The Ly-alpha Properties of Faint Galaxies at z2-3 with Systemic Redshifts and Velocity Dispersions from Keck-MOSFIRE

We study the Ly alpha profiles of 36 spectroscopically detected Ly alpha-emitters (LAEs) at z similar to 2-3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical nebular emission lines. The sample has a median optical magnitude R = 26.0, and ranges from R similar or equal to 23 to R > 27, corresponding to rest-frame UV absolute magnitudes M-UV similar or equal to -22 to M-UV > -18.2. Dynamical masses range from M-dyn < 1.3 x 10(8) M circle dot to M-dyn = 6.8 x 10(9) M circle dot, with a median value of M-dyn = 6.3 x 10(8) M circle dot. Thirty of the 36 Ly alpha emission lines are redshifted with respect to the systemic velocity with at least 1 sigma significance, and the velocity offset with respect to systemic Delta v(Ly alpha) is correlated with the R-band magnitude, M-UV, and the velocity dispersion measured from nebular emission lines with >3 sigma significance: brighter galaxies with larger velocity dispersions tend to have larger values of Delta v(Ly alpha). We also make use of a comparison sample of 122 UV-color-selected R < 25.5 galaxies at z similar to 2, all with Ly alpha emission and systemic redshifts measured from nebular emission lines. Using the combined LAE and comparison samples for a total of 158 individual galaxies, we find that Delta v(Ly alpha) is anti-correlated with the Ly alpha equivalent width with 7 sigma significance. Our results are consistent with a scenario in which the Ly alpha profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Lya photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars.