Anomalous HI in NGC 2997

Abstract

We present deep H I observations of the moderately inclined spiral galaxy, NGC 2997. The goal of these observations was to search for H I clouds in the vicinity of NGC 2997 analogous to the high-velocity clouds of the Milky Way and gain insight into their origins. We find evidence for the presence of a galactic fountain as well as the accretion of intragalactic material, however we do not identify any large clouds of H I far from the disk of the galaxy. NGC 2997 has a thick, lagging H I disk that is modeled with a vertical velocity gradient of 18-31 km s⁻¹ kpc⁻¹. Anomalous velocity H I clouds with masses of order 10⁷ M_⊙, which cannot be explained by galactic fountain models allow us to estimate a lower limit to the accretion of extragalactic gas of 1.2 M_⊙ yr⁻¹. The number and mass of these clouds have implications for cosmological simulations of large-scale structure and the presence of dark matter halos. We have used values from the literature to estimate a star formation rate of 5 ± 1 M_⊙ yr⁻¹ and to derive a new distance to NGC 2997 of 12.2 ± 0.9 Mpc using published Tully-Fisher relations.