New detections of Mn, Ti and Mg in damped Lyα systems: Toward reconciling the dust/nucleosynthesis degeneracy

Abstract

We have combined new high resolution UVES-VLT observations of a sample of four damped Lyα systems (DLAs) at redshifts between zabs = 1.7-2.5 with the existing HIRES-Keck spectra to undertake a comprehensive study of their physical conditions and their abundances of up to 15 elements. In this paper, we present abundance measurements for Mn, Ti and Mg which are among the first presented in the literature at these redshifts. We confirm the underabundance of Mn with respect to Fe as observed in lower redshift DLAs and a trend with decreasing metallicity very similar to the one observed in Galactic metal-poor stars. This agreement between the Mn/Fe ratios in DLAs and in Galactic stars suggests that the DLAs and the Milky Way share some similarities in their star formation histories. However, these similarities must be cautiously interpreted and investigated in light of all of the elements observed in the DLAs. We have obtained a first measurement and a significant upper limit of the Ti abundance at zabs ∼ 2 from the Ti II lines at λrest > 3000 Å, and we discuss how the relative abundance of this highly depleted element can be used to prove unambiguously any enhancement of the abundance of the α-elements relative to Fe-peak elements. We present the abundances of Mg for two DLAs in addition to the single Mg measurement existing in the literature. Contrary to the trend expected from differential depletion, the [Mg/Si, S] ratios tend to be over-solar. The effect is at the level of the measurement errors, but worth investigating in a larger sample because it could be suggestive of a peculiar nucleosynthesis effect.