The Astrophysical Journal Supplement Series, 155:101–121, 2004 November # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
AN OPTICAL SURVEY OF SUPERNOVA REMNANTS IN M831 William P. Blair Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218;
[email protected]
and Knox S. Long Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218;
[email protected] Receivved 2004 May 17; accepted 2004 July 7
ABSTRACT Observations of the face-on spiral galaxy M83 ( NGC 5236) performed at the Cerro Tololo Inter-American Observatory in Chile have yielded a catalog of optical supernova remnant (SNR) candidates. These observations were performed with the 4 m Blanco telescope and a prime focus CCD imaging system using narrowband interference filters centered on the light of [S ii], H, [O iii], and red and blue continuum bands. Based on strong relative [S ii]:H emission, 71 emission nebulae have been identified as SNR candidates. Positions and H fluxes of the candidates are presented. Follow-up spectra of 25 of the SNR candidates, also performed at CTIO, have confirmed many of the SNR identifications, although the spectra of a few objects are discrepant, perhaps because of inaccurate aperture placement. In addition, the low mean excitation of M83 H ii regions has allowed a separate search for young oxygen-dominated (core collapse) SNRs similar to Cas A in our Galaxy, using [O iii]:H. This search found a number of the same objects as the [S ii]:H search, indicating that many of these SNRs have shock velocities in excess of 100 km s1. However, no bona fide young core-collapse SNRs were detected with this technique, with the possible exception of the independent recovery of SN 1957D, which had been seen previously. We have also attempted to identify optical counterparts for the six historical supernovae that have occurred in M83. Except for SN 1957D, none of the historical supernovae have been detected by this survey. We compare our SNR candidate list against the Chandra X-ray source list of Soria and Wu and identify 15 X-ray sources as likely SNRs, based on positional coincidence within 100 . The sources identified have hardness ratios that are soft compared to the general X-ray source population in M83. Subject headingg s: galaxies: individual (M83) — galaxies: ISM — H ii regions — supernova remnants
1. INTRODUCTION
SNRs in a galaxy can be used to investigate global properties of the galaxy’s ISM and SNR evolution. They also provide a rough estimate of the SN rate, which can be used to determine the structure, kinematics, and composition of the ISM. Van den Bergh & Tammann (1991) have shown that SNe (and consequently their remnants) are more difficult to detect in galaxies that are nearly edge-on (i > 60 ), affecting estimates of the actual SN rate in these galaxies. By studying galaxies that are nearly face-on, the effects of extinction can be minimized. Extragalactic searches for SNRs were first pioneered by Mathewson & Healy (1964) and then Mathewson & Clark (1973). They used the fact that the optical spectra of SNRs have elevated [S ii]:H emission-line ratios, as compared to the spectra of normal H ii regions. This emission ratio has proven to be an accurate means of differentiating between shockheated SNRs (ratios >0.40, but often considerably higher) and photoionized nebulae ( 0:4. The spectra of these objects were possibly contaminated by overlying H ii emission.
Finally, in Figure 19 we show two plots of line ratios versus galactocentric distance. The top panel shows the [ N ii]:H ratio and the bottom panel shows the [S ii]:H ratio. While neither of these ratios should be construed as showing the true abundances, the [ N ii]:H ratio often shows evidence of an enhanced value toward the center of spiral galaxies ( Blair &
Fig. 18.—[S ii] : H ratio ( y-axis) vs. [ N ii]:H ratio (top) and observed flux in H (bottom). Open circles indicate SNR candidates and filled circles indicate H ii regions. The semifilled circles show the position of SNR candidates (from imagery) whose spectra did not confirm ½S ii:H > 0:4.
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Fig. 19.—Line ratios as a function of galactocentric distance. Top: [ N ii]:H ratio. Bottom: [S ii]:H ratio. Symbols are the same as in Fig. 18.
