Transient radio emission from low-redshift galaxies at z<0.3 revealed by VLASS and FIRST surveys

TL;DR

This paper reports the discovery of 18 low-redshift galaxies with transient nuclear radio emission, likely caused by newly formed radio jets from sporadic black hole activity, revealed through VLASS and FIRST surveys.

Contribution

It presents the first identification of low-redshift galaxies with transient radio emission linked to sporadic black hole activity and new-born jets, using multi-epoch radio surveys.

Findings

18 galaxies show significant radio brightening between 1993-2019

Most galaxies are massive, with large black hole masses

Transient radio emission likely caused by new-born jets triggered by sporadic fueling

Abstract

We present the discovery of a sample of 18 low-redshift (z<0.3) galaxies with transient nuclear radio emission. These galaxies are not or weakly detected in the Faint Images of the Radio Sky at Twenty cm survey performed on 1993-2009, but have brightened significantly in the radio flux (by a factor of >5) in the epoch I (2017-2019) observations of Very Large Array Sky Survey (VLASS). All the 18 galaxies have been detected in the epoch II VLASS observations in 2020-2021, for which the radio flux is found to evolve slowly (by a factor of ~40%) over a period of about three years. 15 galaxies have been observed in the Rapid ASKAP Continuum Survey, and a flat or inverted spectral slope between 888 MHz and 3 GHz is found. Based on the Sloan Digital Sky Survey spectra taken before the radio brightening, 14 out of 18 can be classified to be LINERs or normal galaxies with weak or no nuclear activity. Most galaxies are red and massive, with more than half having central black hole masses above 10^8Msun. We find that only one galaxy in our sample displays optical flare lasting for at least two months, and a long decay in the infrared light curve that can be explained as the dust-heated echo emission of central optical flare, such as a stellar tidal disruption event. We discuss several possibilities for the transient radio emission and conclude that it is likely associated with a new-born radio jet triggered by short sporadic fueling of supermassive black hole. Such a scenario can be tested with further multi-frequency radio observations of these sources through measuring their radio flux variability and spectral evolution.