Leo P: An Unquenched Very Low-Mass Galaxy
  Leo P is a low-luminosity dwarf galaxy discovered through the blind HI
Arecibo Legacy Fast ALFA (ALFALFA) survey. The HI and follow-up optical
observations have shown that Leo P is a gas-rich dwarf galaxy with active star
formation, an underlying older population, and an extremely low oxygen
abundance. We have obtained optical imaging with the Hubble Space Telescope to
two magnitudes below the red clump in order to study the evolution of Leo P. We
refine the distance measurement to Leo P to be 1.62+/-0.15 Mpc, based on the
luminosity of the horizontal branch stars and 10 newly identified RR Lyrae
candidates. This places the galaxy at the edge of the Local Group, ~0.4 Mpc
from Sextans B, the nearest galaxy in the NGC 3109 association of dwarf
galaxies of which Leo P is clearly a member. The star responsible for ionizing
the HII region is most likely an O7V or O8V spectral type, with a stellar mass
>25 Msun. The presence of this star provides observational evidence that
massive stars at the upper-end of the initial mass function are capable of
being formed at star formation rates as low as ~10^-5 Msun/yr. The best-fitting
star formation history derived from the resolved stellar populations of Leo P
using the latest PARSEC models shows a relatively constant star formation rate
over the lifetime of the galaxy. The modeled luminosity characteristics of Leo
P at early times are consistent with low-luminosity dSph Milky Way satellites,
suggesting that Leo P is what a low-mass dSph would look like if it evolved in
isolation and retained its gas. Despite the very low mass of Leo P, the imprint
of reionization on its star formation history is subtle at best, and consistent
with being totally negligible. The isolation of Leo P, and the total quenching
of star formation of Milky Way satellites of similar mass, implies that local
environment dominates the quenching of the Milky Way satellites.
