We present new , comprehensive sets of elemental abundances for seven damped Ly \alpha 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 ) , 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 [ \alpha /Fe , Zn ] ratios at low dust depletion levels , 0 \leq { [ Zn / Fe ] } \leq 0.2 , and is characterized by an \alpha -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 3 \sigma , 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 \lambda 1317 transition .