The first detection of [O III] emission from high-redshift damped Lyman-α galaxies

We present the detection of [O-III] emission lines from the galaxies responsible for two high-redshift z > 1.75 damped Lyman-alpha (DLA) absorption lines. We find two sources of [O-III] emission corresponding to the z = 1.92 DLA absorber towards the quasar Q 2206-1958, and we also detect [O-III] emission from the galaxy responsible for the z = 3.10 DLA absorber towards the quasar 2233.9+1381. These are the first detections of rest-frame optical emission lines from high-redshift DLA galaxies. Unlike the Ly alpha line, the [O-III] line provides a measure of the systemic velocity of the galaxy. We compare the [O-III] redshifts with the velocity profile of the low-ionization metal lines in these two absorbers, with the goal of distinguishing between the model of Prochaska and Wolfe of DLA absorbers as large rapidly rotating cold thick discs, and the standard hierarchical cold dark matter model of structure formation, in which DLAs arise in protogalactic fragments. We find some discrepancies with the predictions of the former model. Furthermore, the image of the DLA galaxy towards Q 2206-1958 shows a complex disturbed morphology, which is more in accord with the hierarchical picture. We use the properties of the rest-frame optical emission lines to further explore the question posed by Moller et al.: are high-redshift DLA galaxies Lyman-break galaxies (LBGs) selected by gas cross-section ? The measured velocity dispersions of the DLA galaxies are in agreement with this picture, while the data on the [O-III] luminosities and the velocity differences between the Ly alpha and [O-III] lines are inconclusive, as there are insufficient LBG measurements overlapping in luminosity. Finally, we estimate the star formation rates in these two DLA galaxies, using a variety of diagnostics, and include a discussion of the extent to which the [O-III] line is useful for this purpose.