Long 1997; Matonick et al. 1997; Gordon et al. 1998). In M83 we see little or no evidence of a gradient or a trend in [ N ii]:H or [S ii]:H , for either the SNRs or H ii regions sampled. 3.4. Comparison with X-Ray Observvations Soria & Wu (2002, 2003) analyzed a 50 ks ACIS-S3 exposure of M83 with the Chandra X-Ray Observatory. (Soria & Wu 2002 concentrate on the nuclear region.) These authors detect 127 point sources (>3 ) with a luminosity limit of 3 ; 1036 ergs s1 in the 0.3–8.0 keV band. Of these, 15 are within the bright starburst nuclear region where our optical search for SNRs is confusion-limited. The majority of bright (>1036 ergs s1) X-ray sources in nearby normal galaxies are compact binaries, and that is surely true of M83 as well. However, SNRs also emit X-rays with luminosities that can exceed 1036 ergs s1. As a result, X-ray source emission can be used to confirm that a nebula with the optical properties of an SNR is indeed an SNR. We have compared the Soria & Wu (2003) source list with Table 5 and find for the nonnuclear sources, that 15 X-ray sources align with optical SNR candidates within a 100 radius.4 A simple estimate based on the number of sources in the X-ray and optical samples in the region between 0A5 and 60 of the nucleus indicates that less than one of these is a chance coincidence. The properties of these sources are summarized in Table 9. Here we have taken the total counts listed by Soria & Wu (2003, Table A.1) and converted to an equivalent luminosity by assuming a soft ( Raymond-Smith) 1 keV model and a fixed representative absorption column of N (H) ¼ 1:0 ; 1021 cm2, using the PIMMS tool on the Chandra Web site. We have also derived the hardness ratios listed for each target in the last column of Table 9 by comparing the counts listed 4 Soria & Wu (2003) compared against an early version of the optical SNR candidate list described here and noted that 10 of the X-ray sources lay within 500 of sources in that list. However, the analysis here is to be preferred in that regard.
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TABLE 9 M83 Optical Supernova Remnants with X-Ray Counterparts
Optical ID BL BL BL BL BL BL BL BL BL BL BL BL BL BL BL
7 .............. 15 ............ 18 ............ 24 ............ 29 ............ 31 ............ 35 ............ 37 ............ 40 ............ 41 ............ 46 ............ 52 ............ 53 ............ 58 ............ 63 ............
SW03 IDa
LX(0.3–8 keV) (1037 ergs s1)
J133648.2295244 J133654.1295209 J133655.0295239 J133655.6295303 J133659.3295508 J133659.5295203 J133701.6295410 J133701.7295113 J133702.2294952 J133702.4295126 J133706.0295514 J133707.0295320 J133707.4295133 J133708.5295135 J133711.9295215
0.35 0.23 0.55 0.89 0.21 0.66 0.24 0.50 0.51 1.45 1.34 0.21 0.31 0.21 1.03
(M S)/(M + S)b 0.27 0.68 0.72 0.46 0.78 0.53 0.64 0.75 0.49 0.55 0.55 0.82 0.94 0.80 0.69
0.28 0.16 0.13 0.18 0.21 0.18 0.20 0.16 0.19 0.10 0.10 0.18 0.06 0.19 0.10
a
Chandra source identification from Soria & Wu (2003). X-ray hardness ratio made using medium (1–2 keV ) and soft (0.3– 1 keV ) energy bands. b
by Soria & Wu (2003) in defined energy bands (medium, 1.0– 2.0 keV, and soft, 0.3–1.0 keV ). In Figure 20 we show a plot of this hardness ratio for the Soria & Wu source list (dashed line) and for the optical / X-ray SNR overlaps (solid line). As might be expected a priori, the sources identified with optical SNRs are quite soft, and it is clear that soft X-ray sources [with ( M S)=( M þ S) of 1.0 to 0.4] are preferentially identified with optical SNR candidates. In Figure 21 we show a histogram of the number of SNR candidates in various H flux bins, using the imaging H
Fig. 20.—Histogram of X-ray hardness ratios calculated using data from Table A.1 of Soria & Wu (2003) from the medium (1-2 keV ) and soft (0.3-1 keV ) count rate Chandra data. The dashed line is for the entire listing of X-ray sources, and the solid line is for the subset identified with optical SNR candidates. It is likely that many of the other soft sources are also SNRs.
