From HI to Stars: HI Depletion in Starbursts and Star-Forming Galaxies
  in the ALFALFA H-alpha Survey
  HI in galaxies traces the fuel for future star formation and reveals the
effects of feedback on neutral gas. Using a statistically uniform, HI-selected
sample of 565 galaxies from the ALFALFA H-alpha survey, we explore HI
properties as a function of star formation activity. ALFALFA H-alpha provides
R-band and H-alpha imaging for a volume-limited subset of the 21-cm ALFALFA
survey. We identify eight starbursts based on H-alpha equivalent width and six
with enhanced star formation relative to the main sequence. Both starbursts and
non-starbursts have similar HI to stellar mass ratios (MHI/M*), which suggests
that feedback is not depleting the starbursts' HI. Consequently, the starbursts
do have shorter HI depletion times (t_dep), implying more efficient HI-to-H2
conversion. While major mergers likely drive this enhanced efficiency in some
starbursts, the lowest mass starbursts may experience periodic bursts,
consistent with enhanced scatter in t_dep at low M*. Two starbursts appear to
be pre-coalescence mergers; their elevated MHI/M* suggest that HI-to-H2
conversion is still ongoing at this stage. By comparing with the GASS sample,
we find that t_dep anti-correlates with stellar surface density for disks,
while spheroids show no such trend. Among early-type galaxies, t_dep does not
correlate with bulge-to-disk ratio; instead, the gas distribution may determine
the star formation efficiency. Finally, the weak connection between galaxies'
specific star formation rates and MHI/M* contrasts with the well-known
correlation between MHI/M* and color. We show that dust extinction can explain
the HI-color trend, which may arise from the relationship between M*, MHI, and
metallicity.
