1. INTRODUCTION 2. SAMPLE SELECTION

THE ASTROPHYSICAL JOURNAL, 529 : 859È865, 2000 February 1 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.

COMPACT RADIO SOURCES WITH THE STEEPEST SPECTRA D. L. KAPLAN1 Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, NY 14853 ; kaplan=spacenet.tn.cornell.edu

J. M. CORDES Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, NY 14853 ; cordes=spacenet.tn.cornell.edu

J. J. CONDON National Radio Astronomy Observatory,2 520 Edgemont Road, Charlottesville, VA 22903 ; jcondon=nrao.edu

AND S. G. DJORGOVSKI Palomar Observatory 105-24, California Institute of Technology, Pasadena, CA 91125 ; george=deimos.caltech.edu Received 1999 May 24 ; accepted 1999 September 15

ABSTRACT We used the 1.4 GHz NRAO VLA Sky Survey (NVSS) and 365 MHz Texas survey catalogs to select relatively compact (h ¹ 25@@) radio sources having exceptionally steep spectral indices a º 1.5 (where the Ñux density S at frequency l scales as S P l~a). Our sample of 74 sources (of D30,000 total sources l surveys) with S(365 MHz) l º 200 mJy in the 10 sr between d \ [35¡30@ and d \ 71¡30@ common to both should be complete for spectra as steep as a \ 3.2, but we found only four sources with a [ 2.5. Most known sources with such steep spectra are pulsars (coherent radio sources), relic radio galaxies, and high-redshift radio galaxies (synchrotron sources with spectra steepened by radiation losses). In fact, six of our sources have been identiÐed with known pulsars and one with a high-redshift (z \ 3.79) radio galaxy. In order to Ðnd possible new populations of steep-spectrum sources and classify the rest, we made VLA images of our sources with 1A. 5 and 0A. 5 resolution at 1.4 and 5 GHz, respectively. There are 25 doubles, 24 multicomponent sources, 16 unresolved (h [ 0A. 2) sources, and six known pulsars. Three sources have undetermined morphologies as they were not reobserved with the VLA. The smallest sources have exceptionally steep spectra for their angular sizes. New pulsars, if any, are likely to be of special interest since they are strong but were missed by pulse-detection surveys. Preliminary radio, infrared, and optical observations suggest that most are high-redshift extragalactic sources. Subject headings : catalogs È galaxies : active È pulsars : general È radio continuum : galaxies È radio continuum : stars È surveys 1.

INTRODUCTION

The nomenclature for our sources follows the form of the Texas sources (see Douglas et al. 1996) : the sources are identiÐed as TXS HHMM^DD.D, where HHMM is the truncated B1950 right ascension and DD.D is the truncated B1950 declination. We use the Texas names as they are unambiguous for a sample of this size and they allow easy comparison with previous samples (which often used B1950 names). For identiÐers of the form HHMM^DD.D ““ TXS ÏÏ is implied. Except for these names, all coordinates are equinox J2000.0.

Steep-spectrum samples have been used for years to identify several types of sources such as pulsars, high-redshift radio galaxies, and relic radio galaxies. These sources explore environments and source types of intense interest in modern astronomy : neutron stars and the aftermaths of supernovae, the early universe and the history of galaxy formation, and the oldest galaxies and the centers of dense clusters. To select the most extreme examples of these objects and to discover possible new classes of steepspectrum sources, we constructed the Ðrst statistically useful sample of sources with spectral indices a(365,1400) º 1.5 (where S P l~a). Not only do we sample the steepest tail of l spectral-index distribution, but many of our the radio sources are also more compact than the majority of radio sources, making this sample unique. This is the Ðrst paper in a series concerning this sample. Here we describe the general sample characteristics and scientiÐc motivations. In Kaplan, Cordes, & Condon (2000, hereafter Paper II) we will present the complete source catalog, with all of the radio data. S. G. Djorgovski et al. (2000, in preparation, hereafter Paper III) will report on optical and infrared imaging and optical spectroscopy for a subset of these sources.

2.