Fig. 21.—Histogram of the imaging H fluxes of SNR candidates. All SNR candidates from Table 5 are shown by the dashed line, and those sources with X-ray counterparts are shown by the solid line. This comparison clearly shows that the brighter H candidates are being systematically identified in the Chandra data.
fluxes from Table 5. The dashed line shows the entire candidate list and the solid line highlights the sources coincident with X-ray sources. From this comparison, it is clear that the brighter H SNR candidates are being systematically identified as X-ray sources. This is consistent with the expectation that the brightest X-ray SNRs are currently evolving in denser than average regions and that, at least at the current level of exposure, Chandra is not detecting the fainter SNR population. To further investigate the X-ray sources, we retrieved the X-ray data from the Chandra archive and processed it ourselves. After point-source identification, we combined the data for various subsets of sources, as shown in Figure 22. The right panel in Figure 22 shows summed background spectra from annular regions around each defined set of source positions. The annular region selected for each source ranged from 200 to 1000 beyond the source radius, depending on the off-axis source position, following instructions from the ACIS support Web site.5 The left panel shows the ‘‘summed source’’ spectra after subtraction of the appropriate background. The top panel at left shows the summed X-ray data from the 15 positions coincident with matched optical SNR candidates from Table 9. The second panel shows the summed data for 35 X-ray sources not coincident with optical SNRs, but which have soft spectra ( hardness ratios 0.4). Finally, the bottom panel shows the summed X-ray spectrum at the locations of the 55 optical SNR candidates that did not have explicit individual X-ray source detections at greater than 3 . Although the signal is low in the summed optical / X-ray panel, a relative excess below 1 keV is clearly present. Furthermore, the shape of the summed ‘‘soft source’’ spectrum is 5
See http://www.astro.psu.edu/xray/acis/memos/memoindex.html.
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Fig. 22.—Summed count spectra for a number of subsets of the M83 Chandra X-ray source data (left) and appropriate background regions (right). Top left: Summed X-ray data after background subtraction from the positions coincident with matched optical SNR candidates. Second left: Similar, for X-ray sources that are not coincident with optical SNRs but have soft spectra ( hardness ratios 0.4). Bottom left: Summed, background-subtracted X-ray spectrum at the locations of all optical SNR candidates that did not have explicit X-ray source detections at greater than 3 . Right panels: Background spectra that were subtracted for each subset. (See text for details.)
consistent with that of the matched sources, implying that many of the soft X-ray sources are also likely to be SNRs. The sources with a harder ratio show a peak near 0.8 keV, but the global spectral shape does not drop off as quickly to higher energy. Unfortunately, even by co-adding the faint X-ray emission at the positions of the non–X-ray detected optical SNRs, the resulting signal is too weak to provide useful information. Since many of the other soft X-ray sources may be SNRs, we have inspected our optical images at the locations of the 35 softspectrum X-ray sources that were not coincident with optical SNR candidates from Table 5. We found a broad range of results from this exercise. Some sources were in unconfused regions but simply had no discernible nebular counterparts. Some sources aligned with portions of very bright nebular regions but were not coincident with specific knots or features, and five sources were within the bright nuclear region. These could represent buried SNRs in confused regions, but it is not possible to tell with current data. Some sources aligned within a few pixels of relatively isolated but faint emission regions for which the imaging [S ii]:H ratio could not be determined with accuracy. A