We present new elemental abundance studies of seven damped Lyman-alpha
systems (DLAs). Together with the four DLAs analyzed in Dessauges-Zavadsky et
al. (2004), we have a sample of eleven DLA galaxies with uniquely comprehensive
and homogeneous abundance measurements. These observations allow one to study
the abundance patterns of 22 elements and the chemical variations in the
interstellar medium of galaxies outside the Local Group. Comparing the
gas-phase abundance ratios of these high redshift galaxies, we found that they
show low RMS dispersions, reaching only up 2-3 times the statistical errors for
the majority of elements. This uniformity is remarkable given that the quasar
sightlines cross gaseous regions with HI column densities spanning over one
order of magnitude and metallicities ranging from 1/55 to 1/5 solar. The
gas-phase abundance patterns of interstellar medium clouds within the DLA
galaxies detected along the velocity profiles show, on the other hand, a high
dispersion in several abundance ratios, indicating that chemical variations
seem to be more confined to individual clouds within the DLA galaxies than to
integrated profiles. The analysis of the cloud-to-cloud chemical variations
within seven individual DLAs reveals that five of them show statistically
significant variations, higher than 0.2 dex at more than 3 sigma. The sources
of these variations are both the differential dust depletion and/or ionization
effects; however, no evidence for variations due to different star formation
histories could be highlighted. These observations place large constraints on
the mixing timescales of protogalaxies and on scenarios of galaxy formation
within the CDM hierarchical theory. Finally, we provide an astrophysical
determination of the oscillator strength of the NiII 1317 transition.Comment: 45 pages, 33 figures (high-resolution figures available on request
from the authors or in the A&A journal). Accepted for publication in A&