On the kinematics of damped Lyman-α systems

We report on high spectral resolution observations of five damped Lyman-alpha absorbers: z(abs) = 3.0248 toward Q 0347-383, z(abs) = 2.1411 and 2.8112 toward Q 0528-250, z(abs) = 2.6184 toward Q 0913+072, and z(abs) = 2.5226 toward Q 1213+093. 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] approximate to -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 Lyman-alpha systems in the redshift range 1.17-4.38. We note a strong correlation between the velocity broadenings of the Si II lambda 1808 and Fe II lambda 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(-1). 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(-1). The systems with Delta V > 200 km s(-1) are peculiar with kinematics consistent with random motions. They show sub-systems as those expected if the objects are in the process of merging. Although the sample is not large enough to draw firm conclusions, there is a trend for the mean velocity broadening to decrease with redshift from 80 km s(-1) at z < 2.2 to 50 km s(-1) at z > 2.2. The kinematics of the low and high ionization species are found to be statistically correlated, though the high-ionization phase has a much more disturbed kinematical field than the low-ionization phase. This should be taken into account in any model of high redshift damped Lyman-alpha systems.