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1.2. Definitions and Samples

We use the term compact steep spectrum instead of steep spectrum core (SSC) to describe the CSS sources, since they are not just cores but are full-fledged radio sources complete with jets and lobes on small scales.

Currently, our understanding of these sources is based on samples selected from (rather bright) flux density-limited samples. Similar samples of CSS sources drawn from the 3C and Peacock & Wall (1982, hereafter PW) samples have been constructed by Fanti et al. (1990b), Spencer et al. (1989), and Sanghera et al. (1995). The selection criteria for the Fanti et al. sample are (1) projected linear sizes less than 20 kpc, (2) flux density at 178 MHz of at least 10 Jy (1) (the flux densities are extrapolated for the sources that turn over before 178 MHz), (3) |b| > 10° and delta > 10°, and (4) log power at 178 MHz of at least 26.75 W Hz-1. The selection frequency of the 3C sample, 178 MHz, favors sources whose spectra peak around 178 MHz or lower. The PW sample is selected at 2.7 GHz and contains sources with spectral peaks at higher frequencies than the 3C selected objects. However, the PW criterion that the spectral index (2) between 2.7 and 5 GHz be steeper than -0.5 selects sources that peak below 2.7 GHz and in practice below about 1 GHz (Fanti et al. 1990b). Thus, although it is often stated that the GPS sources are a subset of the CSS sources, there are very few sources with turnover frequencies above 1 GHz in the existing well-studied CSS samples.

The published lists of GPS sources are heterogeneous but are predominantly based on surveys at 5 GHz (S4 and 1 Jy - see, e.g., Pauliny-Toth et al. 1978; Kühr et al. 1981a, 1981b).

The high selection frequency allows sources with higher frequency spectral peaks than in the existing CSS samples to be included. However, sources with spectral peaks much above 5 GHz are underrepresented in these samples, since the flux density is declining at 5 GHz in these sources.

Lists of GPS sources have been published by Gopal-Krishna and collaborators (Gopal-Krishna et al. 1983; Spoelstra, Patnaik, & Gopal-Krishna 1985; Gopal-Krishna & Spoelstra 1993), Cersosimo et al. (1994), O'Dea et al. (1991), Stanghellini et al. (1990c, 1996), and King et al. (1996). A combined list of GPS and CSS sources has been collated by Dallacasa & Stanghellini (1990). Lists of GPS sources selected to have lower flux densities than the other major lists are presented by Snellen et al. (1995), Marecki et al. (1996), and Fanti, Vigotti, & Di Paolo (1996). Stanghellini's sample is well defined: (1) flux density at 5 GHz above 1 Jy, (2) turnover frequency between 0.4 and 6 GHz, (3) spectral index above the peak of steeper than -0.5, (4) delta > -25° and |b| > 10°. The most comprehensive list of GPS sources so far is that of O'Dea et al. (1991).

The selection effects in the list and the possible redshift evolution of the radio spectral peaks are discussed by de Vries et al. (1997a). The Fanti et al. (1990b) CSS sample and Stanghellini et al. (1990c, 1996) GPS sample are given in Table 1. Future lists of GPS and CSS sources are likely to be generated from follow-ups to surveys now in progress, e.g., the VLA 20 cm surveys NVSS and FIRST, and the WSRT survey WENSS. Selection of sources from VLBI surveys on the basis of compact symmetric object (CSO) morphology (see, e.g., Wilkinson et al. 1994; Taylor et al. 1994; Readhead et al. 1996b; Taylor et al. 1996c) is very successful in identifying GPS galaxies because of the strong correlation between GPS-type radio spectra and CSO or compact double (CD) morphology in radio galaxies.