1 Present address : 105-24 California Institute of Technology, Pasadena, CA 91125. 2 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

859

SAMPLE SELECTION

We constructed our sample by searching for positional coincidence between sources stronger than 2.5 mJy in the 1.4 GHz NRAO VLA Sky Survey (NVSS ; Condon et al. 1998) and 200 mJy in the 365 MHz Texas (Douglas et al. 1996) survey catalogs, requiring that each source be nearly unresolved (h ¹ 25@@ FWHM), the spectral index be at least 1.5, and the positional o†set be less than 3 times the quadratic sum of the individual positional uncertainties. We rejected a number of candidates that appear to be lobe-shifted or confused at 365 MHz. Table 1 presents a source list, and Table 2 summarizes our classiÐcations (see ° 3). There are D30,000 sources with h ¹ 25@@ common to both the NVSS and the Texas survey, and we found 74 objects satisfying all of our constraints, representing D0.3% of the total population. Figure 1 indicates how strict our spectral criterion is, sampling only the very steepest spectra. Our search should be complete for Ñux densities S º 200 mJy and 365MHz

TABLE 1 SUMMARY OF SOURCE DATA

Name TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS TXS

0001[021 . . . . . . 0004]364 . . . . . . 0139[143 . . . . . . 0201]656 . . . . . . 0208]621 . . . . . . 0225]615 . . . . . . 0240]210 . . . . . . 0304[332 . . . . . . 0307]365 . . . . . . 0329]544 . . . . . . 0352]045 . . . . . . 0409]620 . . . . . . 0431[292 . . . . . . 0444[044 . . . . . . 0508]604 . . . . . . 0519]315 . . . . . . 0602[202 . . . . . . 0628[285 . . . . . . 0744]370 . . . . . . 0825[128 . . . . . . 0855[302 . . . . . . 0859]183 . . . . . . 0908[222 . . . . . . 0936]039 . . . . . . 0951]146 . . . . . . 1001]052 . . . . . . 1040]243 . . . . . . 1051]048 . . . . . . 1125]486 . . . . . . 1133]161 . . . . . . 1135[353 . . . . . . 1149[299 . . . . . . 1211]022 . . . . . . 1222]548 . . . . . . 1223]130 . . . . . . 1340]568 . . . . . . 1346[205 . . . . . . 1419]678 . . . . . . 1447]251 . . . . . . 1541[048 . . . . . . 1548[102 . . . . . . 1602[026 . . . . . . 1626]161 . . . . . . 1634[194 . . . . . . 1640[347 . . . . . . 1642[032 . . . . . . 1723[320 . . . . . . 1725[319 . . . . . . 1730]467 . . . . . . 1730[320 . . . . . . 1749[281 . . . . . . 1844[009 . . . . . . 1922]319 . . . . . . 1926]029 . . . . . . 1929]108 . . . . . . 1948]102 . . . . . . 2011[188 . . . . . . 2018]082 . . . . . . 2027]365 . . . . . . 2027]416 . . . . . . 2027[091 . . . . . . 2050[061 . . . . . . 2058[004 . . . . . . 2105]026 . . . . . .

RA (J2000) 00 00 01 02 02 02 02 03 03 03 03 04 04 04 05 05 06 06 07 08 08 09 09 09 09 10 10 10 11 11 11 11 12 12 12 13 13 14 14 15 15 16 16 16 16 16 17 17 17 17 17 18 19 19 19 19 20 20 20 20 20 20 21 21

04 07 41 04 11 29 42 06 10 32 55 14 33 46 12 22 04 30 47 27 57 01 10 39 54 03 43 53 28 36 37 51 13 24 26 42 49 20 49 44 51 04 28 37 43 45 27 29 31 34 52 46 24 28 32 50 14 21 29 29 29 52 01 08

