Revealing the host galaxy of a quasar 2175 \AA$ $ dust absorber at z =
  2.12
  We report the first detection of the host galaxy of a strong 2175 \AA$ $ dust
absorber at z = 2.12 towards the background quasar SDSS J121143.42+083349.7
using 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 $\pm$ 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 $\pm$ 2.6
M$_{\odot}$yr$^{-1}$ assuming an [O\III]/H$\alpha$ ratio of 1. The moderate
4000 \AA$ $ break (Dn(4000) index $\sim$ 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) $\times$ 10$^{10}$ M$_{\odot}$. The SFR
and M$_*$ of the host galaxy are comparable to, though slightly lower than,
those of typical emission-selected galaxies at $z$ $\sim$ 2. As inferred from
our absorption analysis in Ma et al. (2015, 2017, 2018), 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.
