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

ABSTRACT

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 ≈3 deg², is 50 per cent complete to 0.2 mJy. Over 50 per cent of the radio sources are found to have optical counterparts brighter than R≈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. Hα and [O ii] λ3727) 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 Hα 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≲1) radio sources at fainter flux densities. The Hα LF of the faint radio population agrees with published Hα LFs derived from local samples selected by Hα emission. This suggests that the star-forming faint radio population is coincident with the Hα-selected population.

The 1.4-GHz and Hα 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≲1. Radio selection appears to be as effective as Hα selection in finding the galaxies that dominate star formation at a given epoch. Although the sample contains many galaxies lying beyond z≈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≈0.3 are generally fainter than R≈21.