Cold gas & mergers: fundamental difference in HI properties of different types of radio galaxies?

TL;DR

This study reveals that different types of radio galaxies exhibit distinct large-scale neutral hydrogen properties, suggesting varied formation histories and triggering mechanisms, with implications for understanding galaxy evolution.

Contribution

It provides new observational evidence linking HI properties with radio galaxy types and their formation processes, highlighting the role of galaxy mergers and accretion modes.

Findings

Low power compact sources often have large HI disks/rings.

FR-I galaxies generally lack large-scale HI detection.

FR-II galaxies frequently contain large HI structures indicative of mergers.

Abstract

We present results of a study of large-scale neutral hydrogen (HI) gas in nearby radio galaxies. We find that the early-type host galaxies of different types of radio sources (compact, FR-I and FR-II) appear to contain fundamentally different large-scale HI properties: enormous regular rotating disks and rings are present around the host galaxies of a significant fraction of low power compact radio sources, while no large-scale HI is detected in low power, edge-darkened FR-I radio galaxies. Preliminary results of a study of nearby powerful, edge-brightened FR-II radio galaxies show that these systems generally contain significant amounts of large-scale HI, often distributed in tail- or bridge-like structures, indicative of a recent galaxy merger or collision. Our results suggest that different types of radio galaxies may have a different formation history, which could be related to a difference in the triggering mechanism of the radio source. If confirmed by larger studies with the next generation radio telescopes, this would be in agreement with previous optical studies that suggest that powerful FR-II radio sources are likely triggered by galaxy mergers and collisions, while the lower power FR-I sources are fed in other ways (e.g. through the accretion of hot IGM). The giant HI disks/rings associated with some compact sources could - at least in some cases - be the relics of much more advanced mergers.