Portrait of a Dark Horse: a Photometric and Spectroscopic Study of the
  Ultra-faint Milky Way Satellite Pegasus III
  Pegasus III (Peg III) is one of the few known ultra-faint stellar systems in
the outer halo of the Milky Way. We present the results from a follow-up
campaign with Magellan/IMACS and Keck/DEIMOS. Deep stellar photometry down to
$r_0\approx 25$ mag at 50% completeness level has allowed accurate measurements
of its photometric and structural properties. The color-magnitude diagram of
Peg III confirms that the stellar system is well described by an old
($\gtrsim12$ Gyr) and metal-poor ([Fe/H]$\lesssim-2.0$ dex) stellar population
at a heliocentric distance of $215\pm12$ kpc. The revised half-light radius
$r_{h}=53\pm14$ pc, ellipticity $\epsilon=0.38^{+0.22}_{-0.38}$, and total
luminosity $M_{V}=-3.4\pm0.4$ are in good agreement with the values quoted in
our previous paper. We further report on the spectroscopic identification of
seven, possibly eight member stars of Peg III. The Ca II triplet lines of the
brightest member stars indicate that Peg III contains stars with metallicity as
low as [Fe/H]=$-2.55\pm0.15$ dex. Peg III has a systemic velocity of $-222.9
\pm 2.6$ km s$^{-1}$ and a velocity dispersion of $5.4^{+3.0}_{-2.5}$ km
s$^{-1}$. The inferred dynamical mass within the half-light radius is
$1.4^{+3.0}_{-1.1} \times 10^6\rm{M_{\odot}}$ and the mass-to-light ratio
$\rm{M/L}$$_{V} = 1470^{+5660}_{-1240}$ $\rm{M_{\odot}/L_{\odot}}$, providing
further evidence that Peg III is a dwarf galaxy satellite. We find that Peg III
and another distant dwarf satellite Pisces II lie relatively close to each
other ($\Delta d_{spatial}=43\pm19$ kpc) and share similar radial velocities in
the Galactic standard-of-rest frame ($\Delta v_{GSR}=12.3\pm3.7$ km s$^{-1}$).
This suggests that they may share a common origin.