21.87 02.94 38.02 57.34 54.05 39.67 56.666 39.78 54.86 59.383 12.78 03.03 35.70 37.74 54.86 33.01 57.58 49.43 29.38 51.18 53.47 60.00 34.15 11.72 08.65 55.31 43.27 45.673 05.18 03.19 33.34 59.41 53.484 52.31 12.26 31.21 02.91 03.13 49.990 04.89 30.98 59.59 19.128 44.79 18.58 02.34 05.06 12.61 46.19 06.32 58.69 41.359 13.88 41.65 13.940 55.33 52.03 17.59 25.43 37.20 49.61 49.93 18.78 26.74

decl. (J2000) [01 ]36 [14 ]65 ]62 ]61 ]21 [33 ]36 ]54 ]04 ]62 [29 [04 ]60 ]31 [20 [28 ]36 [13 [30 ]18 [22 ]03 ]14 ]05 ]24 ]04 ]48 ]15 ]60 [30 ]01 ]54 ]12 ]56 [20 ]67 ]24 [05 [10 [02 ]16 [19 [34 [03 [32 [31 ]46 [32 [28 [00 ]32 ]03 ]10 ]10 [18 ]08 ]36 ]41 [08 [05 [00 ]02

50 41 06 55 23 46 14 04 44 34 40 08 06 21 30 33 15 34 54 03 26 09 28 43 24 01 04 33 22 51 30 13 56 36 48 36 49 35 55 02 24 48 03 31 52 18 05 59 40 04 06 55 01 03 59 25 43 23 43 51 58 55 16 49

16.9 56.0 08.8c 30.5 14.3c 25.3c 54.96 32.4c 01.9c 43.48 41.1c 38.7c 02.2 04.2c 52.0c 30.2 56.9c 42.7 38.5c 39.0 35.0 03.5c 43.3c 59.0c 25.8c 25.1c 47.3 44.92 56.7 09.1 52.0c 40.4 00.51 39.5 46.7c 43.8c 11.8 09.1c 09.79 46.8 40.3 47.8c 59.63 22.9 19.7 10.5c 24.4 31.5c 02.0c 54.9c 37.5 14.64 36.7c 00.4 32.28 14.2c 35.3c 17.8 24.5c 41.5c 49.3 49.0c 16.9c 46.4c

S NVSS mJy 23.0 27.9 30.0 31.2 126.7 25.6 28.3 27.4 23.6 150.7 255.5 52.6 274.1 24.7 156.8 32.3 28.9 13.8 35.6 28.4 54.3 54.5 53.8 21.2 27.0 36.2 50.1 28.9 28.9 21.2 37.8 52.1 22.8 68.3 44.9 49.9 25.7 33.0 44.1 36.6 35.5 45.5 53.2 36.0 61.2 8.3 38.1 60.8 15.3 33.5 41.6 24.6 31.8 28.3 27.1 117.9 19.8 36.6 100.6 46.7 46.1 40.7 22.9 64.0

^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

0.8 1.2 1.3 1.6 3.8 0.9 0.9 0.9 0.8 5.4 7.7 2 9.7 1.2 4.7 1.1 1.3 0.6 1.1 0.9 1.7 2.1 1.7 1.1 0.9 1.2 1.6 1.0 1.0 0.7 1.2 1.6 0.8 2.1 1.8 1.5 0.9 1.4 1.7 1.5 1.1 1.4 1.6 1.2 1.9 1.5 1.2 2.3 0.6 1.1 1.3 0.9 1.4 1.3 0.9 3.6 0.7 1.5 3.1 1.5 1.4 1.3 1.1 2.0

a1365 TXS 1.72 2.04 1.46 1.61 2.15 1.26 1.84 1.53 1.54 1.52 1.51 1.58d 1.58 2.42 1.54d 1.54 1.48 1.63 1.60 1.42 1.57 1.57 1.62 1.79 1.66 1.56 1.74 1.65 1.63 1.67 1.69d 1.56 1.63 1.86 0.94 1.56 1.63 1.68 1.55 1.67 1.53 1.62 1.90 1.83 1.54 3.28d 1.56 1.69 1.43 0.94 3.01 0.84 1.58 1.84 2.03 1.69 1.94 1.82 1.71 1.56 1.66 1.49 2.19 1.62

h a M (arcsec)

Classb

1.8 \0.30 38 \0.57 6.4 10.4 \0.12 7 7 \0.08 2.7 \25 2.5 6 \25 \0.17 19 \0.53 1.8 0.6 \0.24 20 14 23 10 4 3.2 \0.11 0.7 \0.21 \25 \0.23 \0.11 1.0 25 13 2.8 14 \0.12 0.31 1.3 14 \0.14 \0.38 \0.25 \25 1.00 13 16 30 \0.9 \0.17 23 \0.2 \0.06 20 3.2 1.3 14 3 1.3 9 26 15

