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

© 2016 The Authors.We present our multiwavelength analysis of a prototype HI-excess galaxy, GASS 3505, selected based on having a large gas content (MHI = 109.9 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 HI 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 HI 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 MHI/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.