Australia Telescope National Facility

Summary of the ``Sub-microJansky Radio Sky'' workshop
Andrew Hopkins, Ron Ekers, Carole Jackson, Lawrence Cram, Anne Green, Dick Manchester, Lister Staveley-Smith and Ray Norris, PASA, 16 (2), in press.

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Galaxy source density - Andrew Hopkins

At bright flux densities radio galaxies are dominated by sources with high radio luminosity and the traditional double-lobed morphology of the FRI and FRII classes. However, as objects with ever fainter radio flux densities are observed the dominant population changes to one whose radio emission is derived, not from ultra-relativistic jets, but from extremely high rates of star-formation (Condon 1989; Benn et al. 1993). This new population of starburst galaxies starts becoming dominant at flux densities of a few mJy. As flux density continues to decrease, more distant starburst populations will become evident, as will less vigorous local starbursts, until, it is assumed, the measurements of radio emission are sensitive enough to detect the star-forming processes in ``normal" galaxies.

As an element in a Square Kilometre Array design study, a model to simulate the radio sky down to arbitrarily faint flux densities is useful for a number of reasons. One of the main ones, perhaps, is that a simulation of the radio sky can be applied to model telescope designs to distinguish the optimum solution. However, there is also the opportunity to provide models for a range of different scenarios, all consistent with current observational results, but between which Square Kilometre Array observations will be able to distinguish: Different cosmologies, different rates of galaxy evolution, different population fractions as a function of flux density, and so on.

Such a model has been constructed using the known 1.4GHz source counts (Windhorst et al. 1993; Hopkins et al. 1998) as a starting point for a preliminary investigation. Since the Square Kilometre Array will have a sensitivity enabling it to detect sources as faint as $1\,\mu$ Jy or even fainter, 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 (a $2^{\circ}$ diameter survey at 1.4GHz cataloguing over 1000 sources to 0.1mJy, see Hopkins (1997) for details) 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 modelled simply by a uniform distribution between values of 0.2 and 1, an aspect of the method which obviously needs refinement.

With the angular size and the axial ratio for each source, an image is 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.

In a simulation of a region the size and shape of the Hubble Deep Field (HDF), (Figure 3), with a flux density limit of $0.1\mu$ Jy, over 2200 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.

There are many elements which have been neglected in this preliminary effort which it would be desirable to have included in further refinements of such modelling. One major refinement will be to base the simulations on known radio luminosity functions for different populations rather than using the source counts. This will allow the modelling of different evolutionary scenarios and cosmologies. Extending the models to cover a range of frequencies is also desirable.

However, while the initial model excludes many things, it still provides a very useful first estimation of the nature of the radio sky down to flux densities several orders of magnitude fainter than has ever been observed. As a result of the model presented, several preliminary suggestions relating to the details of the Square Kilometre Array design have been proposed. To minimise the effects of confusion a synthesised beam FWHM of about

$0\hbox{$.\!\!^{\prime\prime}$}1$ is desirable. Also, with a field of view of a square degree, (compared to the few square arcminutes for the HDF) new techniques will need to be developed to catalogue, characterise and analyse the sheer number of sources which will be detected.

**Figure 3:** A simulation of AGN and starburst galaxies brighter than $0.1\,\mu$ Jy at 1.4GHz. Only 150 of the 2200 total sources predicted are AGN.
$\begin{figure} \centerline{\psfig{figure=submicro2.ps,angle=-90,height=10cm}}\par\end{figure}$

Next Section: The Galaxy - Anne
Title/Abstract Page: Summary of the ``Sub-microJansky
Previous Section: Primordial HI - Lister
Contents Page: Volume 16, Number 2

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