The 1.4-GHz and Hα luminosity functions and star formation rates from faint radio galaxies

A sample of over 1000 objects selected from a 1.4-GHz survey made by the Australia Telescope Compact Array (ATCA) is used to study the properties of the faint radio source population. The sample, covering an area of approximate to 3 deg(2), is 50 per cent complete to 0.2mJy, Over 50 per cent of the radio sources are found to have optical counterparts brighter than R approximate to 21.5. Spectroscopic observations of 249 optically identified radio sources have been made, using the 2-degree Field (2dF) facility at the Anglo-Australian Telescope (AAT). Redshifts and equivalent widths of several spectral features (e.g. H alpha and [OII] lambda 3727) sensitive to star formation have been measured. On the basis of the photometric and spectroscopic data, the optically identified radio sources are classified as (i) absorption-line galaxies, (ii) star-forming galaxies and (iii) Seyfert-like galaxies. The spectroscopic sample is corrected for incompleteness and used to estimate the 1.4-GHz and H alpha luminosity functions (LFs) and luminosity density distributions. The 1.4-GHz LF of the star-forming population has a much steeper faint-end slope (1.85) than the ellipticals (1.35). This implies an increasing preponderance of star-forming galaxies among the optically identified (i.e. z less than or similar to 1) radio sources at fainter flux densities. The H alpha LF of the faint radio population agrees with published H alpha LFs derived from local samples selected by Ha emission. This suggests that the star-forming faint radio population is coincident with the H alpha-selected population. The 1.4-GHz and H alpha luminosity densities have been used to estimate the star formation rates (SFRs). The two SFRs agree, both giving a SFR density of 0.032 M.yr(-1)Mpc(-3) in the range z less than or similar to 1. Radio selection appears to be as effective as Ha selection in finding the galaxies that dominate star formation at a given epoch. Although the sample contains many galaxies lying beyond z approximate to 0.3, it does not reveal a significant rise in the global star formation rate with increasing redshift. This result suggests that the optical counterparts of galaxies undergoing vigorous star formation at redshifts beyond z approximate to 0.3 are generally fainter than R approximate to 21.