Measurements of Ca ii absorption, metals and dust in a sample ofz≃ 1 DLAs and subDLAs★

Abstract

We present observations of Ca ii, Zn ii and Cr ii absorption lines in 16 damped Lyman alpha (DLA) systems and six subDLAs at redshifts 0.6 < zabs < 1.3, obtained for the dual purposes of (i) clarifying the relationship between DLAs and absorption systems selected from their strong Ca ii lines, and (ii) increasing the still limited sample of Zn and Cr abundance determinations in this redshift range. We find only partial overlap between current samples of intermediate redshift DLAs (which are drawn from magnitude-limited surveys) and strong Ca ii absorbers: approximately 25 per cent of known DLAs at these redshifts have an associated Ca iiλ3935 line with a rest-frame equivalent width greater than 0.35 A, the threshold of the Sloan Digital Sky Survey sample assembled by Wild and her collaborators. The lack of the strongest Ca ii systems (with equivalent widths greater than 0.5 A) is consistent with these authors' conclusion that such absorbers are often missed in current DLA surveys because they redden and dim the light of the background quasi-stellar objects. We rule out the suggestion that strong Ca ii absorption is associated exclusively with the highest column density DLAs. Furthermore, we find no correlation between the strength of the Ca ii lines and either the metallicity or degree of depletion of refractory elements, although the strongest Ca ii absorber in our sample is also the most metal-rich DLA yet discovered, with [Zn/H]≃ solar. We conclude that a complex mix of parameters must determine the strengths of the Ca ii lines, including the density of particles and ultraviolet photons in the interstellar media of the galaxies hosting the DLAs. We find tentative evidence (given the small size of our sample) that strong Ca ii systems may preferentially sample regions of high gas density, perhaps akin to the DLAs exhibiting molecular hydrogen absorption at redshifts z > 2. If this connection is confirmed, strong Ca ii absorbers would trace possibly metal rich, H2 bearing columns of cool, dense gas at distances up to tens of kpc from normal galaxies.