HI absorption in 3C 49 and 3C 268.3. Probing the environment of Compact Steep Spectrum and GHz Peaked Spectrum sources

TL;DR

This study uses VLBI observations to localize and analyze HI absorption in two compact steep spectrum galaxies, revealing interactions between radio jets and dense gas clouds that influence their asymmetrical appearance.

Contribution

It provides the first localization of HI absorption in 3C 49 and 3C 268.3 and links this absorption to emission line gas, highlighting environmental interactions.

Findings

HI absorption detected in specific lobes of both galaxies.

Absorbing lobes are brighter, closer to the core, and depolarized.

HI absorption is associated with optical emission line gas.

Abstract

We present and discuss European VLBI Network UHF band spectral line observations, made to localise the redshifted 21cm HI absorption known to occur in the subgalactic sized compact steep spectrum galaxies 3C 49 and 3C 268.3. We have detected HI absorption towards the western radio lobe of 3C 49 and the northern lobe of 3C 268.3. However, we cannot rule out the presence of similar amounts of HI towards the opposite and much fainter lobes. The radio lobes with detected HI absorption (1) are brighter and closer to the core than the opposite lobes; (2) are depolarized; and (3) are associated with optical emission line gas. The association between the HI absorption and the emission line gas, supports the hypothesis that the HI absorption is produced in the atomic cores of the emission line clouds. Our results are consistent with a picture in which compact steep spectrum sources interact with clouds of dense gas as they propagate through their host galaxy. We suggest that the asymmetries in the radio and optical emission can be due to interaction of a two sided radio source with an asymmetric distribution of dense clouds in their environment.