. DDB 80 apparently did not check against continuum images of NGC 300, and therefore it is not surprising that some of the objects in their list of candidates are dominated by continuum emission. DDB 2 and DDB 5 are by far the two highest surface brightness objects in our NGC 300 sample ; only one new SNR is within a factor of D2 of these two objects, with the remainder of the new SNRs being fainter by factors of 4È40 (see Table 3A). As noted earlier,

NGC 7793 has not been surveyed previously for SNRs. We Ðnd a range of surface brightnesses (Table 3B) similar to that found for NGC 300, with the highest surface brightness objects being comparable to those in NGC 300 and, if anything, a few more objects at the high surface brightness end of the distribution (see Figs. 6a, 6b). As shown in Figures 7a and 7b, many of the SNR candidates appear to show enhanced values of [N II] : Ha ratio compared with H II regions. Although not terribly well documented in the literature, this is a characteristic often seen when comparing SNRs and H II regions within a galaxy (cf. Blair et al. 1981 ; Smith et al. 1993). This ratio is seen to vary in both SNR samples and H II region samples as a function of GCD. We show this comparison for our galaxies in Figures 9a and 9b, where again solid circles represent SNR candidates and open circles represent H II region data. The systematic trend is most obvious within the inner 1.5È2 kpc of each galaxy. We take this as additional evidence that most of the candidates observed spectroscopically are indeed SNRs. For completeness, we show in Figures 10a and 10b the [S II] : Ha ratio for SNRs and H II regions in each galaxy as a function of GCD. Other than the characteristic o†set between the two types of nebulae, no identiÐable trend with galactic radius is apparent. Although neither [S II] : Ha nor [N II] : Ha can be used to determine abundances without an analysis of ionization conditions, radial gradients even in line intensity ratios are often seen, especially for [N II] : Ha. The inclusion of many lower surface brightness H II regions may partially mask the o†set between SNRs and H II regions normally seen in this ratio in Local Group galaxies. Even though the optical criterion we use to Ðnd extragalactic SNRs appears to be more e†ective than radio or X-ray techniques, determining SNR diameters from optical data alone can be misleading. In Local Group galaxies, where the scale is more favorable than in the Sculptor galaxies, we know that optical diameter measurements can be fraught with difficulties (Blair & Davidsen 1993). Nevertheless, the majority of the nebulae we have identiÐed appear

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FIG. 9a

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FIG. 9b

FIG. 9.ÈObserved [N II] : Ha ratios vs. deprojected galactocentric distance for SNRs and H II regions in (a) NGC 300 and (b) NGC 7793. (Symbols are as described in Fig. 7.) Note the trend toward higher values of this ratio at small GCDs, as seen in many other galaxies. There is also a tendency for SNRs to have higher values of [N II] : Ha compared with H II regions, although this is masked somewhat by observational scatter.

to be resolved and have a reasonably well deÐned extent ; Tables 3A and 3B list the diameters to the extent that we can measure them. The smallest SNRs in each galaxy are essentially at the seeing disk value of D1A. 5 (note that 1A \ 10.2 pc in NGC 300 and 16.5 pc in NGC 7793 at the assumed distances), and the apparent diameters should be considered upper limits. Figures 11a and 11b show the distribution of the SNR surface brightnesses with our measured diameters. There is a very slight trend for the relatively small diameter objects to have higher surface brightnesses. However, there are relatively few objects at or near the low-diameter end of the

distribution, perhaps indicating that we are systematically missing the smallest diameter objects. Low to moderate surface brightness objects smaller than the seeing disk could be smeared out and drop below our detection threshold. The majority of the objects we identify as SNRs in these galaxies have apparent diameters in the 25È75 pc range, implying that we are mostly Ðnding the well-evolved, ISMdominated population of SNRs for which the search technique works best. However, there are a signiÐcant number of objects that, at least at the adopted distances for these galaxies, have diameters between 75 and 200 pc. (The object in NGC 7793 with a listed diameter of 260 pc will be dis-

FIG. 10a

FIG. 10b

FIG. 10.ÈObserved [S II] : Ha ratios vs. deprojected galactocentric distance for SNRs and H II regions in (a) NGC 300 and (b) NGC 7793. (Symbols are as described in Fig. 7.) Although the general o†set between SNRs and H II regions in each galaxy is apparent, no trend with galactic radius is apparent for this ratio.

