Extragalactic radio sources cover a wide range of luminosity extending from 1019 Watts/Hz for normal spiral galaxies to more than 1028 Watts/Hz for the powerful FRII radio galaxies and radio loud quasars. Intermediate in luminosity are the less powerful FRI radio galaxies, the radio quiet quasars, and galaxies with active star formation.
With few exceptions the observed radio emission is non thermal synchrotron radiation. The nature of individual radio sources, e.g., normal galaxies, starbursts, AGNs, FRI or FRII radio galaxies, may be distinguished by their luminosity, the observed radio morphology, the radio spectral index, variability, the optical counterpart and spectral features, and observed characteristics in other wavelength bands, particularly x-ray and infrared.
In the powerful radio galaxies and quasars, the source of energy is thought to lie in a massive central engine, whereas in many of the sources of intermediate luminosity the energy source appears to lie in regions of active star formation and supernovae activity which accelerate relativistic electrons into the interstellar medium. This starforming activity may be found in a population of faint blue galaxies (Windhorst et al. 1985, Thuan and Condon 1987) or associated with the strong IR galaxies detected in the IRAS survey at 60 microns (Danese et al. 1987, Franceschini et al. 1989). Because of the close association of 60 micron and radio wavelength emission (Helou, Soifer, & Rowan-Robinson 1985), both believed to be closely linked to star forming activity, deep radio observations are sensitive to star formation at early epochs, unaffected by the obscuration which plagues optical and infrared observations. Star forming rates may be estimated from the observed radio flux density (Condon 1992), but only if the contribution to the observed radio flux density from a massive central engine can be determined from the radio or optical data.
Typically, both the FRI and FRII radio galaxies, as well as radio loud and radio quiet quasars have linear dimensions of the order of a few hundred kiloparsecs with structure characterized by radio lobes which are well separated from the optical counterpart, but often joined to the optical counterpart by a thin jet. FRI and FRII radio galaxies are optically thin with radio spectral indices, ~ -0.8, but may have a self absorbed flat spectrum core. The compact radio cores of quasars and AGN typically have angular dimensions much less than an arcsecond, and due to self absorption have flat radio spectra with observed spectral indices near zero. These very compact radio sources are often variable, and are identified with galaxies showing broad emission line spectra in their nuclei.
Radio emission from galaxies with active star formation is typically confined to dimensions comparable to that of the galactic disk. Optical counterparts are often unusually bright at far infrared wavelengths as a result of the absorption of uv emission from young massive stars by dust and its subsequent thermal reradiation.
Radio source surveys made over a wide range of wavelengths and flux density have catalogued about two million discrete radio sources. Figure 1 shows the normalized differential radio source counts compiled from surveys made at 1.4, 5, and 8.4 GHz as presented by Windhorst et al. (1993). Optical identification and spectroscopic redshifts show that most catalogued radio sources stronger than a few millijauskys are relatively distant powerful radio galaxies or quasars with radio luminosities greater than 1025 Watts/Hz, or nearby normal or nearly normal galaxies with much weaker radio emission. The space density of powerful radio galaxies and quasars quickly converges beyond redshifts of unity, (e.g., Condon 1984, 1989) so that nearly all of these powerful sources are included in the radio source counts above one millijansky.
Figure 1. Differential number flux density relation at 1.4, 5, and 8.4 GHz compiled by Windhorst et al. (1993). Data are shown multiplied by S2.5 so that the expected count, in a uniformly filled static universe with Euclidean geometry, is represented by a horizontal line.
At submillijansky levels, the normalized radio source count begins to turn up, corresponding to a new population of radio sources (e.g., Windhorst et al. 1985, Fomalont et al. 1991, Windhorst et al. 1993). Identification of these sub-millijansky radio sources are with a mixture of faint starforming galaxies and low luminosity AGN which are often found in pairs or small groups (e.g., Weistrop et al. 1987, Benn et al. 1993, Windhorst et al. 1995, Hammer et al. 1995). These previous programs of optical identification of weak radio sources, which have been done with 4-m class telescopes and with HST prior to refurbishment, typically reach a limiting R magnitude of about 25. The radio observations of the HDF offers the possibility of examining the optical counterparts of microjansky radio sources down to 29-30 magnitude with angular resolution up to an order of magnitude better than previous work. Moreover, the extensive ground-based follow-up work has been important in obtaining spectroscopic classification of the optical counterparts, as well as for determining redshifts needed to constrain the radio luminosity function.