Ly Line Properties at z=3.78 and Their Environmental Dependence: A Case Study around a Massive Protocluster

Ly alpha-emitting galaxies (LAEs) are easily detectable in the high-redshift universe and are potentially efficient tracers of large-scale structure at early epochs, as long as their observed properties do not depend strongly on environment. We investigate the luminosity and equivalent width functions of LAEs in the overdense field of a protocluster at redshift z similar or equal to 3.78. Using a large sample of LAEs (many spectroscopically confirmed), we find that the Ly alpha luminosity distribution is well represented by a Schechter function with log (L*/erg s(-1)) = 43.26(-0.22)(+0.20) and log(phi*/Mpc(-3)) = -3.40(-0.04)(+0.03) with alpha = -1.5. Fitting the equivalent width distribution as an exponential, we find a scale factor of omega = 79(-15)(+15)angstrom. We also measured the Ly alpha luminosity and equivalent width functions using the subset of LAEs lying within the densest cores of the protocluster, finding similar values for L* and omega. Hence, despite having a mean overdensity more than 2x that of the general field, the shapes of the Ly alpha luminosity function and equivalent width distributions in the protocluster region are comparable to those measured in the field LAE population by other studies at similar redshift. While the observed Ly alpha luminosities and equivalent widths show correlations with the UV continuum luminosity in this LAE sample, we find that these are likely due to selection biases and are consistent with no intrinsic correlations within the sample. This protocluster sample supports the strong evolutionary trend observed in the Ly alpha escape fraction and suggests that LAEs at lower redshift can be on average significantly more dusty that their counterparts at higher redshift.