HI 21-cm absorption in radio-loud AGN with double-peaked [OIII] emission

TL;DR

This study investigates neutral hydrogen gas in galaxy centers with double-peaked emission lines, using radio observations to understand processes like mergers and outflows in galaxy evolution.

Contribution

First pilot survey of HI 21-cm absorption in radio-loud AGN with double-peaked [OIII] lines, revealing higher detection rates in disturbed systems.

Findings

HI 21-cm absorption detected in 3 out of 8 sources.

Higher absorption detection rate in interacting or disturbed systems.

Tentative correlation between galaxy disturbance and HI absorption presence.

Abstract

Different physical processes in galaxy evolution, such as galaxy mergers that lead to coalescence of dual Active Galactic Nuclei (AGN) and outflows emanating from the narrow line region, can leave their imprint on the optical spectra of AGN in the form of double-peaked narrow emission lines. To investigate the neutral gas in the centres of such AGN, we have conducted a pilot survey of HI 21-cm absorption, using the upgraded Giant Metrewave Radio Telescope (uGMRT), in radio-loud AGN whose optical spectra show double-peaked [OIII] emission lines at z<=0.4 (median z~0.14). Among the eight sources for which we could obtain clean spectra, we detect HI 21-cm absorption in three sources (detection rate of 38(+36/-20)%) and find tentative indication of absorption in two other sources. The detection rate of HI 21-cm absorption is tentatively higher for the systems that show signatures of interaction or tidal disturbance (>~50%) in the ground-based optical images than that for the systems that appear single and undisturbed (~25%). This is consistent with the high incidence of HI 21-cm absorption observed in z<=0.2 galaxy mergers. Higher spatial resolution spectroscopy is required to confirm the origin of the HI absorbing gas, i.e. either gas infalling onto the radio-loud AGN, outflowing gas ejected by the AGN, or gas in rotation on the galactic-scale or circumnuclear discs.