High-resolution radio observations of TeV candidate sources

TL;DR

This study uses high-resolution radio observations to identify faint active galactic nuclei in optically passive galaxies, aiding the characterization of potential TeV gamma-ray sources for future CTA detections.

Contribution

First dual-frequency VLBI and e-MERLIN observations of faint TeV candidate sources reveal hidden AGN in optically passive galaxies.

Findings

Both sources host faint compact radio-emitting AGN

Radio spectra are steep, indicating non-thermal emission

Results help identify obscured AGN in TeV source candidates

Abstract

Radio-loud active galactic nuclei (AGN) with their jets pointed close to our line of sight constitute the majority of extragalactic $\gamma$-ray sources and significantly contribute to the radiation observed in the even higher energy regime. The upcoming Cherenkov Telescope Array (CTA) is expected to detect fainter TeV objects, leading to an anticipated increase in the proportion of non-blazar extragalactic high-energy sources. Here we present the results of our dual-frequency (1.7 and 5~GHz) European VLBI Network (EVN) and enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) observations of two faint radio sources from the list of TeV candidate sources. They do not show signs of nuclear activity in their optical spectra, but they were hypothesized to contain faint AGN that is outshone by the host galaxy. We used the mas-scale resolution radio data to try to pinpoint the location of the compact radio emitting feature, determine its spectral index, radio power, brightness temperature and radio-X-ray luminosity ratio and thus identify the origin of the radio emission. Our results suggest that both optically passive-looking galaxies host faint compact radio-emitting AGN with steep spectra.