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Remnant radio galaxies discovered in a multi-frequency survey

Quici, B, Hurley-Walker, N, Seymour, N, Turner, RJ ORCID: 0000-0002-4376-5455, Shabala, SS ORCID: 0000-0001-5064-0493, Huynh, M, Andernach, H, Kapinska, AD, Collier, JD, Johnston-Hollitt, M, White, SV, Prandoni, I, Galvin, TJ, Franzen, T, Ishwara-Chandra, CH, Bellstedt, S, Tingay, SJ, Gaensler, BM, O'Brien, A, Rogers, J, Chow, K, Driver, S and Robotham, A 2021 , 'Remnant radio galaxies discovered in a multi-frequency survey' , Publications of the Astronomical Society of Australia, vol. 38 , pp. 1-21 , doi: 10.1017/pasa.2020.49.

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Abstract

The remnant phase of a radio galaxy begins when the jets launched from an active galactic nucleus are switched off. To study the fraction of radio galaxies in a remnant phase, we take advantage of a 8.31 deg2 subregion of the GAMA 23 field which comprises of surveys covering the frequency range 0.1-9 GHz. We present a sample of 104 radio galaxies compiled from observations conducted by the Murchison Widefield Array (216 MHz), the Australia Square Kilometer Array Pathfinder (887 MHz), and the Australia Telescope Compact Array (5.5 GHz). We adopt an ‘absent radio core’ criterion to identify 10 radio galaxies showing no evidence for an active nucleus. We classify these as new candidate remnant radio galaxies. Seven of these objects still display compact emitting regions within the lobes at 5.5 GHz; at this frequency the emission is short-lived, implying a recent jet switch off. On the other hand, only three show evidence of aged lobe plasma by the presence of an ultra-steep-spectrum (α rem ≲ 10% the lower limit comes from the limiting case in which all remnant candidates with hotspots are simply active radio galaxies with faint, undetected radio cores. Finally, we model the synchrotron spectrum arising from a hotspot to show they can persist for 5-10 Myr at 5.5 GHz after the jets switch off - radio emission arising from such hotspots can therefore be expected in an appreciable fraction of genuine remnants.

Item Type: Article
Authors/Creators:Quici, B and Hurley-Walker, N and Seymour, N and Turner, RJ and Shabala, SS and Huynh, M and Andernach, H and Kapinska, AD and Collier, JD and Johnston-Hollitt, M and White, SV and Prandoni, I and Galvin, TJ and Franzen, T and Ishwara-Chandra, CH and Bellstedt, S and Tingay, SJ and Gaensler, BM and O'Brien, A and Rogers, J and Chow, K and Driver, S and Robotham, A
Keywords: remnants, AGN, galaxies
Journal or Publication Title: Publications of the Astronomical Society of Australia
Publisher: CSIRO Publishing
ISSN: 1323-3580
DOI / ID Number: 10.1017/pasa.2020.49
Copyright Information:

Copyright The Author(s), 2021. Published by Cambridge University Press on behalf of the Astronomical Society of Australia

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