Delayed Star Formation in Isolated Dwarf Galaxies: HST Star Formation
  History of the Aquarius Dwarf Irregular
  We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius
dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope (HST) Advanced
Camera for Surveys (ACS). The resulting color-magnitude diagram (CMD) reaches
more than a magnitude below the oldest main-sequence turnoff, allowing us to
derive the star formation history (SFH) over the entire lifetime of the galaxy
with a timing precision of ~10% of the lookback time. Using a maximum
likelihood fit to the CMD we find that only ~10% of all star formation in
Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift
z ~2). The star formation rate increased dramatically ~6-8 Gyr ago (z ~
0.7-1.1) and then declined until the present time. The only known galaxy with a
more extreme confirmed delay in star formation is Leo A, a galaxy of similar
M(HI)/M(stellar), dynamical mass, mean metallicity, and degree of isolation.
The delayed stellar mass growth in these galaxies does not track the mean dark
matter accretion rate from CDM simulations. The similarities between Leo A and
Aquarius suggest that if gas is not removed from dwarf galaxies by interactions
or feedback, it can linger for several gigayears without cooling in sufficient
quantity to form stars efficiently. We discuss possible causes for the delay in
star formation including suppression by reionization and late-time mergers. We
find reasonable agreement between our measured SFHs and select cosmological
simulations of isolated dwarfs. Because star formation and merger processes are
both stochastic in nature, delayed star formation in various degees is
predicted to be a characteristic (but not a universal) feature of isolated
small galaxies.
