Star Formation in Partially Gas-Depleted Spiral Galaxies
  Broadband B and R and H-alpha images have been obtained with the 4.1-m SOAR
telescope atop Cerro Pachon, Chile for 29 spiral galaxies in the Pegasus I
galaxy cluster and for 18 spirals in non-cluster environments. Pegasus I is a
spiral-rich cluster with a low density intracluster medium and a low galaxy
velocity dispersion. When combined with neutral hydrogen (HI) data obtained
with the Arecibo 305-m radiotelescope, acquired by Levy et al. (2007) and by
Springob et al. (2005a), we study the star formation rates in disk galaxies as
a function of their HI deficiency. To quantify HI deficiency, we use the usual
logarithmic deficiency parameter, DEF. The specific star formation rate (SSFR)
is quantified by the logarithmic flux ratio of H-alpha flux to R band flux, and
thus roughly characterizes the logarithmic SFR per unit stellar mass. We find a
clear correlation between the global SFR per unit stellar mass and DEF, such
that the SFR is lower in more HI-deficient galaxies. This correlation appears
to extend from the most gas-rich to the most gas-poor galaxies. We also find a
correlation between the central SFR per unit mass relative to the global
values, in the sense that the more HI-deficient galaxies have a higher central
SFR per unit mass relative to their global SFR values than do gas-rich
galaxies. In fact, approximately half of the HI-depleted galaxies have highly
elevated SSFRs in their central regions, indicative of a transient evolutionary
state. In addition, we find a correlation between gas-depletion and the size of
the H-alpha disk (relative to the R band disk); HI-poor galaxies have truncated
disks. Moreover, aside from the elevated central SSFR in many gas-poor spirals,
the SSFR is otherwise lower in the H-alpha disks of gas-poor galaxies than in
gas-rich spirals.
