Low-luminosity radio sources in early-type galaxies

We have made a sensitive radio continuum survey of 114 nearby E and S0 galaxies to search for weak sources. The radio detection rate is 42 per cent, with a flux limit of 0.8 mJy at 5 GHz. By deriving the radio luminosity function for a complete sample, we show that most bright early-type galaxies have low-luminosity |$({10}^{19}-{10}^{21}\,\text{W Hz}^{-1})$| non-thermal radio sources. Galaxies of similar optical luminosity vary widely in radio luminosity, but a characteristic radio power (the 30th percentile) rises roughly as the optical luminosity squared. S0 galaxies have weaker radio sources on average than elliptical galaxies, but this can be explained by the low luminosity of most S0 bulges relative to ellipticals. We find no correlation between radio power and axial ratio for galaxies with radio luminosities below |${10}^{23}\,\text{W Hz}^{-1}.$| Most early-type galaxies have modest amounts of ionized gas in their central regions, but there is no correlation between optical emission-line luminosity and the presence or strength of radio emission at the low luminosities considered here.