D U G U G G U G G P D X D G X U G P D D U G G G G D D U D P X U U D G G D G U D D G U U U P D G G G P U G U P G D D G D D D G G

COMPACT RADIO SOURCES

861

TABLE 1ÈContinued

Name

RA (J2000)

TXS 2143]584 . . . . . . . . \TXS 2210]677 . . . . . . TXS 2212]144 . . . . . . . . TXS 2217[046 . . . . . . . . TXS 2219]328 . . . . . . . . TXS 2225[171 . . . . . . . . TXS 2229]169 . . . . . . . . TXS 2307]587 . . . . . . . . TXS 2312]206 . . . . . . . . TXS 2346]543 . . . . . . . .

21 22 22 22 22 22 22 23 23 23

45 11 14 19 22 28 31 10 14 48

10.39 54.58 51.32 39.14 15.19 00.00 44.48 01.54 56.048 58.33

S NVSS mJy

decl. (J2000) ]58 ]68 ]14 [04 ]33 [16 ]17 ]59 ]20 ]54

43 02 42 26 05 52 14 01 53 35

11.1 20.9c 37.8c 15.9c 44.1 30.9 51.9c 22.0c 37.10 43.5

105.6 27.9 34.8 33.5 30.0 30.0 18.8 14.2 45.6 28.4

^ ^ ^ ^ ^ ^ ^ ^ ^ ^

3.2 0.9 1.4 1.1 1.3 1.0 0.7 0.7 1.4 1.0

a1365 TXS 1.51 1.58 1.65 1.58 1.62 1.56 2.13 1.08 1.54 1.57

h a M (arcsec)

Classb

2.3 5 15 6 0.3 \0.19 23 2.6 \0.14 1

D D G D D U G D U D

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a Approximate major axis of radio source. b Morphology classes : ““ U ÏÏ is unresolved, ““ D ÏÏ is compact double, ““ G ÏÏ is galaxy-like, ““ P ÏÏ is pulsar, and ““ X ÏÏ is undetermined. c Positions are from NVSS. d For the ““ X ÏÏ sources without a 1365 MHz Ñux density, the NVSS Ñux density was used.

spectra between the frequencies of 365 MHz and 1.4 GHz as steep as a \ [log (2.5/200)/log (1400/365) \ 3.2 (this limit arises from the completeness limit of the NVSS) over the 10 sr of sky with 71¡30@ º d º [35¡30@. However, only four 4

3

10

S

Texas

(mJy)

10

2

10 S

NVSS

5 1.

1.

α= −

5 0.

α= −

.0

α= −

.5

α= 0

.0

α= 0

.5

α= 1

α= 1 1

10

0

2

10

3

4

10

10

(mJy)

FIG. 1.ÈS vs. S for D30,000 sources common to both surveys. The D30,000 NVSS compact TXS sources common to both the NVSS and the Texas surveys (points) and the sources from this paper (squares) are shown. Also shown are lines are of constant spectral index a \ [ log (S /S )/log (1400/365), for a \ [1.5, [1.0, . . . , 1.5. The points NVSS above the line TXS a \ 1.5 that are not squares were removed from the sample

because of confusion, lobe-shifts, etc. The dashed line is at S \ 200 mJy, and we are complete for sources above this line (with a º 1.5,TXS of course).

