Evidence for Late-Time Feedback from the Discovery of Multiphase Gas in a Massive Elliptical at $z=0.4$

Abstract

We report the first detection of multiphase gas within a quiescent galaxy beyond $z\approx 0$. The observations use the brighter image of doubly lensed QSO HE 0047$-$1756 to probe the ISM of the massive ($M_{\mathrm{star}}\approx 10^{11}{\mathrm{M}}_{\odot }$) elliptical lens galaxy at $z_{\mathrm{gal}}=0.408$. Using Hubble Space Telescope's Cosmic Origins Spectrograph (COS), we obtain a medium-resolution FUV spectrum of the lensed QSO and identify numerous absorption features from ${\mathrm{H}}_2$ in the lens ISM at projected distance $d=4.6$ kpc. The ${\mathrm{H}}_2$ column density is $\log N({\mathrm{H}}_2)/\mathrm{c}{\mathrm{m}}^{-2}=17.8_{-0.3}^{+0.1}$ with a molecular gas fraction of $f_{{\mathrm{H}}_2}=2-5\%$, roughly consistent with some local quiescent galaxies. The new COS spectrum also reveals kinematically complex absorption features from highly ionized species O VI and N V with column densities log $N(\mathrm{OVI})/\mathrm{c}{\mathrm{m}}^{-2}=15.2\pm 0.1$ and log $N(\mathrm{NV})/\mathrm{c}{\mathrm{m}}^{-2}=14.6\pm 0.1$, among the highest known in external galaxies. Assuming the high-ionization absorption features originate in a transient warm ($T\sim 10^5$K) phase undergoing radiative cooling from a hot halo surrounding the galaxy, we infer a mass accretion rate of $\sim 0.5-1.5{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. The lack of star formation in the lens suggests the bulk of this flow is returned to the hot halo, implying a heating rate of $\sim 10^{48}\mathrm{erg}\mathrm{y}{\mathrm{r}}^{-1}$. Continuous heating from evolved stellar populations (primarily SNe Ia but also winds from AGB stars) may suffice to prevent a large accumulation of cold gas in the ISM, even in the absence of strong feedback from an active nucleus.

10 pages, 4 figures. Accepted for publication in ApJ Letters following a minor revision

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