The Ly-alpha Properties of Faint Galaxies at z~2-3 with Systemic Redshifts and Velocity Dispersions from Keck-MOSFIRE

Dawn K. Erb, Charles C. Steidel, Ryan F. Trainor, Milan Bogosavljevic, Alice E. Shapley, Daniel B. Nestor, Kristin R. Kulas, David R. Law, Allison L. Strom, Gwen C. Rudie, Naveen A. Reddy, Max Pettini, Nicholas P. Konidaris, Gregory Mace, Keith Matthews, Ian S. McLean

Published 2014-08-15, updated 2014-09-02Version 2

We study the Ly-alpha profiles of 36 Lya-emitters (LAEs) at z~2-3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical nebular emission lines. The galaxies were selected via narrowband imaging of their Lya emission, and have spectroscopic measurements of their Lya profiles from Keck LRIS. The LAE sample has a median optical magnitude R = 26.0, and ranges from R ~ 23 to R > 27, corresponding to rest-frame UV absolute magnitudes M_UV ~ -22 to M_UV > -18.2. Dynamical masses calculated from sizes measured from Hubble Space Telescope images and nebular line velocity dispersions range from M_dyn < 1.3e8 Msun to M_dyn = 6.8e9 Msun, with a median value of M_dyn = 6.3e8 Msun. Thirty of the 36 Lya emission lines in this sample are redshifted with respect to the systemic velocity with at least 1 sigma significance, and the velocity offset with respect to systemic dv_Lya is correlated with 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 dv_Lya. We also make use of a comparison sample of 122 UV-color-selected R < 25.5 galaxies at z~2, all with Lya 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 dv_Lya is anti-correlated with the Lya equivalent width with 7 sigma significance. Our results are consistent with a scenario in which the Lya 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. [Abridged]