GASS 3505: The prototype of HI-excess, passive galaxies

We present our multiwavelength analysis of a prototype H I-excess galaxy, GASS 3505, selected based on having a large gas content (M_{H I} = 10^{9.9} M☉) compared to its little associated star formation activity (̃0.1 M☉ yr-1) in the GALEX Arecibo SDSS Survey (GASS). Very Large Array observations show that the H I in GASS 3505 is distributed in a regularly rotating, extended (̃50 kpc radius) gas ring. In the SDSS optical image GASS 3505 appears as a bulge-dominated galaxy, however deep optical imaging reveals low surface brightness (≳25 mag arcsec-2) stellar emission around the central bulge. Direct evidence for accretion is detected in form of an extended (̃60 kpc) stellar stream, showing that GASS 3505 has experienced a minor merger in the recent past. We investigate the possibility that the H I ring in GASS 3505 was accreted in such a merger event using N-body and smoothed particle hydrodynamic simulations. The best model that reproduces the general properties (i.e. gas distribution and kinematics, stellar morphology) of the galaxy involves a merger between the central bulge and a gas-rich (M⋆ = 109 M☉ and M_{H I}/M⋆ = 10) disc galaxy. However, small discrepancies in the observed and modelled properties could suggest that other sources of gas have to be involved in the build-up of the gas reservoir. This work is the first step towards a larger program to investigate the physical mechanisms that drive the large scatter in the gas scaling relations of nearby galaxies.