sources with any spectral index greater than 2.5 (based on the initial data or the auxiliary data) were actually found, so our sample of 74 compact sources may contain those with the steepest spectra in existence. 2.1. Auxiliary Radio Observations and Cross-References to Other Catalogs In order to conÐrm the spectra and positions of our sources and to determine their radio morphologies on scales much smaller than the 45A NVSS beam, we observed them with the Very Large Array (VLA) A conÐguration in 1998 AprilÈJune. The 1.365 and 4.86 GHz snapshots were imaged with resolutions of 1A. 5 and 0A. 5, respectively. These observations also yield 1.4 GHz Ñux densities that constrain source variability, be it either intrinsic or caused by interstellar scintillation. We checked the Digitized Sky Survey (DSS) images for optical counterparts and searched the SIMBAD database for published identiÐcations of the sample sources. Six of the 74 sample sources are strong pulsars (PSRs B0329]54, B0628[28, B1133]16, B1642[03, B1749[28, and B1929]10) already known to have a º 1.5. Their NVSS and Texas Ñuxes are close to the cataloged Ñuxes (see Kaplan et al. 1998 ; Han & Tian 1999), and the di†erences are consistent with interstellar scintillation. One source, TXS 0508]604, (with a \ 1.53) is the radio galaxy 4C 60.07 at z \ 3.79 (Chambers et al. 1996a, 1996b). No other sources have published identiÐcations. We augmented our data with Ñux measurements from other radio surveys as well as IR and X-ray surveys. While some surveys such as the 325 MHz Westerbork Northern

TABLE 2 SUMMARY OF SOURCE CLASSIFICATIONS Class

Number

Size

Radio Morphologies

Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compact unclassiÐed sources . . . . . . Compact doubles . . . . . . . . . . . . . . . . . . . Radio galaxies . . . . . . . . . . . . . . . . . . . . . . . Undetermined . . . . . . . . . . . . . . . . . . . . . . .

6 16 24 25 3

[0A. 15 [0A. 15 0A. 2È4A [8A \25A

Unresolved Unresolved Two unresolved components Two resolved/unresolved lobes Not reobserved

862

KAPLAN ET AL.

Sky Survey (WENSS ; Rengelink et al. 1997) and the 1.4 GHz Faint Images of the Radio Sky at 20 cm (FIRST) survey (White et al. 1997) have resolution and sensitivity superior to the primary surveys that we used, we found our choices preferable for their greater sky coverage and sensitivity to extended sources. We therefore used the WENSS and FIRST catalogs, along with others, only to provide additional data. Low-frequency radio catalogs constrain the turnover frequency l below which the power-law highm frequency spectrum Ñattens. Only one of the sources detected at 151 MHz appears to turn over above 100 MHz (and this is the only source with only an upper limit, at 151 MHz), indicating l [ 50 MHz for the majority of sources, m and some that were detected at 38 MHz have not started to turn over even there. For example, the spectrum of the unresolved source TXS 0004]364 seen in Figure 2 remains linear from 0.15 GHz to 5 GHz with a \ 2.0.

6C 1000 WENSS Bologna Miyun 100

10

1

100

Vol. 529

1000

FIG. 2.ÈRadio spectrum of the unresolved source TXS 0004]364, including data from this paper (triangles) and other surveys (squares). The straight line is a power-law Ðt, with spectral index a \ 2.0 from 4.86 GHz to 151 MHz. The errors on the Ñuxes from this paper lie within the plot symbols. The other surveys are 6C (Hales, Baldwin, & Warner 1993), Miyun (Zhang et al. 1997), WENSS (Rengelink et al. 1997), and Bologna (Colla et al. 1973).

2.2. Optical and IR Observations We made preliminary optical/IR observations with the Palomar 5 m telescope. Optical observations were done with the coherent optical system of modular imaging collectors (COSMIC) CCD camera. A subset of sources was observed with the r Ðlter, and selected others were observed with combinations of i and g. Typical integration times were D600 s. The IR observations were performed with the Prime-Focus Infrared Camera (PFIRCAM). The primary observations used the J and K Ðlters, with some additional images taken with the H and Fe II (1.644 km) Ðlters. Integra-

FIG. 3.ÈOptical i-band image of TXS 0004]364. The optical source (circled) has multiple components and extends beyond the radio source, which is \0A. 1 in size and therefore smaller than 1 pixel. The scale bar is 5A long. The image is a 3660 s exposure using the COSMIC camera on the Palomar 5 m telescope. The conditions were near-photometric, and the seeing was D1A. 6.

