A new comprehensive set of elemental abundances in DLAs. II. Data analysis and chemical variation studies

We present new, comprehensive sets of elemental abundances for seven damped Lyα systems (DLAs) in the redshift range z_abs = 1.8-2.5. These were derived from UVES/VLT spectra combined with existing HIRES/Keck spectra. We detected 54 metal-line transitions, and obtained the column density measurements of 30 ions from 22 elements, - B, C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, As, Kr. Together with the four DLAs analyzed in Dessauges-Zavadsky et al. (2004, A&A, 416, 79), we have a sample of eleven DLA galaxies with uniquely comprehensive and homogeneous abundance measurements. These observations allow one to study in detail the abundance patterns of a wide range of 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 H i column densities spanning over one order of magnitude and metallicities ranging from 1/55 to 1/5 solar. The uniformity is also remarkable since DLAs are expected (and observed at low redshift) to be associated with a wide range of galaxy types. This implies the respective star formation histories seem to have conspired to yield one set of relative abundances. We examined the gas-phase abundance patterns of interstellar medium "clouds" within the DLA galaxies detected along the velocity profiles. By considering all the clouds of all the DLAs studied together, we observe 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. We found unambiguous correlations between [Si/Fe], [S/Fe] and [S/Si] versus [Zn/Fe], and anti-correlations between [Si/Zn] and [S/Zn] versus [Zn/Fe]. These trends are primarily the result of differential dust depletion effects, which also explain the cloud abundance ratio dispersion. The signature of the nucleosynthesis enrichment contribution is observed in the [ α/Fe,Zn] ratios at low dust depletion levels, 0≤ [Zn/Fe]≤ 0.2, and is characterized by an α-enhancement in individual clouds. Quite surprisingly, however, while the [Si/Fe] ratios are supersolar in clouds with low depletion level, the [S/Zn] ratios remain almost solar, suggesting that [S/Zn] may not be a reliable tracer of nucleosynthesis enrichment. Analysis of the cloud-to-cloud chemical variations within seven individual DLA systems reveals that five of them show statistically significant variations, higher than 0.2 dex at more than 3σ, but only two DLAs show extreme variations. 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 Ni ii λ 1317 transition.