Origin of the Counter-Rotating Gas in NGC 1596

Abstract

We present Australia Telescope Compact Array (ATCA) HI imaging of the edge-on galaxy NGC 1596, which was recently found to have counter-rotating ionized gas in its center (<15"). We find a large HI envelope associated with a nearby companion, the dwarf irregular galaxy NGC 1602. The HI covers a region \~11.9'x13.4' (62x70 kpc^2) and the total HI mass detected is 2.5+/-0.1x10^9 Msun (assuming an 18 Mpc distance). The HI is centered on NGC 1602 but appears to have two tidal tails, one of which crosses over NGC 1596. The HI located at the position of NGC 1596 has a velocity gradient in the same sense as the ionized gas, i.e. opposite to the stellar rotation. Both the existence of a large gas reservoir and the velocity gradient of the HI and the ionized gas strongly suggest that the ionized gas in NGC 1596 originated from NGC 1602. From the length of the HI tails we conclude that the interaction started at least 1 Gyr ago, but the unsettled, asymmetric distribution of the ionized gas suggests that the accretion occured more recently. NGC 1596 thus provides a good example where the presence of counter-rotating gas can be directly linked to an accretion event. After the accretion has stopped or the merging is complete, NGC 1596 may evolve to a system with more extended counter-rotating gas but no obvious signature of interaction. There is a substantial local HI peak in one of the two tails, where we also find a faint stellar counterpart. The M_HI/L_B ratio in this region is too high for a normal dwarf elliptical or a low surface brightness galaxy, so we conclude that a tidal dwarf is currently forming there.