few sources aligned with moderately bright knots of emission in the outskirts of H ii region complexes where the complicated backgrounds affected our search technique. ( That is, derived ratios may be marginally below the dividing line because of errors in background subtraction.) In short, a number of these soft X-ray sources could be SNRs that have escaped clear detection with the current optical data set and search criteria. It is likely that a new, deeper optical search in better seeing could clarify a number of these possible associations. Finally, Soria & Wu (2003) identified four bright sources (their sources 3, 8, 27, and 56) that were bright enough for detailed spectral analysis and that show evidence of line emission. Surprisingly, none of these are associated with an optical emission nebula with high [S ii]:H ratio. Source 56 is buried
in the bright nuclear region, but the other three sources are in relatively unconfused regions in the images and have no clearly associated optical nebular counterparts at all. Soria & Wu (2003) mention evidence that source 8 may have varied by a factor of 2, which if true would be very unlikely for a SNR, especially on the timescale of the observations. Hence, the true nature of these sources remains unclear. 4. SUMMARY While observations of M83 have been made at nearly all wavelengths, this is the first optical search for SNRs in this galaxy. The search method has proven effective for identifying a large group of nebulae (71) that, based on a ratio of [S ii] to H emission greater than 0.4, are almost certainly SNRs. This conclusion is strengthened by the reported spectroscopic observations, where 20 of the ‘‘normal’’ candidates in Table 5 have been confirmed. With six SNe observed since 1923, M83 has an implied supernova rate of approximately one SN every 15 years. Given a typical SNR lifetime of tens of thousands of years, the total number of SNRs in M83 could be well above 500 and perhaps as high as 1000. Clearly we have only detected the optically brightest population of SNRs in this initial survey, and our list is incomplete. Further limitations have been caused by the seeing conditions for these observations, as well as the intrinsic complexity of M83’s spiral arm structure. SNRs buried in H ii regions or hidden behind dust lanes would not consistently have been detected by this optical survey. We have also searched for evidence of young oxygendominated SNRs that arise from core-collapse supernovae (objects for which Cas A is often declared the prototype). This search was unsuccessful, with the possible exception of SN 1957D, which was known previously. This negative result
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indicates that there are no exceptionally bright young SNRs, such as the object in NGC 4449 ( Blair et al. 1983), present in M83, but that fainter, perhaps more typical young SNRs like 1E 01027219 in the Small Magellanic Cloud could have been missed due to faintness and the relatively poor spatial resolution of the current data set. Nonetheless, we have shown that optical techniques used on nearby galaxies are effective for identifying a significant sample of SNRs, even at the distance of M83. A more sensitive search in excellent seeing conditions, as well as deeper Chandra imaging in X-rays, could lead to a substantial improvement in SNR detections over the list provided here.
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It is a pleasure to thank the staff at CTIO for their assistance with the observations. Also, we thank Frank Winkler, who assisted with the imaging run, and Chris Smith, who assisted on the spectroscopy run. Johns Hopkins University undergraduate Joseph McCullough and summer student Lauren Jones also worked on this project, and their work is gratefully acknowledged. The authors thank Gabe Brammer and Parviz Ghavamian for discussions and assistance with the figures and with the comparison to the Chandra data. Publication of this work has been supported by the Center for Astrophysical Sciences at JHU, by SAO grant AR3-4003B to JHU, and by SAO grant AR3-4003X to STScI.
