Daniel R. Weisz, Daniel B. Zucker, Andrew E. Dolphin, Nicolas F. Martin, Jelte T. A. de Jong, Jon A. Holtzman, Julianne J. Dalcanton, Karoline M. Gilbert, Benjamin F. Williams, Eric F. Bell, Vasily Belokurov, N. Wyn Evans
We present the star formation history (SFH) of the faintest known
star-forming galaxy, Leo T, based on imaging taken with the Hubble Space
Telescope (HST) Wide Field Planetary Camera 2 (WFPC2). The HST/WFPC2
color-magnitude diagram (CMD) of Leo T is exquisitely deep, extending ~ 2
magnitudes below the oldest main sequence turnoff, permitting excellent
constraints on star formation at all ages. We use a maximum likelihood CMD
fitting technique to measure the SFH of Leo T assuming three different sets of
stellar evolution models: Padova (solar-scaled metallicity) and BaSTI (both
solar-scaled and alpha-enhanced metallicities). The resulting SFHs are
remarkably consistent at all ages, indicating that our derived SFH is robust to
the choice of stellar evolution model. From the lifetime SFH of Leo T, we find
that 50% of the total stellar mass formed prior to z ~ 1 (7.6 Gyr ago).
Subsequent to this epoch, the SFH of Leo T is roughly constant until the most
recent ~ 25 Myr, where the SFH shows an abrupt drop. This decrease could be due
to a cessation of star formation or stellar initial mass function sampling
effects, but we are unable to distinguish between the two scenarios. Overall,
our measured SFH is consistent with previously derived SFHs of Leo T. However,
the HST-based solution provides improved age resolution and reduced
uncertainties at all epochs. The SFH, baryonic gas fraction, and location of
Leo T are unlike any of the other recently discovered faint dwarf galaxies in
the Local Group, and instead bear strong resemblance to gas-rich dwarf galaxies
(irregular or transition), suggesting that gas-rich dwarf galaxies may share
common modes of star formation over a large range of stellar mass (~ 10^5-10^9
Msun).
View original:
http://arxiv.org/abs/1201.4859
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