NGC 5719/13: interacting spirals forming a counter-rotating stellar disc

Context: When a galaxy acquires material from the outside, it is likely that the resulting angular momentum of the accreted material is decoupled from that of the pre-existing galaxy. The presence of stars counter-rotating with respect to other stars and/or gas represents an extreme case of decoupling. Aims: NGC 5719, an almost edge-on Sab galaxy with a prominent skewed dust lane, shows a spectacular on-going interaction with its face-on Sbc companion NGC 5713. Observations of such interacting systems provide insight into the processes at work in assembling and reshaping galaxies. Methods: Studies were made of the distribution and kinematics of neutral hydrogen in the NGC 5719/13 galaxy pair and the ionised gas and stellar kinematics along the major axis of NGC 5719. Results: Two HI tidal bridges that loop around NGC 5719 and connect to NGC 5713, and two HI tidal tails departing westward from NGC 5713 were detected. There is a correspondence between the HI condensations and the location of clumps of young stars within and outside the disc of NGC 5719. The low-mass satellite PGC 135857 at the tip of the northern tail was detected in HI, and is likely a by-product of the interaction. The neutral and ionised hydrogen in the disc of NGC 5719 are counter-rotating with respect to the main stellar disc. The counter-rotating stellar disc contains about 20% of the stars in the system, and has the same radial extension as the main stellar disc. This is the first interacting system in which a counter-rotating stellar disc has been detected. Conclusions: The data support a scenario where HI from the large reservoir available in the galaxy's surroundings was accreted by NGC 5719 onto a retrograde orbit and subsequently fuelled the in situ formation of the counter-rotating stellar disc.