Revealing the host galaxy of a quasar 2175 \AA$ $ dust absorber at z = 2.12

TL;DR

This study reports the first direct imaging and spectroscopic detection of a galaxy hosting a 2175 Å dust absorber at z=2.12, revealing a massive, evolved, star-forming disk galaxy with specific morphological and stellar properties.

Contribution

First direct detection and characterization of a host galaxy of a 2175 Å dust absorber at high redshift using HST imaging and spectroscopy.

Findings

Host galaxy is a disk-like, massive galaxy at z=2.12.

Galaxy has an SFR of about 9.4 M$_{ ext{odot}}$/yr.

Galaxy shows signs of being evolved and transitioning from star-forming to quiescent.

Abstract

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.