Classical radio source propagating into outer HI disc in NGC 3801

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.

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