Line Emitting Galaxies Beyond a Redshift of 7: An Improved Method for
  Estimating the Evolving Neutrality of the Intergalactic Medium
  The redshift-dependent fraction of color-selected galaxies revealing Lyman
alpha emission has become the most valuable constraint on the evolving
neutrality of the early intergalactic medium. However, in addition to resonant
scattering by neutral gas, the visibility of Lyman alpha is also dependent on
the intrinsic properties of the host galaxy, including its stellar population,
dust content and the nature of outflowing gas. Taking advantage of significant
progress we have made in determining the line emitting properties of $z \simeq
4-6$ galaxies, we propose an improved method, based on using the measured
slopes of the rest-frame ultraviolet continua of galaxies, to interpret the
growing body of near-infrared spectra of $z>7$ galaxies in order to take into
account these host galaxy dependencies. In a first application of our new
method, we demonstrate its potential via a new spectroscopic survey of $7<z<8$
galaxies undertaken with the Keck MOSFIRE spectrograph. Together with earlier
published data our data provides improved estimates of the evolving visibility
of Lyman alpha, particularly at redshift $z\simeq 8$. As a byproduct, we also
present a new line emitting galaxy at a redshift $z=7.62$ which supersedes an
earlier redshift record. We discuss the improving constraints on the evolving
neutral fraction over $6<z<8$ and the implications for cosmic reionization.
