Revealing the Host Galaxy of a Quasar 2175 Å Dust Absorber at z =  2.12

Abstract

We report the first detection of the host galaxy of a strong 2175 Å dust absorber at z  = 2.12 toward the background quasar SDSS J121143.42+083349.7 using Hubble Space Telescope /Wide Field Camera 3 ( HST /WFC3) IR F140W direct imaging and G141 grism spectroscopy. The spectroscopically confirmed host galaxy is located at a small impact parameter of ∼5.5 kpc (∼0.″65). The F140W image reveals a disk-like morphology with an effective radius of 2.24 ± 0.08 kpc. The extracted 1D spectrum is dominated by a continuum with weak emission lines ([O  iii ] and [O  ii ]). The [O  iii ]-based unobscured star formation rate (SFR) is 9.4 ± 2.6 M ⊙ yr −1 , assuming an [O  iii ]/H α ratio of 1. The moderate 4000 Å break (Dn(4000) index ∼1.3) and Balmer absorption lines indicate that the host galaxy contains an evolved stellar population with an estimated stellar mass M * of (3–7) × 10 10 M ⊙ . The SFR and M * of the host galaxy are comparable to, though slightly lower than, those of typical emission-selected galaxies at z  ∼ 2. As inferred from our absorption analysis in Ma et al., the host galaxy is confirmed to be a chemically enriched, evolved, massive, and star-forming disk-like galaxy that is likely in the transition from a blue star-forming galaxy to a red quiescent galaxy.