The GLEAM 4-Jy (G4Jy) Sample: I. Definition and the catalogue

The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, d < 30°) at low radio frequencies, over the range 72 231 MHz. These observations constitute the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we use the extragalactic catalogue (EGC) (Galactic latitude, |b| > 10°) to define the GLEAM 4-Jy (G4Jy) Sample. This is a complete sample of the 'brightest' radio sources (S151 MHz > 4 Jy), the majority of which are active galactic nuclei with powerful radio jets. Crucially, low-frequency observations allow the selection of such sources in an orientation-independent way (i.e. minimising the bias caused by Doppler boosting, inherent in high-frequency surveys). We then use higher-resolution radio images, and information at other wavelengths, to morphologically classify the brightest components in GLEAM. We also conduct cross-checks against the literature and perform internal matching, in order to improve sample completeness (which is estimated to be > 95.5%). This results in a catalogue of 1863 sources, making the G4Jy Sample over 10 times larger than that of the revised Third Cambridge Catalogue of Radio Sources (3CRR; S178 MHz > 10.9 Jy). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam (∼ 2 arcmin at 200 MHz), and we label 67% of the sample as 'single', 26% as 'double', 4% as 'triple', and 3% as having 'complex' morphology at ∼ 1 GHz (45 arcsec resolution). We characterise the spectral behaviour of these objects in the radio and find that the median spectral index is a = -0.740 ± 0.012 between 151 and 843 MHz, and a = -0.786 ± 0.006 between 151 MHz and 1400 MHz (assuming a power-law description, S°a), compared to a = -0.829 ± 0.006 within the GLEAM band. Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6 resolution at 3.4μm) for the radio emission, as identified through visual inspection and thorough checks against the literature. As such, the G4Jy Sample can be used as a reliable training set for cross-identification via machine-learning algorithms. We also estimate the angular size of the sources, based on their associated components at ∼ 1 GHz, and perform a flux density comparison for 67 G4Jy sources that overlap with 3CRR. Analysis of multiwavelength data, and spectral curvature between 72 MHz and 20 GHz, will be presented in subsequent papers, and details for accessing all G4Jy overlays are provided at https://github.com/svw26/G4Jy.