No. 2, 2000

COMPACT RADIO SOURCES

tions were typically background limited, with exposures of D180 s. The optical/IR counterparts that we detected are quite faint (r D 23) and very red (J[KD2, r[i D 1.5). Some are extended, and several seem to be associated with companion objects. For example, the source TXS 0004]364 appears to be in an interacting pair (although this could be just a coincidence ; see Fig. 3) and has J[K \ 1.7. The infrared magnitudes and colors suggest, upon comparison with the sources in De Vries et al. (1998), that the source has a redshift z Z 0.5. 2.3. Completeness and Comparison with Other Steep-Spectrum Samples Our sample should be representative and at least 75% complete for sources with S º 200 mJy, increasing to 365MHz over 90% complete for sources with S º 500 mJy. 365MHz While we cannot precisely assess the overall completeness because of the variety of factors involved, we believe that it is in general about 85%È90% for sources with S Z 365MHz 250 mJy, which is true for almost all of the sources. Source variability could a†ect our sample completeness. However, our pairs of 1.4 GHz Ñux densities generally show good agreement, except for four of the known pulsars (which scintillate considerably) and a few extended sources. This suggests that variability does not signiÐcantly alter our spectral indices, even though the Ñux densities used to select the sample were measured at times di†ering by many years. The spectral indices between 365 MHz and 1.4 GHz often di†er from those between 1.4 GHz and 4.85 GHz, the latter generally being steeper. These di†erences might be inÑuenced by intrinsic source variability or interstellar scintillation but most likely indicate genuine spectral curvature (e.g., Lacy et al. 1994). The earliest published samples of steep-spectrum sources have much higher Ñux limits, often by a factor of 40 at 1400 MHz (Chambers et al. 1996b ; Rhee et al. 1996) and much lower spectral-index limits (usually a [ 0.5 or a [ 0.7), appropriate for the majority of radio galaxies. Even recently published samples emerging from the new generation of surveys (e.g., OÏDea 1998 ; Blundell et al. 1998 ; White et al. 1997 ; De Breuck et al. 1998) are less sensitive to the objects with the steepest spectra, given their limited sky coverage and modest spectral-index criteria. The present sample extends both the Ñux and spectral-index limits while still being large enough for statistical use. As seen in Figure 1, we sample the very steepest spectrum and most compact radio sources, whereas surveys with less stringent spectralindex limits (such as a [ 1.0) still contain many more typical sources. 3.

CLASSIFICATION

3.1. Possible Source Classes Very few sources were previously known to have a º 1.5. Most of these are pulsars, relic radio galaxies (Komissarov & Gubanov 1994), and high-redshift radio galaxies (Chambers et al. 1996b). The last can also be separated by size and morphology into classical radio galaxies (Fanaro† & Riley classes I and II, hereafter FR I and FR II ; Fanaro† & Riley 1974), compact steep-spectrum sources (CSSs ; OÏDea 1998), and gigahertz-peakedÈspectrum (GPS) sources (Phillips & Mutel 1982 ; OÏDea 1998). Pulsars have the steepest known radio spectra, with spectral indices between 0 and 3 (e.g., Lorimer et al. 1995 ;

