Annu. Rev. Astron. Astrophys. 1997. 35: 607-36 Copyright © 1997 by Annual Reviews. All rights reserved

2.4. Compact Steep Spectrum Sources and Compact Symmetric Objects

On the order of 20-30% of compact sources in flux-density-limited surveys at lower frequencies (e.g. 3CR) belong to the compact steep spectrum (CSS) class (Fanti et al 1990). These galaxies or quasars are typically small (< 2 arcsec or linear sizes 10-15 h^-1 kpc) and exhibit a steep radio spectrum ( > -0.5); few have been found to show evidence for the presence of relativistic components. The giga-Hertz-peaked spectrum sources (O'Dea et al 1991, O'Dea 1996), subkiloparsec-size objects with radio spectra that peak at giga-Hertz frequencies, are considered part of this class.

In a morphologic sense, three subgroups have been identified: compact symmetric objects (CSO, size < 0.5 h^-1 kpc), medium-size symmetric objects (MSO, size 0.5 kpc), and complex sources (Readhead 1995, Fanti & Spencer 1996). The latter are mostly identified as quasars with core-jet structures where the jets dominate (Spencer 1994).

The distortions in the sources with complex structures have been attributed to beaming and/or "frustration" by interaction with a dense interstellar medium (Spencer 1994) that would inhibit the growth of the structures to large dimensions. Figure 5 shows VLBI polarization images of the superluminal CSS quasar 3C 138 (Cotton et al 1997). The magnetic field in this complex jet is aligned along the jet axis and wraps around the head of the jet. The nuclear region is depolarized at 1.7 GHz, and there is no evidence for significant magnetized plasma in front of the end of the jet. At least in this case, the data suggest that this is not a jet that is frustrated by a dense interstellar medium.

Figure 5. The core and jet in the CSS quasar 3C 138 (from Cotton et al 1997). (a) Image at 18 cm, made with the EVN and the VLBA. The peak in the image is 241 mJy/beam and contours are drawn at -5, 5, 8, 14, 19, 27, 54, 81, 135, 189, and 270 mJy/beam. The lengths of the polarization vectors are proportional to the polarized intensity and have the orientation of the electric field vectors. (b) Image at 5 GHz, made with the VLBA. The resolution is 5 milliarcsec and the beam is shown in the lower left corner. The peak in the image is 215 mJy/beam and contours are drawn at -2, 2, 3, 5, 7, 10, 20, 30, 50, 70, 100, and 200 mJy/beam. Vectors as in a. Note that total intensity images of this source also show a weak feature on the opposite side from the jet.

The CSO sources comprise 5-10% of all high-luminosity AGN and are probably related to the "compact double" type (Phillips & Mutel 1982). Figure 6 shows an example, 2352+495 (Taylor & Vermeulen 1996). The identification of the center of activity in this and other CSOs is often difficult due to low core fractions and complex structures. Most objects in this class are identified with galaxies (but see Perlman et al (1994) for a possible BL Lac object with CSO properties and a parsec-scale counterjet). Typically the CSOs show a very weak core, two symmetric lobes or hot spots, and resolved jet-like emission on one side. The ratio of jet length to counterjet length is close to unity, and higher pressure and smaller hot spots are seen on the jet side. It is still unclear if this asymmetry is intrinsic or instead caused by beaming and/or environmental effects. Within a simple unification model of radio sources, the CSOs and MSOs appear likely to be young and intermediate progenitors of large-scale FR II objects that show decreasing luminosity as they expand (Readhead et al 1996a, b, Fanti et al 1995).

Figure 6. A 15-GHz VLBA image of the CSS source 2352+495 (from Taylor et al 1996). Contours are drawn at -0.75, 0.75, 1.5, 3, 6, 12, 24, 48, 96 mJy/beam, and the beam is 0.91 × 0.67 milliarcsec. Component D is identified with the core, and A and C with hot spots.