The little Galaxies that could (reionize the universe): predicting faint end slopes & escape fractions at z>4
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abstract
The sources that reionized the universe are still unknown, but likely
candidates are faint but numerous galaxies. In this paper we present results
from running a high resolution, uniform volume simulation, the Vulcan, to
predict the number densities of undetectable, faint galaxies and their escape
fractions of ionizing radiation, $f_\mathrm{esc}$, during reionization. Our
approach combines a high spatial resolution, a realistic treatment of feedback
and hydro processes, a strict threshold for minimum number of resolution
elements per galaxy, and a converged measurement of $f_\mathrm{esc}$. We
calibrate our physical model using a novel approach to create realistic
galaxies at z=0, so the simulation is predictive at high redshifts. With this
approach we can (1) robustly predict the evolution of the galaxy UV luminosity
function at faint magnitudes down to $M_\mathrm{UV}$~-15, two magnitudes
fainter than observations, and (2) estimate $f_\mathrm{esc}$ over a large range
of galaxy masses based on the detailed stellar and gas distributions in
resolved galaxies. We find steep faint end slopes, implying high number
densities of faint galaxies, and the dependence of $f_\mathrm{esc}$ on the UV
magnitude of a galaxy, given by the power-law: log $f_\mathrm{esc} = (0.51 \pm
0.04)M_\mathrm{UV} + 7.3 \pm 0.8$, with the faint population having
$f_\mathrm{esc}$~35%. Convolving the UV luminosity function with
$f_\mathrm{esc}$($M_\mathrm{UV}$), we find an ionizing emissivity that is (1)
dominated by the faintest galaxies and (2) reionizes the universe at the
appropriate rate, consistent with observational constraints of the ionizing
emissivity and the optical depth to the decoupling surface tau_es, without the
need for additional sources of ionizing radiation.