863

Kaplan et al. 1998) and median SaT D 1.5. Some, such as the Ðrst millisecond pulsar (Backer et al. 1982), were discovered via spectral-index selection. While the Ñux limits of our sample are much higher than those of most current pulsar searches, the latter are sensitive only to periodic pulsations. Their sensitivity depends strongly on the pulsarÏs period and dispersion measure (DM ; e.g., Cordes & Cherno† 1997). Since our sample depends only on the average continuum Ñux density, it is not limited by those quantities and can be used to look at the global pulsar population in a new way (e.g., Kaplan et al. 1997). In addition to sampling the known pulsar population in a complete manner, it may contain new and intrinsically interesting pulsar types. These could be such sources as pulsars with spin periods P \ 1.5 ms, in very tight binary systems, that are very highly dispersed or with very little intrinsic modulation. Most radio lobes produced by galaxies or quasars have spectral indices a D 0.8 ^ 0.2. Relic radio galaxies whose central engines turned o† D108 yr ago have spectra as steep as a D 1.5 owing to synchrotron and inverse Compton losses (Komissarov & Gubanov 1994). Many relic sources are found in the centers of large clusters, where conÐnement by cluster gas pressure slows adiabatic expansion losses (Komissarov & Gubanov 1994). Relic sources typically have angular sizes h Z 10@@ and have detectable optical counterparts (Chambers et al. 1996b). Spectral aging occurs more rapidly at higher frequencies in the source frame, so high-luminosity radio galaxies often have convex radio spectra that appear steeper at high redshifts (e.g., Lacy et al. 1994). This can increase the observed spectral index to D1.5. De Breuck et al. (1998) noted that searching for steep-spectrum sources is the most e†ective way of Ðnding high-redshift radio galaxies. Two subclasses of high-redshift radio galaxies distinguished on the basis of size and turnover frequency l due m to synchrotron self-absorption are compact steep-spectrum (CSS) and gigahertz-peakedÈspectrum (GPS) sources (OÏDea 1998). CSS sources are typically 1A in size and have l D 100 MHz, while GPS sources are smaller thanD0A. 1 m have l D 1 GHz. Both types generally have complex and double or mtriple radio morphologies and very low integrated linear polarizations (OÏDea 1998). Current theories (OÏDea 1998) place GPS sources at the head of an evolutionary sequence leading through CSS sources to traditional large-scale FR I/FR II double radio sources. Almost all known CSS/GPS sources have spectral indices below 1.5, and none have a Z 2.2 (OÏDea 1998). Any such CSS/GPS sources in our sample are therefore more extreme cases than those identiÐed previously. 3.2. Initial ClassiÐcation We have provisionally classiÐed our sources on the basis of angular size (at 1.4 GHz if resolved there, otherwise at 4.86 GHz) and radio morphology from the follow[up VLA observations (see Tables 1 and 2). Additional source data will follow in Paper II. The apparent angular size distribution (Fig. 4) appears to have a peak for h Z 10@@ and then Ñatten for h [ 10@@, although it is strongly a†ected by the resolution of the VLA. The objects with h Z 10@@ are morphologically similar to double-lobed radio galaxies, and we therefore call these ““ large galaxies.ÏÏ These sources may have very small bright lobes or large di†use ones, but their lobe separations are typically 10A. They are likely distant radio galaxies or quasars, with some of the more di†use

864

KAPLAN ET AL.

12

unresolved compact double large galaxy 10

Number

8

6

4

2

0 −2 10

−1

10

0

10

1

θ (arcseconds)

10

2

10

3

10

FIG. 4.ÈDistribution of angular sizes of the sources, with source type indicated. The angular sizes were determined from the image that had the best dynamic range while still resolving the source, if possible. While the galaxy-like sources do appear separated from the rest, the unresolved and compact double sources have a quasi-continuous distribution of sizes, suggesting that they are similar populations.

ones perhaps being relic sources (where the lobes have dissipated into the intergalactic medium). There then appears a slight decline in the total number of sources in Figure 4 toward h D 3@@. It is here that we separate the large galaxies from our next classiÐcation : ““ compact doubles.ÏÏ The compact doubles have sizes D1AÈ3A with two unresolved components and appear morphologically similar to many of the CSS/GPS sources in OÏDea (1998). The distinction between the large galaxies and the compact doubles is not always clear : while some of the large galaxies (such as those with large, di†use lobes) are very di†erent from the compact doubles, others have unresolved lobes separated by D15A (instead of D2A for the compact doubles). For these sources, the distinction is mainly one of convenience, for ease of comparison with other studies. Their angular size distribution blends into that of the smallest objects, which we call ““ unresolved.ÏÏ These all have radio morphologies unresolved by our highest resolution observations (so h [ 0A. 2). Six are known pulsars. The rest seem clearly di†erent, having much lower linear (and circular) polarizations than the known pulsars and showing no signs of interstellar scintillation. We believe that most of our steep-spectrum sources, both resolved and unresolved, are extragalactic. The one source that is not a pulsar but has a conÐrmed classiÐcation, TXS 0508]604 (4C 60.07), is a very distant and faint (z \ 3.79, R \ 23.2) radio galaxy (Chambers et al. 1996a). Our resolved sources are morphologically similar to radio galaxies, and their density on the sky is independent of Galactic latitude. All of our preliminary optical and IR identiÐcations are very faint and have colors similar to known objects with z Z 1 (Chambers et al. 1996a ; de Vries et al. 1998). However, some of these sources could still be Galactic, and their magnitudes and colors could be a†ected by absorption and reddening by the interstellar dust at low latitudes. All of the arguments against identiÐcation of new pulsars (i.e., lack of polarization, lack of variability) are statistical in nature and therefore still allow a small number of