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FIG. 11a

273

FIG. 11b

FIG. 11.ÈAverage Ha surface brightnesses of our SNR samples (as derived from our imagery) plotted vs. SNR diameter for (a) NGC 300 and (b) NGC 7793. Although the higher surface brightness SNRs tend to have smaller diameters, there is a large scatter at any given diameter. The same trend is seen in the M33 SNR sample of Gordon et al. (1996b).

cussed separately below.) These objects have no obvious Galactic counterparts.4 A handful of M33 SNRs have diameters in the 80È120 pc range. In fact, the largest NGC 300 SNR, N300-S2, is reminiscent in appearance of the largest M33 SNR, 013007]30252 (see LBKW). These objects are found at relatively large GCDs, perhaps indicating a trend toward larger diameters in regions away from the galactic 4 In actuality, the distances to individual Galactic SNRs are often not known well enough to permit accurate diameter determinations. However, to the extent they are available, the known Galactic SNRs seem to have diameters less than D60È65 pc ; see Green (1984) for a discussion.

nucleus. Figures 12a and 12b show plots of our diameter measurements versus GCD for our SNR samples. A trend toward larger diameters at larger GCDs is suggested in both galaxies, but is by no means conclusive. The surface brightnesses of the SNRs show no strong trend with either diameter (Figs. 11a, 11b) or GCD (Figs. 13a, 13b), so this may be a real e†ect. However, it is not clear that the energetics of these large-diameter objects are compatible with single SNR explosions. They may represent a class of object that is intermediate between supershells (i.e., kiloparsecscale nebulae with D1053 ergs of inferred kinetic energy) and isolated SNRs. Measurements of the actual expansion

FIG. 12a

FIG. 12b

FIG. 12.ÈSNR diameters plotted vs. deprojected galactocentric distance for (a) NGC 300 and (b) NGC 7793. There is a tendency for large-diameter SNRs to be found at large GCD in both samples.

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FIG. 13b

FIG. 13.ÈSurface brightnesses of the SNR samples vs. GCD for (a) NGC 300 and (b) NGC 7793. These plots show that there is no strong trend with GCD and no particular tendency for low surface brightness SNRs missing from the samples at low GCD (although an e†ect within 1 kpc is not precluded).

velocities of some of these large shells are needed to pursue this further. The region of N7793-S26 deserves individual comment. The SNR candidate listed as N7793-S26 in Table 3B is the bright, high surface brightness portion of a much more extended, roughly oval emission region that maintains a high [S II] : Ha ratio. The appearance of the region gives the distinct impression that this is one physical structure. The listed diameter of the extended region (260 pc) is a mean of the measured major and minor axes of the ellipse containing the emission (180 pc ] 340 pc), with the major axis lying roughly along a southeast-northwest direction. Although considerable structure is present in this nebula, it does not have the multiple loop structure of some of the other largediameter objects mentioned above, and the surface brightness is higher and more spatially variable. This region may represent a location where multiple SNs have been caught in the process of blowing out a large bubble of shockexcited gas, perhaps eventually becoming a superbubble, as seen, for instance, in the LMC (Chu & Mac Low 1990). We note, however, that there is no evidence of an interior star cluster or associated continuum emission in this case. This region is of interest for further study. In principle, we could use the density-sensitive ratio of [S II] jj6716, 6731 to estimate the electron densities in each object. In practice, our data are not of sufficient quality in most cases to determine this ratio accurately. Most of the listed values in Tables 4A, 4B, 5A, and 5B show this ratio to be at or near the low-density limit value of 1.43 (see Blair & Kirshner 1985), as would be expected, particularly for the H II regions (but often seen in SNRs as well). Some objects have unphysical ratios º1.43, presumably due to noise a†ecting the measurements of some of the fainter objects, while a few others have values near 1.0. In particular, we note N300-S26 (\DDB 5), with a ratio of 0.92 and an inferred electron density near 500 cm~3 (see Blair & Kirshner 1985). This object is presumably expanding into dense preshock material (N \ 10È20 cm~3), resulting in a high e emissivity that permitted the object to be identiÐed pre-