Table 1. Combined CSS and GPS Complete Samples
S5 GHz theta num P5 GHz Size num(1 + z)
Source Catalog Sample ID Mag z (Jy) (arcsec) (MHz) (W Hz-1) (kpc) (MHz)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
0019-000   G G 21.1 0.305 1.1 0.06 700. 26.4 0.240 914.
0108+388   G G 22.0 0.669 1.26 0.006 4000. 27.3 0.037 6676.
0127+233 3C43 C Q 20.0 1.459 1.1 2.60 20. 28.0 21.054 49.
0134+329 3C48 C Q 16.1 0.367 5.3 0.50 80. 27.3 2.255 109.
0138+138 3C49 C G 22.0 0.621 0.9 0.92 120. 27.0 5.529 195.
0221+276 3C67 C G 18.0 0.309 0.9 2.5 50. 26.3 10.100 65.
0223+341 4C34.07 C S 21.3 1.0 1.3 1.1 250. 27.6 8.004 500.
0237-233   G Q 16.6 2.223 3.34 0.018 1000. 29.0 0.158 3223.
0248+430   G Q 15.5 1.316 1.24 0.06 5000. 27.9 0.474 11580.
0316+161 4C16.09 CG G 22.0 1.0 2.89 0.30 800. 28.1 2.183 1600.
0319+121 OE131 C Q 19.0 2.662 1.10 0.02 400. 28.5 0.180 1465.
0345+337 3C93.1 C G 19.0 0.244 0.8 0.25 60. 26.0 0.858 75.
0404+769 4C76.03 C G 22.0 0.5985 2.82 0.15 600. 27.4 0.886 959.
0428+205 OF247 CG G 20.0 0.219 2.38 0.250 1100. 26.4 0.793 1341.
0429+415 3C119 C G 20.0 1.023 3.5 0.08 150. 28.1 0.587 303.
0457+024   G Q 19.4 2.384 1.57 ... 2100. 28.7 0.000 7106.
0500+019   G Q 20.2 0.583 1.89 0.015 1800. 27.2 0.087 2849.
0518+165 3C138 C Q 18.8 0.760 4.1 0.60 100. 27.8 3.942 176.
0538+498 3C147 C Q 17.8 0.545 8.2 0.55 150. 27.9 3.101 232.
0710+439   G G 20.7 0.518 1.68 0.025 1900. 27.1 0.137 2884.
0738+313   G Q 16.1 0.630 3.62 0.010 5300. 27.6 0.061 8639.
0740+380 3C186 C Q 17.6 1.063 0.3 2.2 40. 27.2 16.330 83.
0742+103   G EF 21.9 1.0 3.46 0.010 2700. 28.1 0.073 5400.
0743-006   G Q 17.5 0.994 2.05 0.005 5800. 27.8 0.036 11565.
0758+143 3C190 C Q 20.3 1.197 0.9 4.1 40. 27.8 31.538 88.
0941-080   G G 19.0 0.228 1.11 0.05 500. 26.1 0.163 614.
1005+077 3C237 C G 21.0 0.877 2.0 1.3 50. 27.8 9.033 94.
1019+222 3C241 C G 22.0 1.617 0.4 0.84 40. 27.8 6.960 105.
1031+567   G Q 20.3 0.459 1.28 0.040 1300. 26.9 0.206 1897.
1117+146 4C14.41 G G 20.0 0.362 1.00 0.08 500. 26.5 0.358 681.
1127-145   G Q 16.9 1.187 3.82 0.003 1000. 28.3 0.023 2187.
1143-245   G Q 18.5 1.950 1.40 0.006 2200. 28.4 0.052 6490.
1153+317 4C31.38 C Q 19.0 1.557 1.0 0.9 100. 28.0 7.397 256.
1203+645 3C268.3 C G 19.0 0.371 1.1 1.36 80. 26.6 6.175 110.
1225+368 ON343 C Q 21.7 1.974 0.77 0.060 1200. 28.5 0.516 3569.
1245-197   G Q 20.5 1.280 2.34 0.500 500. 28.2 3.918 1140.
1250+568 3C277.1 C Q 17.9 0.321 1.0 1.67 100. 26.4 6.918 132.
1323+321 4C32.44 CG G 19.0 0.369 2.39 0.06 500. 26.9 0.272 684.
1328+254 3C287 C Q 17.7 1.055 3.2 0.048 50. 28.1 0.355 103.
1328+307 3C286 C Q 17.2 0.849 7.4 3.2 80. 28.2 21.966 148.
1345+125 4C12.50 G G 17.0 0.122 3.05 0.080 400. 26.0 0.160 449.
1358+624 4C62.22 CG G 19.9 0.429 1.80 0.07 500. 26.9 0.347 714.
1404+286 OQ208 G G 14.0 0.077 2.69 0.007 4200. 25.5 0.009 4523.
1413+349 OQ323 C EF 22.0 1.0 1.02 0.06 1000. 27.5 0.437 2000.
1416+067 3C298 C Q 16.8 1.439 1.5 1.49 80. 28.3 12.026 195.
1442+101 OQ172 CG Q 17.8 3.535 1.20 0.02 900. 29.3 0.185 4081.
1443+77 3C303.1 C G 19.0 0.267 0.5 1.7 100. 26.0 6.219 127.
1447+77 3C305.1 C G 21.0 1.132 0.5 2.34 90. 27.5 17.709 192.
1458+718 3C309.1 C Q 16.8 0.905 3.5 2.11 40. 28.0 14.831 76.
1517+204 3C318 C G 20.3 0.752 0.8 1.05 40. 27.2 6.868 70.
1518+047   G S 22.6 1.296 1.09 0.135 800. 28.1 1.061 1837.
1600+335 OS300 CG EF 23.2 1.0 2.67 0.06 2400. 28.0 0.437 4800.
1607+268 CTD93 CG G 20.7 0.473 1.73 0.05 1100. 27.1 0.262 1620.
1634+628 3C343 C Q 20.6 0.988 1.48 0.20 250. 27.7 1.449 497.
1637+626 3C343.1 C G 20.8 0.750 1.2 0.24 250. 27.4 1.568 438.
1819+39 4C39.56 C G 19.0 0.4 1.0 0.44 100. 26.6 2.091 140.
1829+29 4C29.56 C Q 20.0 0.842 1.1 3.1 100. 27.3 21.212 184.
2008-068   G G 21.1 1.0 1.34 0.030 1400. 27.8 0.218 2800.
2126-158   G Q 17.3 3.270 1.17 0.008 4100. 28.9 0.073 17507.
2128+048   G G 23.4 0.990 2.02 0.030 700. 27.9 0.218 1393.
2134+004   G Q 16.8 1.936 8.50 0.002 4300. 29.2 0.017 12625.
2210+016   G S 21.7 1.0 1.05 0.055 500. 27.6 0.400 1000.
2248+71 3C454.1 C G 22.0 1.841 0.3 1.6 40. 27.9 13.600 114.
2249+185 3C454 C Q 18.5 1.758 0.8 0.66 40. 28.1 5.562 110.
2252+129 3C455 C Q 19.7 0.543 0.8 3.3 40. 26.9 18.570 62.
2342+821   CG Q 20.5 0.735 1.28 0.18 500. 27.4 1.166 868.
2352+495   G G 18.4 0.237 1.49 0.050 700. 26.3 0.168 866.

