The GLEAM 200-MHz local radio luminosity function for AGN and star-forming galaxies

Abstract

AbstractThe GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) is a radio continuum survey at 76–227 MHz of the entire southern sky (Declination$<\!{+}30^{\circ}$) with an angular resolution of${\approx}2$arcmin. In this paper, we combine GLEAM data with optical spectroscopy from the 6dF Galaxy Survey to construct a sample of 1 590 local (median$z \approx 0.064$) radio sources with$S_{200\,\mathrm{MHz}} > 55$mJy across an area of${\approx}16\,700\,\mathrm{deg}^{2}$. From the optical spectra, we identify the dominant physical process responsible for the radio emission from each galaxy: 73% are fuelled by an active galactic nucleus (AGN) and 27% by star formation. We present the local radio luminosity function for AGN and star-forming (SF) galaxies at 200 MHz and characterise the typical radio spectra of these two populations between 76 MHz and${\sim}1$GHz. For the AGN, the median spectral index between 200 MHz and${\sim}1$GHz,$\alpha_{\mathrm{high}}$, is$-0.600 \pm 0.010$(where$S \propto \nu^{\alpha}$) and the median spectral index within the GLEAM band,$\alpha_{\mathrm{low}}$, is$-0.704 \pm 0.011$. For the SF galaxies, the median value of$\alpha_{\mathrm{high}}$is$-0.650 \pm 0.010$and the median value of$\alpha_{\mathrm{low}}$is$-0.596 \pm 0.015$. Among the AGN population, flat-spectrum sources are more common at lower radio luminosity, suggesting the existence of a significant population of weak radio AGN that remain core-dominated even at low frequencies. However, around 4% of local radio AGN have ultra-steep radio spectra at low frequencies ($\alpha_{\mathrm{low}} < -1.2$). These ultra-steep-spectrum sources span a wide range in radio luminosity, and further work is needed to clarify their nature.