viously (DDB80). The other object found previously, N300S10 (\DDB 2), also has a density well above the low-density limit, partially accounting for its brightness and detection. 3.3. Sample (In)completeness and Confusion E†ects The vast majority of the objects identiÐed in our surveys satisfy the conventional optical criterion for shock heating and are likely to be SNRs. Our conÐdence arises from the analysis of the combined imaging and spectroscopy data sets : Even though we thought we were applying a low threshold in [S II] : Ha from the images in order to Ðnd candidate SNRs, the spectra show systematically higher [S II] : Ha ratios, with essentially all the spectroscopically observed candidates being conÐrmed. Even the test objects with much lower image ratios than our threshold have [S II] : Ha ratios above 0.4. We are far less conÐdent, however, about the completeness of the samples ; there are likely numerous objects just below the ratio threshold used in our imaging survey that would have spectroscopic [S II] : Ha ratios above 0.4. Also, because of the relatively large distances of these galaxies compared with the Local Group surveys, we may be systematically missing the smallest diameter objects. Hence we suspect that the SNR samples we have identiÐed are seriously incomplete. However, it is not straightforward to analyze the completeness of our samples (or, should we say, the extent of the incompleteness), for a number of reasons. In unconfused regions away from the galaxy nuclei, there is a surface brightness limit : only one object was found with an Ha surface brightness less than 1.0 ] 10~16 ergs cm~2 s~1 arcsec~2. It would not be surprising if this surface brightness limit was dependent on the size of the object and its GCD ; there is a tendency to overlook large-diameter objects near the low surface brightness limit because they are difficult to distinguish against the background. Likewise, as mentioned earlier, small-diameter objects at low surface brightness will be missed as a result of seeing e†ects. Figures 13a and 13b, however, which show the Ha surface

No. 1, 1997


brightnesses as a function of GCD for the two samples, do not show a strong e†ect with GCD, with the possible exception of the inner kiloparsec of each galaxy. (Indeed, the lowest surface brightness object in NGC 7793 is near 1.3 kpc galactic radius.) The surface brightness range is similar to the much larger M33 sample (Gordon et al. 1996b), in which only four out of 98 SNRs were found with Ha surface brightnesses below 1.0 ] 10~16 ergs cm~2 s~1 arcsec~2. This limit must be higher within or near H II regions. Objects in particularly complex regions of emission could have been missed entirely since we do not have an e†ective way to subtract the contribution of the H II emission. Gordon et al. (1993) used combined radio, X-ray, and optical techniques to Ðnd an SNR buried in the giant H II region NGC 592 in M33, even though this region was within the area surveyed by LBKW. Still, judging from our experience in M33, we do not expect this e†ect to cause serious incompleteness in the sample. We have found a number of SNRs in fairly complex regions of emission and on the edges of H II region complexes, which tends to support this conclusion. Another factor a†ecting completeness is that not all known optical SNRs have [S II] : Ha ratios above 0.4. In our Galaxy, for instance, there are a few conÐrmed SNRs whose observed [S II] : Ha ratios are below 0.4 (e.g., OA 184 ; see Fesen, Blair, & Kirshner 1985 ; also Balmer-dominated SNRs like Tycho and SN 1006 ; see Smith et al. 1991), although such objects are the exception rather than the rule. In addition, for SNRs with relatively low intrinsic [S II] : Ha ratios, H II region emission along the line of sight could ““ contaminate ÏÏ the SNRÏs spectrum and artiÐcially lower the observed ratio below our threshold. In most cases, however, the presence of such contamination can be judged directly from the imagery or long-slit spectroscopy and corrected to Ðrst order. We note that the search criteria that we have applied are biased toward Ðnding conventional, ISMdominated SNRs, which form the bulk of the known SNRs in the Local Group galaxies studied to date. A more serious question for the completeness of our sample is whether the observational test we have deÐned for an SNR remains valid, in particular at the lower surface brightnesses. We have deÐned an SNR as an emission-line object with an [S II] : Ha ratio that exceeds 0.4. As mentioned earlier, this deÐnition is based on the observational fact that most known SNRs have [S II] : Ha ratios that exceed this ratio while most H II regions have [S II] : Ha ratios well below this value, even in galaxies with a wide range of mean heavy-element abundances (see, e.g., DÏOdorico 1978 ; Blair et al. 1982 ; Dopita et al. 1980 ; Blair & Kirshner 1985). This criterion has been used for essentially all optical imaging SNR surveys to date (DDB80 ; Blair et al. 1981 ; Mathewson et al. 1983, 1985 ; LBKW ; Williams et al. 1995). Theoretical models of shock waves (Dopita et al. 1984 ; Hartigan, Raymond, & Hartmann 1987) and H II regions (Stasinska 1982 ; Evans & Dopita 1985 ; Dopita & Evans 1986) support this general picture. Generally speaking, objects identiÐed as SNRs on the basis of [S II] : Ha ratios from imaging surveys have proved to be SNRs when detailed studies were carried out at optical, radio, and/or X-ray wavelengths (Mathewson et al. 1983 ; Smith et al. 1993 ; Gordon et al. 1996a, 1996b ; Long et al. 1996). The success of the [S II] : Ha criterion for identifying SNRs, however, does not lie in the absolute value of the

