On the kinematics of damped Lyman-alpha systems

Abstract

We report on high spectral resolution observations of five damped Ly-alpha absorbers. Line velocity profiles and heavy element abundances are discussed. Nitrogen is found to have abundances less than silicon in the systems toward Q 0347-383, Q 0913+072, and Q 1213+093. The absorber toward Q 0913+072 is the most metal-deficient damped system known, with [Fe/H] < -3.2. The simple kinematical structure of the metal absorptions makes this system ideal to discuss the [O/Si] and [N/O] ratios. We find [O/H] ~ -2.7 and -2.7 < [Si/H] < -2.2. By combining these data with information gathered in the literature, we study the kinematics of the low and high ionization phases in a sample of 26 damped Ly-alpha systems in the redshift range 1.17-4.38. We note a strong correlation between the velocity broadenings of the SiII,1808 and FeII,1608 lines whatever the line optical depth, implying that the physical conditions are quite homogeneous in the sample. Statistically this shows that large variations of abundance ratios and thus large variations of depletion into dust grains are unlikely. The velocity broadening of the absorption lines, Delta V, is correlated with the asymmetry of the lines for Delta V < 150 km/s. The broader the line the more asymmetric it is, as expected in case rotation dominates the line broadening. However this correlation does not hold for larger Delta V suggesting that evidence for rotational motions is restricted to velocity broadenings Delta V < 150 km/s. The systems with Delta V > 200 km/s are peculiar with kinematics consistent with random motions. They show sub-systems as those expected if the objects are in the process of merging (abridged).