Discovery of 178 Giant Radio Galaxies in 1059 deg2 of the Rapid ASKAP Continuum Survey at 888 MHz

Heinz Andernach,Eric F Jiménez-Andrade,Anthony G Willis

doi:10.3390/galaxies9040099

Heinz Andernach, Eric F Jiménez-Andrade + Show 1 more

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https://doi.org/10.3390/galaxies9040099

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Abstract

We report the results of a visual inspection of images of the Rapid ASKAP Continuum Survey (RACS) in search of extended radio galaxies (ERG) that reach or exceed linear sizes on the order of one Megaparsec. We searched a contiguous area of 1059 deg2 from RAJ = 20h20m to 06h20m, and −50∘<DecJ<−40∘, which is covered by deep multi-band optical images of the Dark Energy Survey (DES) and in which previously only three ERGs larger than 1 Mpc had been reported. For over 1800 radio galaxy candidates inspected, our search in optical and infrared images resulted in hosts for 1440 ERG, for which spectroscopic and photometric redshifts from various references were used to convert their largest angular size (LAS) to projected linear size (LLS). This resulted in 178 newly discovered giant radio sources (GRS) with LLS >1 Mpc, of which 18 exceed 2 Mpc and the largest one is 3.4 Mpc. Their redshifts range from 0.02 to ∼2.0, but only 10 of the 178 new GRS have spectroscopic redshifts. For the 146 host galaxies, the median r-band magnitude and redshift are 20.9 and 0.64, while for the 32 quasars or candidates these are 19.7 and 0.75. Merging the six most recent large compilations of GRS results in 458 GRS larger than 1 Mpc, so we were able to increase this number by ∼39% to 636.

Highlights

Giant radio galaxies (GRG) were first discovered in 1974 [1] and were originally defined as those for which their projected linear size (LLS) exceeds 1 Mpc, at a time when the Hubble constant was assumed to be H0 = 50 km s−1 Mpc−1
In 1983 [2] the first two giant radio quasars (GRQs) were found, [HB89] 1146–037 of LLS = 1.06 Mpc, at a redshift of z = 0.341, and [HB89] 1429 + 160 with 1.37 Mpc at z = 1.016, and these authors already raised the issue that inverse Compton losses suffered by the synchrotron-emitting electrons by scattering the cosmic microwave background (CMB) photons at z 1, and the denser Universe at that cosmic epoch, should not let these GRQs expand to the same extent as in the local Universe
About 30 of these were discarded for being likely separate sources, and 178 objects were accepted as new giant radio sources (GRS), of which we mark 14 as likely candidates in Table 2 due to uncertainties in either their largest angular size (LAS), zphot or their host ID

Summary

Introduction

Giant radio galaxies (GRG) were first discovered in 1974 [1] and were originally defined as those for which their projected linear size (LLS) exceeds 1 Mpc, at a time when the Hubble constant was assumed to be H0 = 50 km s−1 Mpc−1. The first GRG found (3C 236) originally assigned an LLS of 5.7 Mpc, is considered to extend over 4.1 Mpc. the first GRG found (3C 236) originally assigned an LLS of 5.7 Mpc, is considered to extend over 4.1 Mpc This change in the adopted value of H0 has led more recent authors to adopt an LLS of 0.7 Mpc as a lower limit for considering a radio galaxy a “giant” one. Many other GRQs have been found, and currently they constitute about 18% of all known giant radio sources (GRS); see Section 4.1. Here we use the terms GRS, GRG, and GRQ for sources with LLS > 1 Mpc

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