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تسجيل مستخدم جديدThe MWA GLEAM 4-Jy (G4Jy) Sample
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Powerful radio-galaxies feature heavily in our understanding of galaxy evolution. However, when it comes to studying their properties as a function of redshift and/or environment, the most-detailed studies tend to be limited by small-number statistics. During Focus Meeting 3, on Radio Galaxies: Resolving the AGN phenomenon, SVW presented a new sample of nearly 2,000 of the brightest radio-sources in the southern sky (Dec. $<$ 30 deg). These were observed at low radio-frequencies as part of the GaLactic and Extragalactic All-sky MWA (GLEAM) Survey, which is a continuum survey conducted using the Murchison Widefield Array (MWA). This instrument is the precursor telescope for the low-frequency component of the Square Kilometre Array, and allows us to select radio galaxies in an orientation-independent way (i.e. minimising the bias caused by Doppler boosting, inherent in high-frequency surveys). Being brighter than 4 Jy at 151 MHz, we refer to these objects as the GLEAM 4-Jy (G4Jy) Sample. The G4Jy catalogue is close to being finalised, with SVW describing how multi-wavelength data have been used to determine the morphology of the radio emission, and identify the host galaxy. In addition, the MWAs excellent spectral-coverage and sensitivity to extended/diffuse emission were highlighted. Both of these aspects are important for understanding the physical mechanisms that take place within active galaxies, and how they interact with their environment.
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The Murchison Widefield Array (MWA) has observed the entire southern sky (Declination, $delta <$ 30 deg) 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 (Galactic latitude, $|b| >$ 10 deg) to define the GLEAM 4-Jy (G4Jy) Sample. This is a complete sample of the brightest radio-sources ($S_{mathrm{151MHz}} >$ 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 1,863 sources, making the G4Jy Sample over 10 times larger than that of the revised Third Cambridge Catalogue of Radio Sources (3CRR; $S_{mathrm{178MHz}} >$ 10.9 Jy). Of these G4Jy sources, 78 are resolved by the MWA (Phase-I) synthesised beam ($sim$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 $sim$1 GHz (45-arcsec resolution). Alongside this, our value-added catalogue provides mid-infrared source associations (subject to 6-arcsec resolution at 3.4 micron) 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. [Abstract abridged for arXiv submission.]
The entire southern sky (Declination, $delta <$ 30 deg) has been observed using the Murchison Widefield Array (MWA), which provides radio imaging of $sim$2-arcmin resolution at low frequencies (72-231 MHz). This is the GaLactic and Extragalactic All-
sky MWA (GLEAM) Survey, and we have previously used a combination of visual inspection, cross-checks against the literature, and internal matching to identify the brightest radio-sources ($S_{mathrm{151MHz}} >$ 4 Jy) in the extragalactic catalogue (Galactic latitude, $|b| >$ 10 deg). We refer to these 1,863 sources as the GLEAM 4-Jy (G4Jy) Sample, and use radio images (of $leq$ 45-arcsec resolution), and multi-wavelength information, to assess their morphology and identify the galaxy that is hosting the radio emission (where appropriate). Details of how to access all of the overlays used for this work are available at https://github.com/svw26/G4Jy. Alongside this we conduct further checks against the literature, which we document in this paper for individual sources. Whilst the vast majority of the G4Jy Sample are active galactic nuclei with powerful radio-jets, we highlight that it also contains a nebula, two nearby, star-forming galaxies, a cluster relic, and a cluster halo. There are also three extended sources for which we are unable to infer the mechanism that gives rise to the low-frequency emission. In the G4Jy catalogue we provide mid-infrared identifications for 86% of the sources, and flag the remainder as: having an uncertain identification (129 sources), having a faint/uncharacterised mid-infrared host (126 sources), or it being inappropriate to specify a host (2 sources). For the subset of 129 sources, there is ambiguity concerning candidate host-galaxies, and this includes four sources (B0424$-$728, B0703$-$451, 3C 198, and 3C 403.1) where we question the existing identification.
Synchrotron emission pervades the Galactic plane at low radio frequencies, originating from cosmic ray electrons interacting with the Galactic magnetic field. Using a low-frequency radio telescope, the Murchison Widefield Array (MWA), we measure the
free-free absorption of this Galactic synchrotron emission by intervening HII regions along the line of sight. These absorption measurements allow us to calculate the Galactic cosmic-ray electron emissivity behind and in front of 47 detected HII regions in the region $250^circ < l < 355^circ$, $|b| < 2^circ$. We find that all average emissivities between the HII regions and the Galactic edge along the line of sight ($epsilon_b$) are in the range of 0.24$,,sim,,$0.70$,,$K$,,$pc$^{-1}$ with a mean of 0.40$,,$K$,,$pc$^{-1}$ and a variance of 0.10$,,$K$,,$pc$^{-1}$ at 88$,,$MHz. Our best model, the Two-circle model, divides the Galactic disk into three regions using two circles centring on the Galactic centre. It shows a high emissivity region near the Galactic centre, a low emissivity region near the Galactic edge, and a medium emissivity region between these two regions, contrary to the trend found by previous studies.
The GaLactic and Extragalactic All-sky MWA survey (GLEAM) is a new relatively low resolution, contiguous 72-231 MHz survey of the entire sky south of declination +25 deg. In this paper, we outline one approach to determine the relative contribution o
f system noise, classical confusion and sidelobe confusion in GLEAM images. An understanding of the noise and confusion properties of GLEAM is essential if we are to fully exploit GLEAM data and improve the design of future low-frequency surveys. Our early results indicate that sidelobe confusion dominates over the entire frequency range, implying that enhancements in data processing have the potential to further reduce the noise.
We investigate the linear radio size properties of the $mu$Jy populations of radio-selected active galactic nuclei (AGN) and star-forming galaxies (SFGs) using a multi-resolution catalog based on the original VLA-COSMOS 3,GHz 0farcs75 resolution mosa
ic and its convolved images (up to a resolution of 2farcs2). The final catalog contains 6,399 radio sources above a 3,GHz total flux density of $S_T>20$ $mu$Jy (median $<S_T>=37$ $mu$Jy), with redshift information (median $<z>=1.0$), and multi-wavelength classification as SFGs, radio-excess AGN (RX-AGN), or non-radio-excess AGN (NRX-AGN). RX-AGN are those whose radio emission exceeds the star formation rate derived by fitting the global spectral energy distribution. We derive the evolution with redshift and luminosity of the median linear sizes of each class of objects. We find that RX-AGN are compact, with median sizes of $sim$ 1-2 kpc and increasing with redshift, corresponding to an almost constant angular size of 0farcs25. NRX-AGN typically have radio sizes a factor of 2 larger than the RX-AGN. The median radio size of SFGs is about 5 kpc up to $zsim 0.7$, and it decreases beyond this redshift. Using luminosity-complete subsamples of objects, we separately investigate the effect of redshift and luminosity dependance. We compare the radio sizes of SFGs with those derived in the rest-frame far-infrared (FIR) and UV bands. We find that SFGs have comparable sizes (within 15%) in the radio and rest-frame FIR, while the sizes measured in the UV-band are systematically larger than the radio sizes.
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