The radio properties of these young sources are described briefly in Section 1, the Introduction. The classic complete review is O'Dea 1998, while a shorter but recent review is Fanti 2009.
Recall that few can grow into large bright sources, because they are nearly as common as the big ones (as selected in the centimeter region), but have very short kinematic and synchrotron-aging lifetimes. Recall also that many of the tiny kinematic ages probably measure just the age of the current stage of activity, which may repeat many times. It's also possible that their birthrate is extremely high, but that most fade out before they grow.
The experts generally seem to agree that the radio-galaxy/Quasar unification holds fairly generally, for the well-studied very radio luminous population. That is, the radio galaxies are in the thermal class. At more modest luminosities, there is less information, and some hints from the infrared that this may not be the case. If so they behave similarly to the giant doubles.
4.1. Radio Properties
Saikia et al (2001) provide several good arguments for small ages and unification by geometry. On the former, kinematic and synchrotron losses ages are small and generally consistent. On the latter, the authors note that the Quasars are more core-dominant in the radio, and they have more asymmetric morphologies consistent with oppositely directed twin jets. Several optical and radio papers present evidence for absorption by molecules and HI, preferentially for the galaxies and thus near the plane according to the Unified Model, e.g., Baker et al 2002; Gupta and Saikia 2006; and Fanti 2009.
Astronomers studied this class of radio source in the infrared with IRAS (Heckman et al 1994) and with ISO (e.g., Fanti et al 2000). These radio emitters show generally high (Quasar-like) power in the aggregate. The ISO mission was able to make many individual detections, but the Fanti et al 2000 paper did not make a comparison of their observed galaxies with GPS/CSS Quasars.
In the Spitzer era, Willett et al (2010) presented data for eight relatively radio-faint "compact symmetric objects," which heavily overlap the GPS class. Only one was a broad-line object (OQ 208); one was a BL Lac Object. Their Fig. 8 shows a plot of the Si strength vs. equivalent width of the 6.2µ PAH feature, demonstrating that hidden AGN (marked by moderate Si absorption and fairly weak PAHs) probably dominate the mid-IR emission of the galaxies in all cases. The Quasar has Si slightly in emission, also as expected for the unified model. However, if considered bolometrically, the AGN luminosities are low (except for the Quasar) and PAH features indicate that star formation may contribute significant luminosity. Ionization levels are low. The authors favor Bondi accretion or black-hole spin energy for most of the galaxies, not a thermal Big Blue Bump, so in our parlance they would fall into the non-thermal class. 33
Our larger Spitzer survey, Ogle et al 2010, contains 13 Quasars and 11 radio galaxies from the 3CR catalog. It contains objects of substantially higher redshift (0.4-1.0) and luminosity (1034-1035 erg s-1 Hz-1 at 1.5 GHz and 5 GHz), as compared with the Willett et al 2010 sample.
The radio luminosities of our sample sound much higher than most FR IIs, where the FR I/II cutoff is ~ 2 × 1032 erg sec-1 Hz-1 at 1.5 GHz, but the CSS and especially the GPS sources are much weaker relative to the big doubles at low frequency. Also note that this and their small sizes indicate that they contain much less energy in particles and fields overall.
It is not so easy to get a complete isotropic sample of GPS sources because they don't have dominant isotropic lobe emission. Possibly one could select by an emission line, preferably in the infrared, after radio classification. We took the GPS sources from Stanghellini et al 1998, a complete GHz-selected sample but likely with beaming effects favoring low inclination. This might not be too bad since we are mainly comparing Quasars and radio galaxies, with the latter still at larger inclinations by hypothesis (shadowing Unified Model); however they may not be at the same level on the luminosity function. For the CSS sources, we could find a roughly isotropic sample in the 3CR, and we took them from Fanti et al 1995.
We find that the GPS/CSS galaxies in our sample are all powerful thermal dust emitters, with vLv(15µ) ~ 5-500 × 1043 erg s-1. The Si features behave somewhat erratically vs. optical type, although most follow the pattern of emission for Quasars and absorption for galaxies. This can be accommodated by clumpy torus models, but since it's specific to the small objects, it might also be from the effects of colder foreground off-nuclear dust. The interpretation of the galaxies as hidden Quasars is greatly strengthened by the [Ne V] and [Ne VI] lines detected in all of the Quasars and many of the galaxies. We detect no PAH or H2 features.
In summary, the infrared evidence favors a (nearly?) ubiquitous shadowing unified model for the most radio-luminous small sources. Remember, however, these are the highest-luminosity members of the class, and it's likely that at lower luminosities, some radio galaxies lack the hidden Quasars, based on Willett et al 2010.
Bright GPS and CSS radio sources tend to have strong line emission. All three 3CR CSS sources studied by Labiano et al (2005), two radio galaxies and one Quasar, show [O III] 5007 / narrow H > 10, so they have high ionization. There is evidence for shocks also in these nice HST spectra.
There is a wealth of information in de Vries et al 1999 and Axon et al 2000 on HST imaging of CSS radio galaxies. These groups imaged in the H or [O III] 5007 line in tens of objects. Only the broad line objects have point continuum sources, in accord with the Unified Model. Thus there are almost no known "bare synchrotron" 34 sources as described above for FR I and FR II galaxies. This is not really a demonstrated difference however since until very recently, only the most luminous GPS/CSS sources have been studied in detail. In fact, very recently Kunert-Bajraszewska and Labiano (2010) have reported on line emission on fainter small radio sources, finding that many have Low Ionization Galaxies spectra, just like for the giant doubles.
Both de Vries et al (1999) and Axon et al (2000) also found that the line emission lies preferentially parallel to the radio axes, and Axon et al add that photon counting arguments require a hidden radiation source. Those alone are powerful arguments for unification with Quasars.
There are several papers on X-rays from small radio sources. Guainazzi et al (2006) reported on a small sample of GPS galaxies at redshifts between 0.2 and 1. All four with adequate SNR have large column densities, "consistent with that measured in High-Excitation FR II galaxies," strongly indicating hidden Quasars. The radio luminosities are around 1034-35 erg s-1 Hz-1 at 1.5 GHz, near the spectral peaks. The 2-10 keV X-ray de-absorbed luminosities are 1044-1045 erg s-1. (The authors give their H0 = 70, but no other cosmological parameters, so this is approximate.) Since that band covers only 0.7 dex in frequency, the X-ray luminosity alone is 1 × 1045 erg s-1, and the bolometric luminosity is likely to be at least a few times higher. There is one exception, which is convincingly argued to be Compton-thick and so opaque in the X-ray. Consistent results were reported by Siemiginowska et al (2008).
This group expanded their survey of GPS galaxies (Tengstrand et al 2009; neither study included Quasars) and confirms and extends these results, again concentrating on the most radio-luminous objects.
33 This refers to the overall SED, and not the infrared emission specifically. Back.
34 An exception is PKS 0116+082, from Cohen et al 1997. This really anomalous object has high and variable polarization like a Blazar and good limits on broad H / 5007, yet very strong narrow emission lines. There is actually a possible broad H line in polarized flux, which is however unexpected in a Blazar. This object is not analogous to the much lower luminosity HST optical point sources studied by Zirbel and Baum, and Chiaberge et al, and discussed in the previous two sections. One point in common though is at least a few percent optical polarization (Capetti et al 2007). Back.