Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission

TL;DR

This study used deep low-frequency radio observations with the GMRT to search for relic radio emissions and episodic activity in galaxies, finding such activity to be rare and most sources having steep spectra.

Contribution

It provides the first comprehensive low-frequency survey for relic radio emission and episodic activity, revealing their scarcity and characterizing the spectral properties of sources.

Findings

No unambiguous evidence of episodic activity was found.

Most sources have a median spectral index of about 0.8, steeper than expected.

14 sources exhibit very steep spectra with α ≥ 1.3.

Abstract

We present deep multifrequency observations using the Giant Metrewave Radio Telescope at 153, 244, 610 and 1260 MHz of a field centered on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as halos and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey, NRAO VLA Sky Survey and the Faint Images of the Radio Sky at Twenty-centimeters survey. Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index, $\alpha\sim$0.8 (S$(\nu)\propto\nu^{-\alpha}$), which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with $\alpha\geq$1.3. We examine their optical fields and discuss the nature of some of these sources.