Origin of the Counter - Rotating Gas in NGC 1596 3

We present Australia Telescope Compact Array (ATCA) H I 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 H I envelope associated with a nearby companion, the dwarf irregular galaxy NGC 1602. The H I covers a region ≈ 11.9×13.4 (62×70 kpc) and the total H I mass detected is 2.5±0.1× 10 M⊙ (assuming an 18 Mpc distance). The H I is centered on NGC 1602 but appears to have two tidal tails, one of which crosses over NGC 1596. The H I 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 H I and the ionized gas strongly suggest that the ionized gas in NGC 1596 originated from NGC 1602. From the length of the H I 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 H I peak in one of the two tails, where we also find a faint stellar counterpart. The MHi/LB 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.