Damped Lyman alpha systems at z=0.68 and z=1.15 towards HE 1122-1649 and HE 0515-4414

Abstract

Detailed analysis of the spectral lines from two low redshift damped Lyman alpha absorbers (DLA) confirms that they are a heterogeneous population. Both systems have low metal abundances of approximately 0.05 to 0.1 solar. The abundance pattern of the DLA at z=1.15 towards HE 0515-4414 shows dust depletion comparable to that found in our Galaxy, while the system at z=0.68 towards HE 1122-1649 shows abundance ratios resembling metal-poor halo stars with no dust depletion, for a comparable H I column. Constraints for N/Fe and N/Si also hint at z=0.68 DLA as a galaxy with recent star formation. The trend of increasing C I /H I with decreasing z is discussed. Only weak absorption from highly-ionized species associated with the DLA at moderate z have been detected and it probably originates in regions distinct from the low ionization gas. The low-ion profiles show very complex structures and are too large to be explained by the rotation of a disk: the system at z=0.68 spans over ~ 300 km s^-1 and the z=1.15 DLA presents substructure over more than 700 km s^-1 , the largest velocity extent found up to date for a DLA. Based on observations performed at ESO/La Silla, on observations with the NASA/ESA Hubble Space Telescope and on observations at Keck Observatory.