275

““ dividing line ÏÏ but in the fact that, for most galaxies observed to date, there is a wide gap in observed ratios between the vast majority of SNRs and H II regions (cf. Blair et al. 1982 ; LBKW ; Smith et al. 1993). Hence the ““ magic ÏÏ value of 0.4 has worked in a wide variety of situations. Relatively few objects have been identiÐed in previous surveys that populate this gap. Blair et al. (1982) noted several emission regions in M31 that appeared to cross the line when all information (e.g., spectra, morphology) was taken into account. In M33, the Ðrst galaxy for which a deeper CCD-based SNR survey has been published (LBKW), the 0.4 criterion was still e†ective, but a number of faint nebulae were found in this imaging survey that populated the intermediate-ratio region (in particular, that had [S II] : Ha ratios of 0.2È0.3). Presumably, most of these nebulae are low-ionization H II regions with somewhat enhanced [S II] emission relative to the brighter H II regions. For some reason, this gap is much less pronounced in these Sculptor Group galaxies, and may not exist at all in NGC 7793. Figures 14a and 14b show the [S II] : Ha ratios for all the nebulae in Tables 4A, 4B, 5A, and 5B as a function of Ha surface brightness. As one looks at fainter H II regions in both galaxies, the observed [S II] : Ha ratio not only approaches but even crosses over the nominal boundary line between H II regions and SNRs. This trend was also obvious in our SNR search, in which a number of emission regions were found with ratios near but below our threshold. Apparently, the low surface brightness H II regions sampled serendipitously in our long-slit spectra have elevated [S II] : Ha ratios in comparison with the norm for Galactic H II regions and for bright Local Group and giant extragalactic H II regions. The situation remains relatively ““ clean ÏÏ in NGC 300, for which a line sloping downward toward higher surface brightnesses in Figure 14a can still divide the H II regions and SNRs. However, there appears to be overlap at low to moderate surface brightnesses in the NGC 7793 sample shown in Figure 14b. Much of this confusion would be alleviated if the four ““ marginal ÏÏ NGC 7793 SNR candidates discussed above were simply reclassiÐed as low surface brightness H II regions that crept into our SNR candidate list by virtue of their low ionization. However, the point remains that the ““ gap ÏÏ so prevalent in previous SNR/H II region comparisons is simply not present in these galaxies. Part of the reason for this di†erence may be that we have sampled systematically fainter H II regions in the Sculptor galaxies than has been the case for other galaxies. Interestingly, relatively few H II region data are published for M33, and what is available is mainly for bright H II regions (cf. Kwitter & Aller 1981 ; Zaritsky, Elston & Hill 1989 reported on 55 H II regions in M33, but did not provide the line intensities). As mentioned above, fainter H II regions in M33 probably have somewhat elevated [S II] : Ha ratios compared with the bright H II regions (LBKW), but whether this e†ect is as apparent as it is in NGC 300 (for instance) is unclear. There is no consistent deÐnition of the cuto† between the faintest H II regions and the truly di†use ionized gas (DIG), but there is evidence that the lowest surface brightness regions of galaxies tend to have higher intrinsic values of [S II] : Ha ratio than the brighter H II regions normally studied. Reynolds (1988) has investigated the properties of di†use Galactic H II regions in an attempt to explain the

