A low HI column density filament in NGC 2403: signature of interaction or accretion

TL;DR

Deep HI observations of NGC 2403 reveal a low-density gas cloud likely resulting from intergalactic accretion or minor galaxy interaction, providing insights into galaxy gas supply mechanisms.

Contribution

First detection of a low HI column density cloud at 16 kpc from NGC 2403's center, supporting models of cold gas accretion or minor interactions.

Findings

Detected a 6.3 x 10^6 Msun HI cloud outside the main disk.

Cloud is associated with an anomalous-velocity HI filament.

Supports the presence of low-density cold flows or minor interactions.

Abstract

Observed HI accretion around nearby galaxies can only account for a fraction of the gas supply needed to sustain the currently observed star formation rates. It is possible that additional accretion happens in the form of low column density cold flows, as predicted by numerical simulations of galaxy formation. To contrain the presence and properties of such flows, we present deep HI observations obtained with the NRAO Green Bank Telescope of an area measuring 4 by 4 degrees around NGC 2403. These observations, with a 5 sigma detection limit of 2.4 x 10^18 cm^-2 over a 20 km/s linewidth, reveal the presence of a low-column density, extended cloud outside the main HI disk, about 17' (~16 kpc or ~2R25) to the NW of the center of the galaxy. The total HI mass of the cloud is 6.3 x 10^6 Msun, or 0.15 percent of the total HI mass of NGC 2403. The cloud is associated with an 8-kpc anomalous-velocity HI filament in the inner disk, previously observed in deep VLA observations by Fraternali et al. (2001, 2002). We discuss several scenarios for the origin of the cloud, and conclude that it is either accreting from the intergalactic medium, or is the result of a minor interaction with a neigbouring dwarf galaxy.