HALO7D I: The Line of Sight Velocities of Distant Main Sequence Stars in
  the Milky Way Halo
  The Halo Assembly in Lambda-CDM: Observations in 7 Dimensions (HALO7D)
dataset consists of Keck II/DEIMOS spectroscopy and Hubble Space
Telescope-measured proper motions of Milky Way halo main sequence turnoff stars
in the CANDELS fields. In this paper, we present the spectroscopic component of
this dataset, and discuss target selection, observing strategy, and survey
properties. We present a new method of measuring line-of-sight (LOS) velocities
by combining multiple spectroscopic observations of a given star, utilizing
Bayesian hierarchical modeling. We present the LOS velocity distributions of
the four HALO7D fields, and estimate their means and dispersions. All of the
LOS distributions are dominated by the "hot halo": none of our fields are
dominated by substructure that is kinematically cold in the LOS velocity
component. Our estimates of the LOS velocity dispersions are consistent across
the different fields, and these estimates are consistent with studies using
other types of tracers. To complement our observations, we perform mock HALO7D
surveys using the synthetic survey software Galaxia to "observe'" the Bullock &
Johnston (2005) accreted stellar halos. Based on these simulated datasets, the
consistent LOS velocity distributions across the four HALO7D fields indicates
that the HALO7D sample is dominated by stars from the same massive (or few
relatively massive) accretion event(s).