Vol. 529

previously unknown pulsars to exist in the sample. There could be as many as 16 new pulsars (as pulsars would be unresolved), but we believe the actual number to be signiÐcantly smaller, quite possibly zero. If extragalactic, the unresolved steep-spectrum sources probably emit incoherent synchrotron radiation. They are exceptionally compact, and no extragalactic sources listed by OÏDea (1998) have sizes, spectral indices, and turnover frequencies comparable to ours. Figure 5 shows a comparison of the turnover Ñux density S versus angular size h for m our sources and those in OÏDea (1998). For our sources, S m was extrapolated from knowledge of the spectral index above the turnover frequency, assuming l D 100 MHz. m While there is some overlap of the two samples, we appear to have many sources that are signiÐcantly stronger at the turnover frequency. Whether this di†erence is quantitative, where we have selected just the strongest and steepest spectrum sources that are nonetheless similar to those from OÏDea (1998), or it represents a qualitative di†erence showing an exceptionally high brightness temperature, remains to be determined. None of our sources exhibits a clear spectral turnover above 365 MHz, and those with 151 MHz data do not turn over above 151 MHz (see Fig. 2). While our sources do not have brightness temperatures exceeding the 1012 K inverse Compton limit (Marscher 1980), they do approach the brightness limits imposed by synchrotron self-absorption (see OÏDea 1998 ; Kellerman & Pauliny-Toth 1981) : h D 9B1@4S1@2 la@2 (1 ] z)1@4l~5@4~a@2 , (1) NVSS NVSS m where S is in janskys, l \ 1.4 GHz, B is in gauss, and h isNVSS in milliarcseconds.NVSS The spectral index a is that above the peak (i.e., for l ? l ). We evaluate this for a typical source : l \ 100 MHz,m S \ 40 mJy, a \ 1.7, NVSS B \ 150 mG (themscale of the equipartition Ðeld ; C. Fanti

1000

100 0.01

0.1

1

10

FIG. 5.ÈS vs. h for the sources in this paper ( Ðlled squares) and those m ; open squares). The sources from this paper have only a in OÏDea (1998 lower bound on S (taking l B 100 MHz) since we have not yet observed m the spectral turnover. Also, mfor the smallest sources the size is an upper limit.

No. 2, 2000

COMPACT RADIO SOURCES

1998, private communication), and z \ 1, which gives h D 0A. 2. The dependences on the unknown redshift z and magnetic Ðeld B are quite weak. This indicates that our unresolved sources cannot be signiÐcantly smaller than their upper limits if they are incoherent synchrotron sources.

4.

CONCLUSIONS

We have constructed a statistically useful sample of the 74 radio sources with S [ 200 mJy having 365MHz 1.5 ¹ a ¹ 3.2, in the 10 sr of sky with 71¡30@ º d º [ 35¡30@, and h ¹ 25@@, the steepest spectrum sources known. Table 1 contains a summary of the radio data for these sources, while Table 2 summarizes our tentative classiÐcations. Preliminary optical/IR imaging suggests that most of the sources are extragalactic, probably lie at very high redshifts, and are in interesting environments. We cannot conclusively classify most of the sources, but ongoing e†orts are likely to produce deÐnitive results soon. We are planning to make VLBI images, pulse searches, and optical/IR spectroscopic observations of subsets of our sample in order to reÐne our classiÐcations. Other future projects include X-ray imaging to detect clusters and near-IR spectroscopy for redshifts.

865

We thank P. Hofner, C. Salter, and T. Ghosh for their assistance with data reduction while D. Kaplan was at the Arecibo Observatory. We also acknowledge the assistance of the sta† of Palomar Observatory and that of S. R. Kulkarni and J. S. Bloom in obtaining the images of TXS 0004]364. D. Kaplan was supported in part by NSF grant AST-95-30397 to Cornell University, an NSF REU grant to the NRAO, and an NSF REU grant to the Arecibo Observatory. S. G. D. was supported in part by the Bressler Foundation. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has also made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation. The VLA is operated by the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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