Zusammenfassung
We present 13 new 2175 \AA 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, Zn II, Mg II,
Si II, Al II, Ni II, Mn II, Cr II, Ti II, and Ca II are present in the same
absorber which gives rise to the 2175 \AA bump. The relative metal abundances
(with respect to Zn) demonstrate that the depletion patterns of our 2175 \AA
dust absorbers resemble that of the Milky Way clouds although some are
disk-like and some are halo-like. The 2175 \AA dust absorbers have
significantly higher depletion levels compared to literature Damped
Lyman-\alpha absorbers (DLAs) and subDLAs. The dust depletion level indicator
Fe/Zn tends to anti-correlate 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, which are larger than
those of most DLAs and subDLAs. Assuming the velocity width is a reliable
tracer of stellar mass, the host galaxies of 2175 \AA dust absorbers are
expected to be more massive than DLA/subDLA hosts. Not all of the 2175 \AA
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.
Nutzer