Abstract

Following our previous work on metal abundances of a double damped Ly? system with a line-of-sight separation ?2000 km s-1 (Ellison & Lopez 2001), we present VLT UVES abundances of 3 new systems spanning a total of ?6000 km s-1 at z ? 2.5 toward the southern QSO CTQ247. These abundances are supplemented with echelle observations of another "double" damped Ly? system in the literature. We propose a definition in terms of velocity shift of the sub-class "multiple damped Ly? system", which is motivated by its possible connection with large-scale structure. We find that the abundance ratio [S/Fe] is systematically low in multiple systems compared with single systems, and with a small scatter. The same behavior is found in 2 more single DLA systems taken from the literature that show evidence of belonging to a galaxy group. Although [Si/Fe] ratios are also generally lower in multiple DLAs than in single DLAs, the effect is less striking since the scatter is larger and there are a number of low [Si/Fe] DLAs in the literature. We suggest that this can be explained with a combination of detection bias and, to a lesser extent, the scatter in ionization corrections for different absorbers. We investigate whether consistently low ?/Fe ratios could be due to dust depletion or ionization corrections and find that the former effect would emphasize the observed trend of low ?/Fe in multiple systems even further. Ionization may have a minor effect in some cases, but at a level that would not change our conclusions. We thus conclude that the low ?/Fe ratios in multiple DLAs have a nucleosynthetic origin and suggest that they could be explained by reduced star formation in multiple damped Ly? systems, possibly due to environmental effects. There seems to be independent evidence for this scenario from the mild odd-even effect and from the relatively high N/? ratios we observe in these multiple systems.