REFERENCES Barbon, R., Cappellaro, E., Ciatti, F., Turatto, M., & Cowal, C. T. 1984, A&AS, Huang, Y. I. 1987, PASP, 99, 461 58, 735 Immler, S., Vogler, A., Ehle, M., & Pietsch, W. 1999, A&A, 352, 415 Barbon, R., Cappellaro, E., & Turatto, M. 1989, A&AS, 81, 421 Jensen, E. B., Talbot, R. J., Jr., & Dufour, R. J. 1981, ApJ, 243, 716 Blair, W. P., Kirshner, R. P., & Chevalier, R. A. 1981, ApJ, 247, 879 Long, K. S., Blair, W. P., Kirshner, R. P., & Winkler, P. F. 1990, ApJS, 72, 61 ———. 1982, ApJ, 254, 50 Long, K. S., Blair, W. P., & Krzeminski, W. 1989, ApJ, 340, L25 Blair, W. P., Kirshner, R. P., & Winkler, P. F. 1983, ApJ, 272, 84 Long, K. S., Winkler, P. F., & Blair, W. P. 1992, ApJ, 395, 632 Blair, W. P., & Long, K. S. 1997, ApJS, 108, 261 Mathewson, D. S., & Clarke, J. N. 1973, ApJ, 180, 725 Blair, W. P., et al. 2000, ApJ, 537, 667 Mathewson, D. S., & Healy, J. R. 1964, in IAU Symp. 20, The Galaxy and the Bohlin, R. C., Cornett, R. H., Hill, J. K., Smith, A. M., & Stecher, T. P. 1983, Magellanic Clouds, ed. F. J. Kerr & A. N. Rodger (Canberra: Australian ApJ, 274, L53 Acad. Sci.) Bohlin, R. C., Cornett, R. H., Hill, J. K., & Stecher, T. P. 1990, ApJ, 363, 154 Matonick, D. M., & Fesen, R. A. 1997, ApJS, 112, 49 Bresolin, F., & Kennicutt, R. C. 2002, ApJ, 572, 838 Matonick, D. M., Fesen, R. A., Blair, W. P., & Long, K. S. 1997, ApJS, Condon, J. J., Condon, M. A., Gisler, G., & Puschell, J. J. 1982, ApJ, 252, 102 113, 333 Cowan, J. J., & Branch, D. 1985, ApJ, 293, 400 Monet, D. G., et al. 1998, VizieR Online Catalog, I/252 de Vaucouleurs, G. 1979, AJ, 84, 1270 Pennington, R. L., Talbot, R. J., Jr., & Dufour, R. J. 1982, AJ, 87, 1538 de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H. G., Jr., Buta, R. J., Paturel, Raymond, J. C. 1979, ApJS, 39, 1 G., & Fouque´, P. 1991, Third Reference Catalogue of Bright Galaxies ( New Roussel, H., et al. 2001, A&A, 369, 473 York: Springer) Rumstay, K. S., & Kaufman, M. 1983, ApJ, 274, 611 D’Odorico, S., Pettini, M., & Ponz, D. 1985, ApJ, 299, 852 Smith, R. C., Kirshner, R. P., Blair, W. P., Long, K. S., & Winkler, P. F. 1993, Dufour, R. J., Talbot, R. J., Jensen, E. B., & Shields, G. A. 1980, ApJ, 236, 119 ApJ, 407, 564 Eck, C. R., Cowan, J. J., & Branch, D. 2002, ApJ, 573, 306 Soria, R., & Wu, K. 2002, A&A, 384, 99 Ehle, M., Pietsch, W., Beck, R., & Klein, U. 1998, A&A, 329, 39 ———. 2003, A&A, 410, 53 Fesen, R. A., Morse, J. A., Chevalier, R. A., Borkowski, K. J., Gerardy, C. L., Stone, R. P. S., & Baldwin, J. A. 1983, MNRAS, 204, 347 Lawrence, S. S., & van den Bergh, S. 2001, AJ, 122, 2644 Talbot, R. J., Jr., Jensen, E. B., & Dufour, R. J. 1979, ApJ, 229, 91 Filippenko, A. 1982, PASP, 94, 715 Telesco, C. M., & Harper, D. A. 1980, ApJ, 235, 392 Gaetz, T. J., Butt, Y. M., Edgar, R. J., Eriksen, K. A., Plucinsky, P. P., Schlegel, Thornley, M. D., Fo¨rster Schriber, N. M., Lutz, D., Genzel, R., Spoon, H. W. W., E. M., & Smith, R. K. 2000, ApJ, 534, L47 & Kunze, D. 2000, ApJ, 539, 641 Gordon, S. M., Kirshner, R. P., Long, K. S., Blair, W. P., Duric, N., & Smith, R. C. Trinchieri, G., Fabbiano, G., & Palumbo, G. G. C. 1985, ApJ, 290, 96 1998, ApJS, 117, 89 van den Bergh, S., & Tammaan, G. 1991, ARA&A, 29, 363 Hartigan, P., Raymond, J., & Hartmann, L. 1987, ApJ, 316, 323
