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 ${≈}2$ 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 $z ≈ 0.064$ ) radio sources with $S_{200 MHz} > 55$ mJy across an area of ${≈}16 700 deg2$ . 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 ${∼}1$ GHz. For the AGN, the median spectral index between 200 MHz and ${∼}1$ GHz, $α_{high}$ , is $-0.600 ± 0.010$ (where $S ∝ ν^{α}$ ) and the median spectral index within the GLEAM band, $α_{low}$ , is $-0.704 ± 0.011$ . For the SF galaxies, the median value of $α_{high}$ is $-0.650 ± 0.010$ and the median value of $α_{low}$ is $-0.596 ± 0.015$ . 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 ( $α_{low} < -1.2$ ). These ultra-steep-spectrum sources span a wide range in radio luminosity, and further work is needed to clarify their nature.
- Publication:
-
Publications of the Astronomical Society of Australia
- Pub Date:
- September 2021
- DOI:
- 10.1017/pasa.2021.36
- arXiv:
- arXiv:2107.09030
- Bibcode:
- 2021PASA...38...41F
- Keywords:
-
- radio continuum: galaxies;
- surveys;
- methods: data analysis;
- catalogues;
- galaxies: active;
- galaxies: starburst;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 28 pages, 18 figures, accepted for publication in PASA