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Remnant radio galaxies discovered in a multi-frequency survey


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, Remnant radio galaxies discovered in a multi-frequency survey, Publications of the Astronomical Society of Australia, 38 Article e008. ISSN 1323-3580 (2021) [Refereed Article]

Copyright Statement

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

DOI: doi:10.1017/pasa.2020.49


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 < -1.2) and a diffuse, low surface brightness radio morphology. The predominant fraction of young remnants is consistent with a rapid fading during the remnant phase. Within our sample of radio galaxies, our observations constrain the remnant fraction to 4%  ≲ 𝑓rem ≲ 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 Details

Item Type:Refereed Article
Keywords:remnants, AGN, galaxies
Research Division:Physical Sciences
Research Group:Astronomical sciences
Research Field:Cosmology and extragalactic astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Turner, RJ (Dr Ross Turner)
UTAS Author:Shabala, SS (Associate Professor Stas Shabala)
UTAS Author:Rogers, J (Mr Jonathan Rogers)
ID Code:143069
Year Published:2021
Web of Science® Times Cited:16
Deposited By:Physics
Deposited On:2021-02-24
Last Modified:2021-09-06

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