Classical radio source propagating into outer HI disc in NGC 3801

TL;DR

This study presents detailed observations of a large-scale neutral hydrogen disc in NGC 3801, linking its properties to galaxy merger history and radio source evolution, providing insights into early phases of radio galaxy development.

Contribution

It provides the first detailed characterization of the HI disc in NGC 3801 and links its properties to galaxy evolution and radio source development.

Findings

The HI disc is 34 kpc in diameter and aligned with the radio jet.

NGC 3801's HI properties resemble those of low-power compact radio sources.

Evidence suggests a past gas-rich merger influencing current galaxy features.

Abstract

We present observations of a large-scale disc of neutral hydrogen (HI) in the nearby Fanaroff & Riley type-I radio galaxy NGC 3801 with the Westerbork Synthesis Radio Telescope. The HI disc (34 kpc in diameter and with M_HI = 1.3 x 10^9 M_sun) is aligned with the radio jet axis. This makes NGC 3801 an ideal system for investigating the evolution of a small radio source through its host galaxy's cold ISM. The large-scale HI disc is perpendicular to a known inner CO disc and dust-lane. We argue that the formation history of the large-scale HI disc is in agreement with earlier speculation that NGC 3801 was involved in a past gas-rich galaxy-galaxy merger (although other formation histories are discussed). The fact that NGC 3801 is located in an environment of several HI-rich companions, and shows indications of ongoing interaction with the nearby companion NGC 3802, strengthens this possibility. The large amounts of ambient cold ISM, combined with X-ray results by Croston et al (2007) on the presence of over-pressured radio jets and evidence for an obscuring torus, are properties that are generally not, or no longer, associated with more evolved FR-I radio sources. We do show, however, that the HI properties of NGC 3801 are comparable to those of a significant fraction of nearby low-power compact radio sources, suggesting that studies of NGC 3801 may reveal important insight into a more general phase in the evolution of at least a significant fraction of nearby radio galaxies.