276

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FIG. 14a

FIG. 14b

FIG. 14.ÈPlot of [S II] : Ha ratio for all the nebulae in Tables 4 and 5 vs. Ha surface brightness for (a) NGC 300 and (b) NGC 7793. Note the large di†erence in the mean ratio for SNRs and H II regions in both galaxies. However, the increase in [S II] : Ha ratio with decreasing surface brightness for H II regions causes confusion at the faintest levels surveyed. The problem is more severe for NGC 7793. The squares in (b) show the observed [S II] : Ha ratios for di†use regions in NGC 7793, extracted from our long-slit data, with typical values near 0.65.

[S II] : Ha ratios of the di†use ISM in our Galaxy. His data suggest a weak correlation between [S II] : Ha ratio and emission measure ; di†use H II regions with emission measures of less than 100 pc cm~6 appear to have [S II] : Ha ratios that are larger by about a factor of 1.5 than do H II regions with emission measures of over 100 pc cm~6, with intrinsic values of [S II] : Ha approaching 0.3 in some cases. Furthermore, Reynolds found a clear correlation between the temperature of the ionizing stars and the [S II] : Ha ratio ; H II regions ionized by stars with T \ 25,000 K have higher [S II] : Ha ratios than do H II regions ionized by stars with temperatures in excess of 35,000 K. For comparison, the faint nebulae in our Sculptor galaxies typically have emission measures of a few hundred ; therefore, it seems unlikely that these nebulae are sufficiently di†use for this to be the primary cause of the observed e†ect. Even higher values of [S II] : Ha have been seen in the most di†use emission regions in nearby galaxies (Walterbos 1991). Walterbos & Braun (1994), for instance, found a mean value of [S II] : Ha \ 0.5 for the DIG in M31 and found ratios as high as 0.7È1.0 in the lowest emission measure gas they could measure. The di†use component in NGC 7793 is very extensive (see Ferguson et al. 1996), and our measurements indicate this component may be even more extreme in [S II] : Ha ratio than the DIG in M31. From the 11 regions listed in Table 5C, we Ðnd a mean [S II] : Ha value of 0.68 ^ 0.20 and a mean [N II] : Ha ratio of 0.52 ^ 0.14. In addition, Walterbos & Braun (1994) deÐned the DIG in M31 to be the material sampled at or below emission measures of 50È100 pc cm~6 (depending on position), whereas the regions listed in Table 5C range from 35 to 200 pc cm~6, with a mean near 110 pc cm~6. Hence the DIG in NGC 7793 is of substantially higher surface brightness than found in M31. In Figure 14b, the di†use regions from Table 5C are plotted as squares for comparison with the NGC 7793 SNR and H II region samples. These di†use regions seem to form a natural extension of the faintest discrete H II regions

toward lower surface brightness and higher [S II] : Ha ratio. This highlights the importance of using the morphology (i.e., discreteness) of suspected SNR candidates in conjunction with the [S II] : Ha ratio in SNR surveys such as the ones we have described. 4.

