A Deeper Look at Leo IV: Star Formation History and Extended Structure
  We present MMT/Megacam imaging of the Leo~IV dwarf galaxy in order to
investigate its structure and star formation history, and to search for signs
of association with the recently discovered Leo~V satellite. Based on
parameterized fits, we find that Leo~IV is round, with $\epsilon < 0.23$ (at
the 68\% confidence limit) and a half-light radius of $r_{h} \simeq 130$ pc.
Additionally, we perform a thorough search for extended structures in the plane
of the sky and along the line of sight. We derive our surface brightness
detection limit by implanting fake structures into our catalog with stellar
populations identical to that of Leo~IV. We show that we are sensitive to
stream-like structures with surface brightness $\mu_{r}\lesssim29.6$ mag
arcsec$^{-2}$, and at this limit, we find no stellar bridge between Leo IV (out
to a radius of $\sim$0.5 kpc) and the recently discovered, nearby satellite Leo
V. Using the color magnitude fitting package StarFISH, we determine that Leo~IV
is consistent with a single age ($\sim$14 Gyr), single metallicity
($[Fe/H]\sim-2.3$) stellar population, although we can not rule out a
significant spread in these value. We derive a luminosity of
$M_{V}=-5.5\pm0.3$. Studying both the spatial distribution and frequency of
Leo~IV's 'blue plume' stars reveals evidence for a young ($\sim$2 Gyr) stellar
population which makes up $\sim$2\% of its stellar mass. This sprinkling of
star formation, only detectable in this deep study, highlights the need for
further imaging of the new Milky Way satellites along with theoretical work on
the expected, detailed properties of these possible 'reionization fossils'.
