Lyman-α at Cosmic Noon II: The relationship between kinematics and Lyman-α in z ∼ 2-3 Lyman break galaxies

We report for the first time a relationship between galaxy kinematics and net Lyman-alpha equivalent width (net Ly alpha EW) in star-forming galaxies during the epoch of peak cosmic star formation. Building on the previously reported broadband imaging segregation of Ly alpha-emitting and Ly alpha-absorbing Lyman break galaxies (LBGs) at z similar to 2 (Paper I in this series) and previously at z similar to 3, we use the Ly alpha spectral type classification method to study the relationship between net Ly alpha EW and nebular emission-line kinematics in samples of z similar to 2 and z similar to 3 LBGs drawn from the literature for which matching rest-frame UV photometry, consistently measured net Ly alpha EWs, and kinematic classifications from integral field unit spectroscopy are available. We show that z similar to 2 and z similar to 3 LBGs segregate in colour-magnitude space according to their kinematic properties and Lyman-alpha spectral type and conclude that LBGs with Ly alpha dominant in absorption (aLBGs) are almost exclusively rotation-dominated (presumably disc-like) systems, and LBGs with Ly alpha dominant in emission (eLBGs) characteristically have dispersion-dominated kinematics. We quantify the relationship between the strength of rotational dynamic support (as measured using v(obs)/2 sigma(int) and v(rot)/sigma(0)) and net Ly alpha EW for subsets of our kinematic sample where these data are available, and demonstrate the consistency of our result with other properties that scale with net Ly alpha EW and kinematics. Based on these findings, we suggest a method by which large samples of rotation- and dispersion-dominated galaxies might be selected using broadband imaging in as few as three filters and/or net Ly alpha EW alone. If confirmed with larger samples, application of this method will enable an understanding of galaxy kinematic behaviour over large scales in datasets from current and future large-area and all-sky photometric surveys that will select hundreds of millions of LBGs in redshift ranges from z similar to 2-6 across many hundreds to thousands of Mpc. Finally, we speculate that the combination of our result linking net Ly alpha EW and nebular emission-line kinematics with the known large-scale clustering behaviour of Ly alpha-absorbing and Ly alpha-emitting LBGs is evocative of an emergent bimodality of early galaxies that is consistent with a nascent morphology-density relation at z similar to 2-3.