Astronomy & Astrophysics

Free access article

Issue		A&A Volume 403, Number 2, May IV 2003
Page(s)		573 - 584
Section		Galactic structure and dynamics
DOI		10.1051/0004-6361:20030403

A&A 403, 573-584 (2003)
DOI: 10.1051/0004-6361:20030403

Distinct abundance patterns in multiple damped Ly $\alpha$ galaxies: Evidence for truncated star formation?

S. Lopez¹ and S. L. Ellison^{2, 3}

¹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
e-mail: sellison@astro.puc.cl

(Received 8 August 2002 / Accepted 17 March 2003 )

Abstract
Following our previous work on metal abundances of a double damped Ly $\alpha$ 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 $z \sim 2.5$ toward the southern QSO CTQ247. These abundances are supplemented with echelle observations of another "double" damped Ly $\alpha$ system in the literature. We propose a definition in terms of velocity shift of the sub-class "multiple damped Ly $\alpha$ 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 $\alpha$ /Fe ratios could be due to dust depletion or ionization corrections and find that the former effect would emphasize the observed trend of low $\alpha$ /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 $\alpha$ /Fe ratios in multiple DLAs have a nucleosynthetic origin and suggest that they could be explained by reduced star formation in multiple damped Ly $\alpha$ 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/ $\alpha$ ratios we observe in these multiple systems.

Key words: quasars: general -- quasars: absorption lines -- quasars: individual: CTQ247 -- galaxies: evolution -- galaxies: clusters: general

Offprint request: S. Lopez, slopez@das.uchile.cl

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Distinct abundance patterns in multiple damped Ly galaxies: Evidence for truncated star formation?

Distinct abundance patterns in multiple damped Ly $\alpha$ galaxies: Evidence for truncated star formation?