Gas Mass Fractions and Star Formation in Blue-Sequence E/S0 Galaxies
  Recent work has identified a population of low-redshift E/S0 galaxies that
lie on the blue sequence in color vs. stellar mass parameter space, where
spiral galaxies typically reside. While high-mass blue-sequence E/S0s often
resemble young merger or interaction remnants likely to fade to the red
sequence, we focus on blue-sequence E/S0s with lower stellar masses (< a few
10^10 M_sun), which are characterized by fairly regular morphologies and
low-density field environments where fresh gas infall is possible. This
population may provide an evolutionary link between early-type galaxies and
spirals through disk regrowth. Focusing on atomic gas reservoirs, we present
new GBT HI data for 27 E/S0s on both sequences as well as a complete tabulation
of archival HI data for other galaxies in the Nearby Field Galaxy Survey.
Normalized to stellar mass, the atomic gas masses for 12 of the 14
blue-sequence E/S0s range from 0.1 to >1.0. These gas-to-stellar mass ratios
are comparable to those of spiral and irregular galaxies and have a similar
dependence on stellar mass. Assuming that the HI is accessible for star
formation, we find that many of our blue-sequence E/S0s can increase in stellar
mass by 10-60% in 3 Gyr in both of two limiting scenarios, exponentially
declining star formation and constant star formation. In a constant star
formation scenario, about half of the blue-sequence E/S0s require fresh gas
infall on a timescale of <3 Gyr to avoid exhausting their atomic gas reservoirs
and evolving to the red sequence. We present evidence that star formation in
these galaxies is bursty and likely involves externally triggered gas inflows.
Our analysis suggests that most blue-sequence E/S0s are indeed capable of
substantial stellar disk growth on relatively short timescales. (abridged)