CONCLUSIONS

Although the NGC 300 and NGC 7793 SNR samples are still likely to be incomplete, they are the Ðrst reasonably large samples of SNRs to have been identiÐed outside the Local Group. All the SNRs in the sample appear to be relatively evolved, with diameters ranging well above 100 pc for the largest objects. It is somewhat surprising that a similar number of SNRs have been found in the more distant (and intrinsically smaller and lower mass) Sd spiral NGC 7793 in comparison with the Sc spiral NGC 300. Overall, however, the distribution of objects with surface brightness and galactic radius seems to parallel the more extensively observed case of M33 in the Local Group. These facts bode well for surveys of even more distant galaxies using this optical technique to identify shock-heated nebulae. Although we have been quite successful in Ðnding SNRs in this Ðrst use of a deep CCD survey beyond the Local Group, we have also found the dividing line between SNRs and H II regions to be much less distinct than in previous surveys of this type. As we have pushed to lower surface brightness nebulae, and in particular as we have included lower surface brightness H II regions for comparison, we have found the [S II] : Ha ratios in H II regions to approach and even cross the normal dividing line between shockheated and photoionized nebulae. It is as yet unclear whether this is a problem speciÐc to these Sculptor galaxies (and especially NGC 7793) or whether the problem is more generic at low surface brightness levels. We stress the importance of obtaining spectra of candidate SNRs to search for secondary indicators of shock heating, and low surface brightness H II regions (comparable to the SNR

No. 1, 1997


candidates) to permit objects near the dividing line in the [S II] : Ha criterion to be properly diagnosed. We would like to thank the director and sta† of the Las Campanas Observatory for permitting us to use the telescopes and instrumentation necessary to carry out these observations. K. S. L. also acknowledges an appointment as a visiting adjunct associate to the Mount Wilson and Las Campanas Observatories when work on this project was

277

begun. We also thank Robert Kirshner for loaning the interference Ðlters used in the imaging portion of this project. We acknowledge useful discussions with Annette Ferguson regarding the di†use emission components of galaxies, and Chris Smith regarding the large-diameter nebulae in these galaxies in comparison with M33. This research has been supported by the Center for Astrophysical Sciences at Johns Hopkins University.

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PLATE 2

BLAIR & LONG (see 108, 263)

FIG. 1.ÈHa collage of the eight Ðelds observed in NGC 300, showing the full extent of the region covered by our survey. A linear transfer function was used, and each Ðeld imaged an area [email protected] on a side. In (a), the SNR candidates identiÐed in our survey are indicated, with numbers cross-referenced to Table 3A ; (b) shows the same Ha mosaic with the H II regions identiÐed and cross-referenced to Table 4B.

FIG. 1a

PLATE 3


FIG. 1b

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FIG. 2.ÈThe two Ðelds used to image NGC 7793 after aligning and merging into a single image. The images are displayed with logarithmic scaling to increase the dynamic range. The full Ðeld shown is [email protected] ] [email protected]. (a) The 6100 Ó continuum image. (b) The Ha image ; circles indicate the H II regions observed spectroscopically, as listed in Table 5B. (c) The [S II] image ; circles indicate candidate SNRs, as listed in Table 3B. (d) The Ha data after a global scaling and subtraction of the continuum image. (e) The [S II] data after a global scaling and subtraction of the continuum image. The circles are the SNR locations as shown in (c), but without the labels. The labels D1ÈD11 indicate the approximate locations of the regions of di†use emission listed in Table 5C. Note the extensive di†use component visible especially in this image.

FIG. 2a

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FIG. 2b

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FIG. 2c

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FIG. 2d

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FIG. 2e

FIG. 3.ÈThe region centered on each NGC 300 SNR candidate shown in the 6100 Ó continuum (left), continuum-subtracted [S II] (middle), and continuum-subtracted Ha (right). The Ðeld shown is 150 pixels (61A. 5) in extent, and the SNR candidate position is centered in each panel. North is up and east is to the left in each Ðgure, and one candidate per panel is shown unless otherwise indicated. Where necessary for clariÐcation, the SNR candidate is indicated with tick marks. BLAIR & LONG (see 108, 265)

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FIG. 3ÈContinued BLAIR & LONG (see 108, 265)

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