GASS 3505: the prototype of HI-excess, passive galaxies
  We present our multiwavelength analysis of a prototype \HI-excess galaxy,
GASS 3505, selected based on having a large gas content ($M_{\rm HI} =
10^{9.9}$ \msun) compared to its little associated star formation activity
($\sim$0.1 \msun\ yr$^{-1}$) in the GALEX Arecibo SDSS Survey (GASS). Very
Large Array (VLA) observations show that the \HI\ in GASS 3505 is distributed
in a regularly rotating, extended ($\sim$50 kpc radius) gas ring. In the SDSS
optical image GASS 3505 appears as a bulge-dominated galaxy, however deep
optical imaging reveals low surface brightness ($\gtrsim25$ mag arcsec$^{-2}$)
stellar emission around the central bulge. Direct evidence for accretion is
detected in form of an extended ($\sim$60 kpc) stellar stream, showing that
GASS 3505 has experienced a minor merger in the recent past. We investigate the
possibility that the \HI\ ring in GASS 3505 was accreted in such a merger event
using N-body and smoothed particle hydrodynamic (SPH) simulations. The best
model that reproduces the general properties (i.e., gas distribution and
kinematics, stellar morphology) of the galaxy involves a merger between the
central bulge and a gas-rich ($M_{\star}$ = 10$^9$ \msun\ and $M_{\rm
HI}$/$M_{\star}$ = 10) disk galaxy. However, small discrepancies in the
observed and modeled 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
toward a larger program to investigate the physical mechanisms that drive the
large scatter in the gas scaling relations of nearby galaxies.
