Dust depletion, chemical uniformity and environment of Ca ii H&K quasar absorbers

ABSTRACT

Ca ii λλ3934, 3969 absorbers, which are likely to be a subset of damped Lyman α systems (DLAs), are the most dusty quasar absorbers known with an order of magnitude more extinction in E(B−V) than other absorption systems. There is also evidence that Ca ii absorbers trace galaxies with more ongoing star formation than the average quasar absorber. Despite this, relatively little is known in detail about these unusual absorption systems. Here, we present the first high-resolution spectroscopic study of 19 Ca ii quasar absorbers, in the range 0.6 ≤z_abs≤ 1.2, with Ca ii λ3934 equivalent widths, W³⁹³⁴₀≥ 0.2 Å. Their general elemental depletion patterns are found to be similar to measurements in the warm halo phase of the Milky Way (MW) and Magellanic Clouds interstellar medium. Dust depletions and α-enrichments profiles of subsamples of seven and three absorbers, respectively, are measured using a combination of Voigt profile fitting and apparent optical depth techniques. Deviations in [Cr/Zn]∼ 0.3 ± 0.1 and [Si/Fe]≳ 0.8 ± 0.1 dex are detected across the profile of one absorber, which we attribute to differential dust depletion. The remaining absorbers have <0.3 dex (3σ limit) variation in [Cr/Zn], much like the general DLA population, though the dustiest Ca ii absorbers, those with W³⁹³⁴₀ > 0.7 Å, remain relatively unprobed in our sample. A limit on electron densities in Ca ii absorbers, n_e < 0.1 cm⁻³, is derived using the ratio of neutral and singly ionized species and assuming a MW-like radiation field. These electron densities may imply hydrogen densities sufficient for the presence of molecular hydrogen in the absorbers. The Ca ii absorber sample comprises a wide range of velocity widths, Δv₉₀= 50–470 km s⁻¹, and velocity structures, thus a range of physical models for their origin, from simple discs to galactic outflows and mergers, would be required to explain the observations.