High Dust Depletion in Two Intervening Quasar Absorption Line Systems with the 2175-Å Extinction Bump at $z\sim 1.4$

Abstract

We present the column densities of heavy-elements and dust depletion studies in twostrong Mg~II absorption systems at $z\sim 1.4$ displaying the 2175-Å dust extinction feature. Column densities are measured from low-ionization absorption lines using Apparent Optical Depth Method on the Keck/ESI spectra. We find the dust depletion patterns resemble to that of cold diffuse clouds in the Milky Way (MW). The values, [Fe/Zn]$\approx -1.5$ and [Si/Zn]$<-0.67$, are among the highest dust depletion measured for quasar absorption line systems. In another 2175-Å absorber at $z$=1.64 toward the quasar SDSS J160457.50+220300.5, Noterdaeme et al. (2009) reported a similar dust depletion measurement ([Fe/Zn]=$-1.47$ and [Si/Zn]=$-1.07$) and detected C~I and CO absorption lineson its VLT/UVES spectrum. We conclude that heavy dust depletion (i.e. a characteristic of cold dense clouds in MW) is required to produce a pronounced 2175-Å extinction bump.

28 Pages, 9 Figures, 5 Tables; Accepted by ApJ

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