Quasar 2175Å dust absorbers - I. Metallicity, depletion pattern and kinematics

We present 13 new 2175Å dust absorbers at z_abs = 1.0-2.2 towards background quasars from the Sloan Digital Sky Survey. These absorbers are examined in detail using data from the Echelle Spectrograph and Imager (ESI) on the Keck II telescope. Many low-ionization lines including Fe II, ZnII, MgII, Si II, Al II, Ni II, MnII, Cr II, Ti II and Ca II are present in the same absorber that gives rise to the 2175Å bump. The relative metal abundances (with respect to Zn) demonstrate that the depletion patterns of our 2175Å dust absorbers resemble that of theMilky Way clouds although some are disc-like and some are halo-like. The 2175Å dust absorbers have significantly higher depletion levels compared to literature damped Lyman α absorbers (DLAs) and sub-DLAs. The dust depletion level indicator [Fe/Zn] tends to anticorrelate with bump strengths. The velocity profiles from the Keck/ESI spectra also provide kinematical information on the dust absorbers. The dust absorbers are found to have multiple velocity components with velocity widths extending from ~100 to ~600 km s^-1, which are larger than those of most DLAs and sub-DLAs. Assuming the velocity width is a reliable tracer of stellar mass, the host galaxies of 2175Å dust absorbers are expected to be more massive than DLA/sub-DLA hosts. Not all of the 2175Å dust absorbers are intervening systems towards background quasars. The absorbers towards quasars J1006+1538 and J1047+3423 are proximate systems that could be associated with the quasar itself or the host galaxy.