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

Abstract

The 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.

GaLactic and Extragalactic All-sky Murchison Widefield Array survey eXtended (GLEAM-X) I: Survey description and initial data release

“…GLEAM has been processed in a multitude of ways: continuum data releases cover most of the extragalactic sky (GLEAM ExGal; Hurley-Walker et al 2017), the Magellanic Clouds (For et al 2018), the Galactic Plane (GLEAM GP; Hurley-Walker et al 2019b), and a deep region over the South Galactic Pole (GLEAM SGP; Franzen et al 2021a); and polarisation products include all-sky circular (Lenc et al 2018) and linear polarisation surveys (Polarised GLEAM Survey (POGS); Riseley et al 2018;Riseley et al 2020). Cross-identifications have been provided for the 1863 brightest radio sources in the mid-infrared (the G4Jy Sample White et al 2020a;White et al 2020b), and for 1590 galaxies in the 6dF Galaxy Survey (Franzen et al 2021b).…”