The redshift distribution of FIRST radio sources at 1 mJy

We present spectra for a sample of radio sources from the FIRST survey, and use them to define the form of the redshift distribution of radio sources at mJy levels. We targeted 365 sources and obtained 46 redshifts (13 per ce nt of the sample). We find that our sample is complete in redshift measurem ent to R similar to 18.6, corresponding to z similar to0.2. Galaxies were a ssigned spectral types based on emission-line strengths. Early-type galaxie s represent the largest subset (45 per cent) of the sample and have redshif ts 0.15 less than or similar toz less than or similar to0.5; late-type gala xies make up 15 per cent of the sample and have redshifts 0.05 less than or similar toz less than or similar to0.2; starbursting galaxies are a small fraction (similar to6 per cent), and are very nearby (z less than or simila r to0.05). Some 9 per cent of the population have Seyfert 1/quasar-type spe ctra, all at z greater than or similar to0.8, and 4 per cent are Seyfert 2 type galaxies at intermediate redshifts (z similar to0.2). Using our measurements and data from the Phoenix survey (Hopkins et al.), w e obtain an estimate for N(z) at S(1.4 GHz)greater than or equal to1 mJy an d compare this with model predictions. At variance with previous conclusion s, we find that the population of starbursting objects makes up less than o r similar to5 per cent of the radio population at S similar to1 mJy.