• White, Sarah V.
;
• Franzen, Thomas M. O.
;
• Riseley, Chris J.
;
• Wong, O. Ivy
;
• Kapińska, Anna D.
;
• Hurley-Walker, Natasha
;
• Callingham, Joseph R.
;
• Thorat, Kshitij
;
• Wu, Chen
;
• Hancock, Paul
;
• Hunstead, Richard W.
;
• Seymour, Nick
;
• Swan, Jesse
;
• Wayth, Randall
;
• Morgan, John
;
• Chhetri, Rajan
;
• Jackson, Carole
;
• Weston, Stuart
;
• Bell, Martin
;
• For, Bi-Qing
;
• Gaensler, B. M.
;
• Johnston-Hollitt, Melanie
;
• Offringa, André
;
• Staveley-Smith, Lister

Abstract

The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, $δ< 30^{\circ}$ ) at low radio frequencies, over the range 72-231MHz. These observations constitute the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, and we use the extragalactic catalogue (EGC) (Galactic latitude, $|b| >10^{\circ}$ ) to define the GLEAM 4-Jy (G4Jy) Sample. This is a complete sample of the `brightest' radio sources ( $S_{\textrm{151 MHz}}>4 Jy$ ), 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; $S_{\textrm{178 MHz}}>10.9 Jy$ ). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam ( $∼2$ arcmin at 200MHz), 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 $α=-0.740 ± 0.012$ between 151 and 843MHz, and $α=-0.786 ± 0.006$ between 151MHz and 1400MHz (assuming a power-law description, $S_{ν} ∝ ν^{α}$ ), compared to $α=-0.829 ± 0.006$ within the GLEAM band. Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6" resolution at 3.4 $μ$ m) 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 multi-wavelength data, and spectral curvature between 72MHz and 20GHz, will be presented in subsequent papers, and details for accessing all G4Jy overlays are provided at https://github.com/svw26/G4Jy.