On the nature of velocity fields in high-z galaxies

We analyze the gas kinematics of damped Ly alpha systems (DLAs) hosting high-z gamma-ray bursts (GRBs) and those toward quasars (QSO-DLAs), focusing on three statistics: (1) Delta v(90), the velocity interval encompassing 90% of the integrated optical depth, and (2) W-1526 and (3) W-1548, the rest equivalent widths of the Si II 1526 and C IV 1548 transitions, respectively. The Delta v(90) distributions of the GRB-DLAs and QSO-DLAs are similar; each has median Delta v(90) approximate to 80 km s(-1) and a significant tail, extending to several hundred km s(-1). This suggests comparable galaxy masses for the parent populations of GRB-DLAs and QSO-DLAs, and we infer that the average dark matter halo mass of GRB galaxies is less than or similar to 10(12) M-circle dot. The unique configuration of GRB-DLA sight lines and the presence (and absence) of fine-structure absorption together give special insight into the nature of high-z protogalactic velocity fields. The data support a scenario in which the Delta v(90) statistic reflects dynamics in the interstellar medium (ISM) and W-1526 traces motions outside the ISM (e.g., halo gas and galactic-scale winds). The W-1526 statistic and gas metallicity [M/H] are tightly correlated, especialLyfor the QSO-DLAs: [M/H] = a + b log (W-1526/1 angstrom) with a = 0.92 +/- 0.05 and b = 1.41 +/- 0.10. We argue that the W-1526 statistic primarily tracks dynamical motions in the halos of high-z galaxies and interpret this correlation as a mass-metallicity relation with very similar slope to the trend observed in local, low-metallicity galaxies. Finally, the GRB-DLAs exhibit systematicalLylarger W-1526 values (> 0.5 angstrom) than the QSO-DLAs (< W-1526 > approximate to 0.5 angstrom), which may suggest that galactic-scale outflows contribute to the largest observed velocity fields.