Bibcode
Cole, A. A.; Weisz, Daniel R.; Dolphin, Andrew E.; Skillman, Evan D.; McConnachie, Alan W.; Brooks, Alyson M.; Leaman, R.
Referencia bibliográfica
The Astrophysical Journal, Volume 795, Issue 1, article id. 54, 11 pp. (2014).
Fecha de publicación:
11
2014
Revista
Número de citas
64
Número de citas referidas
58
Descripción
We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius
dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope
Advanced Camera for Surveys. 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 H I /M sstarf,
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 degrees is predicted to be
a characteristic (but not a universal) feature of isolated small
galaxies.
Based on observations made with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated by
the Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS 5-26555. These observations were obtained under
program GO-12925.
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