Discovery of a Damped Lyα System in a Low-z Galaxy Group: Possible Evidence for Gas Inflow and Nuclear Star Formation

We present a low-redshift (z = 0.029) damped Lyα (DLA) system in the spectrum of a background quasi-stellar object (QSO). The DLA is associated with an interacting galaxy pair within a galaxy group. We detected weak Lyα emission centered at the absorption trough of the DLA. The emission was likely tracing the neutral H i reservoir around the galaxies in the interacting pair, which scattered the Lyα generated by star formation within those galaxies. We also found that the interacting pair is enveloped by a large H i cloud with . We discovered blueshifted 21 cm H i emission, corresponding to M(H i) = , associated with J151225.15+012950.4 - one of the galaxies in the interacting pair. The blueshifted H i was tracing gas flowing into the galaxy from behind and toward us. Gas at similar blueshifted velocities was seen in the QSO sight line, thus suggesting the presence of a filamentary structure of the order of 100 kpc feeding the galaxy. We estimated a mass inflow rate of into the galaxy, which matches the star formation rate estimated from Hα emission. It is likely that the inflow of enormous amounts of gas has triggered star formation in this galaxy. The sudden acquisition of cold gas may lead to a starburst in this galaxy like those commonly seen in simulations.