Source counts and confusion at 72–231 MHz in the MWA GLEAM survey

Franzen, T. M. O.; Vernstrom, Tessa; Jackson, C. A.; Hurley‐Walker, N.; Ekers, R. D.; Heald, G.; Seymour, N.; White, Sarah V.

doi:10.1017/pasa.2018.52

Cited by 40 publications

(42 citation statements)

References 63 publications

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“…Additional GLEAM components enter the sample by association (Section 5.2), and we also search for sources that are brighter than 4 Jy but have been missed from this initial selection (Section 7). Given how the GLEAM source counts vary with flux density (Franzen et al 2019), and that visual inspection and cross-checks are very time consuming, it is currently infeasible to extend this work to a flux density limit lower than S 151 MHz = 4 Jy. Meanwhile, concerning very bright radio sources, the following sub-section lists those that are known to be absent from the GLEAM catalogue in the first instance.…”

Section: Initial Sample Definitionmentioning

confidence: 99%

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

White

Franzen

Riseley

et al. 2020

Publ. Astron. Soc. Aust.

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The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, $\delta< 30^{\circ}$ ) 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 ( $S_{\textrm{151\,MHz}}>4\,\text{Jy}$ ), 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; $S_{\textrm{178\,MHz}}>10.9\,\text{Jy}$ ). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam ( $\sim2$ 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 $\sim1\,\text{GHz}$ (45 arcsec resolution). We characterise the spectral behaviour of these objects in the radio and find that the median spectral index is $\alpha=-0.740 \pm 0.012$ between 151 and 843MHz, and $\alpha=-0.786 \pm 0.006$ between 151MHz and 1400MHz (assuming a power-law description, $S_{\nu} \propto \nu^{\alpha}$ ), compared to $\alpha=-0.829 \pm 0.006$ within the GLEAM band. Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6” resolution at 3.4 $\mu$ 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 $\sim1\,\text{GHz}$ , 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.

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Section: Initial Sample Definitionmentioning

confidence: 99%

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

White

Franzen

Riseley

et al. 2020

Publ. Astron. Soc. Aust.

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“…However, given that GLEAM Exgal is limited by classical confusion in the lowest frequency band, no significant improvement in the sensitivity is expected from combining the GLEAM year 1 and 2 data. The rms noise achieved in the lowest frequency band of GLEAM Exgal is ≈40 mJy beam -1 , while Franzen et al (2019) estimate the classical confusion noise to be 30 mJy beam -1 . In the final source catalogue, we include flux densities extracted from the GLEAM Exgal mosaics below 100 MHz, as explained in Section 4.…”

Section: Data Reductionmentioning

confidence: 88%

“…The sensitivity in GLEAM Exgal is limited by sidelobe confusion, i.e. noise introduced into the image due to the combined sidelobes of undeconvolved sources (Franzen et al 2019), while the flux density calibration is limited by errors in the primary beam model of order 5-20%.…”

Section: Introductionmentioning

confidence: 99%

“…The extremely high surface brightness sensitivity of GLEAM Exgal has made possible the detection and characterisation of large, faint objects such as dying radio galaxies (Duchesne & Johnston-Hollitt 2019), and radio haloes and relics associated with galaxy clusters (Schellenberger et al 2017). Franzen et al (2019) used the GLEAM Exgal catalogue to derive low-frequency source counts above ∼ 100 mJy to high precision, allowing tight constraints on bright radio source population models. More recently, White et al (2020a,b) constructed a complete sample of the 'brightest' radio sources (S 151 MHz > 4 Jy) south of declination +30 • from the GLEAM Exgal catalogue, the majority of which are AGN with powerful radio jets; the GLEAM 4-Jy sample is an order of magnitude larger than the 3CRR sample by Laing, Riley, & Longair (1983) and will be a benchmark for the bright radio galaxy population.…”

Section: Introductionmentioning

confidence: 99%

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GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey III: South Galactic Pole data release

Franzen

Hurley‐Walker

White

et al. 2021

Publ. Astron. Soc. Aust.

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We present the South Galactic Pole (SGP) data release from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. These data combine both years of GLEAM observations at 72–231 MHz conducted with the Murchison Widefield Array (MWA) and cover an area of 5 113 $\mathrm{deg}^{2}$ centred on the SGP at $20^{\mathrm{h}} 40^{\mathrm{m}} < \mathrm{RA} < 05^{\mathrm{h}} 04^{\mathrm{m}}$ and $-48^{\circ} < \mathrm{Dec} < -2^{\circ} $ . At 216 MHz, the typical rms noise is ${\approx}5$ mJy beam–1 and the angular resolution ${\approx}2$ arcmin. The source catalogue contains a total of 108 851 components above $5\sigma$ , of which 77% have measured spectral indices between 72 and 231 MHz. Improvements to the data reduction in this release include the use of the GLEAM Extragalactic catalogue as a sky model to calibrate the data, a more efficient and automated algorithm to deconvolve the snapshot images, and a more accurate primary beam model to correct the flux scale. This data release enables more sensitive large-scale studies of extragalactic source populations as well as spectral variability studies on a one-year timescale.

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“…A widely used method (Franzen et al 2015(Franzen et al , 2019 to identify nonpoint-like sources in the catalogue is based on the ratio S/S peak between the integrated and peak flux density which is expected to be larger than 1 for extended sources. In the XXL survey by Butler et al (2018), sources were classified as resolved if they fulfil the following empirical relation:…”

Section: Estimation Of the Fraction Of Resolved Sourcesmentioning

confidence: 99%

Evolutionary map of the Universe (EMU): Compact radio sources in the scorpio field towards the galactic plane

Riggi

Umana

Trigilio

et al. 2021

Monthly Notices of the Royal Astronomical Society

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We present observations of a region of the Galactic plane taken during the Early Science Program of the Australian Square Kilometre Array Pathfinder (ASKAP). In this context, we observed the scorpio field at 912 MHz with an uncompleted array consisting of 15 commissioned antennas. The resulting map covers a square region of ∼40 deg2, centred on (l, b) = (343.5°, 0.75°), with a synthesized beam of 24 × 21 arcsec2 and a background rms noise of 150–200 μJy beam−1, increasing to 500–600 μJy beam−1 close to the Galactic plane. A total of 3963 radio sources were detected and characterized in the field using the caesar source finder. We obtained differential source counts in agreement with previously published data after correction for source extraction and characterization uncertainties, estimated from simulated data. The ASKAP positional and flux density scale accuracy were also investigated through comparison with previous surveys (MGPS, NVSS) and additional observations of the scorpio field, carried out with ATCA at 2.1 GHz and 10 arcsec spatial resolution. These allowed us to obtain a measurement of the spectral index for a subset of the catalogued sources and an estimated fraction of (at least) 8 per cent of resolved sources in the reported catalogue. We cross-matched our catalogued sources with different astronomical data bases to search for possible counterparts, finding ∼150 associations to known Galactic objects. Finally, we explored a multiparametric approach for classifying previously unreported Galactic sources based on their radio-infrared colours.

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Source counts and confusion at 72–231 MHz in the MWA GLEAM survey

Cited by 40 publications

References 63 publications

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

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

GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey III: South Galactic Pole data release

Evolutionary map of the Universe (EMU): Compact radio sources in the scorpio field towards the galactic plane

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