NOTES. Master list of Fanti et al. CSS and Stanghellini et al. GPS source samples. Column 1. B1950 IAU name. Column 2. Catalog name. Column 3. Membership in CSS (C) or GPS (G) sample. Column 4. ID. Quasar (Q), Galaxy (G), Stellar (S), or Empty Field (EF). Column 5. Optical magnitude. Column 6. Heliocentric Redshift. We adopt z = 1.0 for sources with unknown redshift. Column 7. Flux density at 5 GHz. Column 8. Angular Size. Column 9. Observed frequency of spectral turnover. Column 10. Log of power at 5 GHz. Column 11. Linear Size. Column 12. Rest-frame turnover frequency.

The statistics of occurrence of GPS and CSS sources in several flux density-limited samples is given in Table 2. The CSS and GPS sources make up about 30% and 10%, respectively, of the sources selected at frequencies around 5 GHz. This is a significant fraction of the powerful radio sources!

Table 2. GPS / CSS Demographics
Subsample Parent sample Selection Frequency Fraction (%) Reference
CSS S4 5 GHz 29% 1
CSS PW 2.7 GHz 31% 2
CSS 3CR 178 MHz 12% 2
GPS S4 5 GHz 8.5% 3
GPS 1Jy 5 GHz 10% 3

NOTES. The frequency of occurrence of the GPS and CSS sources in different flux density limited samples. The identification of GPS sources is not straightforward since it requires sufficient spectral data that a peak in the spectrum can be identified. Thus the GPS statistics are probably lower limits.
REFERENCES. (1). Kapahi (1981). (2). Peacock & Wall (1982). (3) this work.

In the rest of this paper, I use the Stanghellini et al. GPS sample and the Fanti et al. CSS sample when I discuss the global properties of GPS and CSS sources. The Fanti and Stanghellini samples contain a few sources that overlap (0316+161, 0428+205, 1323+321, 1358+624, 1442+101, 1600+335, 1607+268, 2342+821), and these are of course only counted once in the combined sample. I have removed a few sources that no longer fit the original sample criteria (3C 216, 3C 299, 3C 346, 3C 380, and 2230+110; see section 3.1). The resulting combined sample contains 67 sources (Table 1).


1 1 Jy = 10-23 ergs s-1 cm-2 Hz-1. Back.

2 We define spectral index alpha such that S propto nualpha. Back.

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