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

Abstract

We present a low-redshift ( z  = 0.029) damped Ly α (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 Ly α 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 Ly α generated by star formation within those galaxies. We also found that the interacting pair is enveloped by a large H i cloud with M ( H I ) = 2 × 10 10 M ⊙ . We discovered blueshifted 21 cm H i emission, corresponding to M (H i ) =  2 × 10 9 M ⊙ , 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 2 M ⊙ yr − 1 into the galaxy, which matches the star formation rate estimated from H α 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.