Kavli Affiliate: Anna Frebel
| First 5 Authors: Alice M. Luna, Alice M. Luna, , ,
| Summary:
Star formation in ultra-faint dwarf galaxies (UFDs, $M_* <10^5M_odot$) is
suppressed by reionization, but may not be completely quenched. The metallicity
distribution function (MDF) of stars in ultra-faint dwarf galaxies could show
these signatures of reionization. However, past studies of UFD MDFs have been
limited, because there are only a few dozen red giant branch (RGB) stars in
such low-mass galaxies. We present low-resolution Magellan/IMACS spectroscopy
of 167 stars in the UFD Reticulum II ($M_* approx 3000 M_odot$), increasing
the number of stellar metallicities by 6.5 times and resulting in the most
populated spectroscopic metallicity distribution function of any UFD. This is
possible because we determined the first spectroscopic metallicities of main
sequence turn-off stars in any UFD. The MDF of Reticulum II is clearly a
bimodal distribution, displaying two peaks with about $80%$ of the stars in
the metal-poor peak at $rm[Fe/H]=-3.0$ and $20%$ of the stars in the more
metal-rich peak at $rm[Fe/H]=-2.1$. Such a large metallicity gap can be
explained by Type Ia supernova enrichment during a long quiescent period. This
supports the currently-favored two-burst star formation history for Reticulum
II and shows that such low-mass galaxies clearly can form stars after
reionization.
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