Deep Continuum Fields

Extragalactic Radio Sources

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, their 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 star forming activity may be found in a population of faint blue galaxies or associated with the strong IR galaxies detected in the IRAS survey at 60 microns. Because of the close association of 60 micron and radio wavelength emission, 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, 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 about -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.

The 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 above a limiting flux density of about 10 microJansky ($\mu$Jy). Optical identification and spectroscopic redshifts show that most catalogued radio sources stronger than a few mJy 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, so that nearly all of these powerful sources are included in the radio source counts above one mJy.

At $\mu$Jy levels, the radio source count again steepens, corresponding to a new population of radio sources. Nearly all $\mu$Jy radio sources can be identified with a mixture of low luminosity AGN and faint star forming galaxies which are often found in pairs or small groups. These $\mu$Jy radio sources mostly have redshifts between zero and one with corresponding radio luminosity between 10²⁰ and 10²⁵ W/Hz. Periods of active star formation may be driven by mergers which may also enhance their disk emission as well as fuel their central engine. The observed peak in the redshift distribution of $\mu$Jy radio sources is comparable with that found for strong radio galaxies, for quasars, and the population of faint star forming galaxies suggesting that the evolutionary scenario is comparable for all of these populations, and that conclusions about star formation rates, the epoch of quasar and galaxy formation are valid and are not the result of obscuration by dust.

The SubmicroJansky Sky

With the improvement in sensitivity given by the SKA, it will be possible detect radio emission as weak as 10 nanoJy in only a few hundred hours integration time, over a factor of a hundred fainter than the weakest sources seen so far with the VLA. At this level, not only will it be possible to detect radio emission from star forming activity out to redshifts of ten or more, if it exists, but for the first time, even normal spiral galaxies with P=10¹⁹ W/Hz will be detected out to cosmologically interesting distances (z=1). High resolution, multi-wavelength observations will be critical to distinguish among the different emission mechanisms for extragalactic radio sources.

Nothing is known about the radio source count below one $\mu$Jy. Extrapolation of the VLA 4 cm and 6 cm deep surveys suggests that there are about 100 sources/sq arcmin above 1 $\mu$Jy at 20 cm. To model the radio sky, the known 1.4GHz source counts (Windhorst et al. 1993; Hopkins et al. 1998) were used as the initial starting point. The known source counts were extrapolated down to a flux density of 1nJy, subject to known limits on the source count slope (due to the CMB) and implied limits from the number of possible optical counterparts (Windhorst et al. 1993). The distribution in apparent size of radio sources at 1.4GHz has been characterised as a function of flux density by Windhorst et al. (1990), and compared with the Phoenix Deep Survey sample (Hopkins, 1997) by way of verification. This distribution has been used to assign apparent sizes to a list of sources with given flux densities. The result of this is to produce a simulated distribution of sources with the same statistical properties (source counts and angular size distribution) as the real sky. The axial ratio of the simulated sources has been modeled simply by a uniform distribution between values of 0.2 and 1.

  

Figure 1.9: Simulated $\lambda$20 cm continuum observation with the SKA of a region the size and shape of the Hubble Deep Field. The radio source population is derived from an extrapolation of the known population above 1 $\mu$Jy. With a 5$\sigma$ detection limit after 8 hours of about 100 nJy, the simulated SKA image contains 2700 sources. Starburst galaxies are shown in blue and Radio Galaxies and Active Galactic Nuclei in red.

With the angular size and the axial ratio for each source, a simulated image was constructed by adding elliptical gaussians at random locations and position angles. The peak value of the gaussian is defined by the flux density of the source. As a first step in refining this very simple model the source counts were divided between two populations, broadly described as ``starbursts" and ``AGNs." This was accomplished by using the known fraction of these populations as a function of flux density (Wall & Jackson 1997; Hopkins et al., 1998). In addition, to mimic the double-lobed nature of many real AGNs, a pair of adjacent elliptical gaussians have been used, rather than the single elliptical gaussian used for starbursts. At brighter flux densities the angular size distribution will not necessarily be valid for the AGN population.

A simulation of an 8-hour SKA observation at 20 cm of a region the size and shape of the Hubble Deep Field is shown in Fig. 1.9. With a flux density limit of 100 nJy, over 2700 sources are predicted (a source density of $\approx 5\times10^9$sr^-1). The different populations are indicated by the colours, starbursts being blue and AGNs being red.

If the source count continues unchanged down to 10 nanoJy, there will be about 10,000 radio sources/sq arcmin above 10 nanoJy, or about three sources/sq arcsec. Assuming for the moment that these are point sources, with a resolution of 0.1 arcsec there will be about 30 beamwidths/source, just adequate to keep the effects of confusion negligible. More likely the nanoJy radio sources have dimensions comparable with those of the optical discs of galaxies, or a significant fraction of an arcsec at cosmologically significant redshifts, and at the full sensitivity of the SKA, individual sources will appear blended at the faintest flux density levels. Resolutions as least as good as 0.1 arcsec at 20 cm (500 km dimensions) will therefore be important not only to separate individual sources, but to image each source with sufficient detail to tell whether the emission comes from the entire disk characteristic of normal galaxies, is concentrated within a few hundred parsecs of the nucleus, characteristic of star formation, or is in a point source at the nucleus characteristic of a massive central engine.

At longer wavelengths, the greater source flux density is roughly canceled by the lower sensitivity of the SKA, so that the limiting source density remains approximately constant with wavelength. But, the required dimensions of the array scales with wavelength to maintain a resolution of 0.1 arcsec, comparable with that of the Next Generation Space Telescope, and adequate to separate individual sources and to image radio emission from distant galaxies. This implies baselines of 2500 km at least for those elements working at 1 meter wavelength (300 MHz).

At the short wavelength limit, the extraordinary sensitivity of the SKA will allow even normal galaxies at cosmologically interesting redshifts to be imaged with a resolution and image quality far superior to that of any other telescope, existing, or planned, operating at any wavelength band in space or on the ground. Such observations will be crucial to outline the early history of the formation of stars, galaxies, and quasars, without uncertainties due to possible obscuration by dust or other material.