Sarah White

We present the GLEAM 4-Jy (G4Jy) Sample, which is a complete sample of the brightest, extragalactic radio-sources in the southern sky, making them ideal for detailed active-galaxy studies in the era of the Square Kilometre Array (the world’s next biggest radio telescope). The majority of these sources have powerful radio jets, and the radio emission that results can extend over large distances, far from the supermassive black hole at the centre of the host galaxy. The fact that these sources are selected at low radio-frequencies means that they are not affected by relativistic beaming, and therefore are without an orientation bias (unlike samples of active galaxies that are selected at higher radio-frequencies).

Earlier this year (2020), we published two papers. The first (https://arxiv.org/abs/2004.13125) defines the G4Jy Sample and describes the associated catalogue, whilst the second (https://arxiv.org/abs/2004.13025) details the host-galaxy identification for individual sources. This involved repeated visual inspection using multiple radio surveys – GLEAM (170-231 MHz), NVSS (1400 MHz), SUMSS (843 MHz), TGSS (150 MHz), and AT20G (20 GHz) – as well as optical positions from 6dFGS and mid-infrared data from AllWISE. Given the complex morphology of these radio sources, and the much higher density of candidate host-galaxies in the mid-infrared (relative to the density of the radio sources), this was followed by thorough literature checks during cross-identification. (As the two papers describe work that was completed in conjunction with one another, they should be cited together.)

The result is a catalogue of 1,863 sources that each have an integrated flux-density greater than 4 Jy at 151 MHz. For reference, the best-studied, low-frequency, radio-source sample – the revised Third Cambridge Catalogue of Radio Sources (3CRR; Laing et al., 1983) – contains 173 active galaxies with an integrated flux-density greater than 10.9 Jy at 178 MHz. By inspecting a histogram of the flux-density distribution for the G4Jy sources, and using a vertical line to indicate the flux-density limit that corresponds to 3CRR sources, it becomes apparent that the larger sample size and wider range in flux densities exhibited by the G4Jy Sample will allow the properties of these radio galaxies to be studied in greater detail (as a function of both intrinsic radio-power and redshift).

Within the catalogue (see https://github.com/svw26/G4Jy), we provide a morphology classification (‘single’, ‘double’, ‘triple’, ‘complex’) that is mostly based upon 45” resolution radio-images at ～1 GHz. We also provide total flux-densities at multiple frequencies, four sets of spectral indices, and mid-infrared identifications for 86% of the sample (1,606 sources). We flag 129 sources as having ambiguous hosts, including four sources where we question the existing identification. Each of these have been followed up using MeerKAT (operated by the South African Radio Astronomy Observatory), which is the precursor telescope for the mid-frequency component of the Square Kilometre Array. [The PI, Sarah White (sarahwhite.astro@gmail.com, Rhodes University), is the lead author of the G4Jy papers, with three quarters of this work being completed whilst at Curtin University in Australia.] Furthermore, we flag 126 radio sources as having a host galaxy that is faint or absent from the AllWISE catalogue, either due to being at high redshift or affected by nearby mid-infrared emission.

To summarise, and whet the appetite for future work, note that the G4Jy Sample includes: at least 8 giant radio-galaxies ( > 1 Mpc), 14 S-/Z-/X-shaped radio sources (one of which is shown to illustrate the radio emission following a snake-like ‘S’ shape in morphology), 23 bent-tail radio-galaxies, 18 head-tail radio-galaxies, two nearby star-forming galaxies, the Flame Nebula, a cluster relic and a halo.