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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.