Near-Field Limits on the Role of Faint Galaxies in Cosmic Reionization
  Reionizing the Universe with galaxies appears to require significant star
formation in low-mass halos at early times, while local dwarf galaxy counts
tell us that star formation has been minimal in small halos around us today.
Using simple models and the ELVIS simulation suite, we show that reionization
scenarios requiring appreciable star formation in halos with $M_{\rm vir}
\approx 10^{8}\,M_{\odot}$ at $z=8$ are in serious tension with galaxy counts
in the Local Group. This tension originates from the seemingly inescapable
conclusion that 30 - 60 halos with $M_{\rm vir} > 10^{8}\,M_{\odot}$ at $z=8$
will survive to be distinct bound satellites of the Milky Way at $z = 0$.
Reionization models requiring star formation in such halos will produce dozens
of bound galaxies in the Milky Way's virial volume today (and 100 - 200
throughout the Local Group), each with $\gtrsim 10^{5}\,M_{\odot}$ of old stars
($\gtrsim 13$ Gyr). This exceeds the stellar mass function of classical Milky
Way satellites today, even without allowing for the (significant)
post-reionization star formation observed in these galaxies. One possible
implication of these findings is that star formation became sharply inefficient
in halos smaller than $\sim 10^9 \,M_{\odot}$ at early times, implying that the
high-$z$ luminosity function must break at magnitudes brighter than is often
assumed (at ${\rm M_{UV}} \approx -14$). Our results suggest that JWST (and
possibly even HST with the Frontier Fields) may realistically detect the
faintest galaxies that drive reionization. It remains to be seen how these
results can be reconciled with the most sophisticated simulations of early
galaxy formation at present, which predict substantial star formation in
$M_{\rm vir} \sim 10^8 \, M_{\odot}$ halos during the epoch of reionization.
