HST Detection of Extended Neutral Hydrogen in a Massive Elliptical at z = 0.4

We report the first detection of extended neutral hydrogen (H I) gas in the interstellar medium (ISM) of a massive elliptical galaxy beyond z∼ 0. The observations utilize the doubly lensed images of QSO HE 0047-1756 at {z}_{QSO}=1.676 as absorption-line probes of the ISM in the massive ({M}_{star}≈ {10}¹¹ {M}_⊙ ) elliptical lens at z = 0.408, detecting gas at projected distances of d = 3.3 and 4.6 kpc on opposite sides of the lens. Using the Space Telescope Imaging Spectrograph, we obtain UV absorption spectra of the lensed QSO and identify a prominent flux discontinuity and associated absorption features matching the Lyman series transitions at z = 0.408 in both sightlines. The H I column density is log N({{H}} {{I}})=19.6{--}19.7 at both locations across the lens, comparable to what is seen in 21 cm images of nearby ellipticals. The H I gas kinematics are well-matched with the kinematics of the Fe II absorption complex revealed in ground-based echelle data, displaying a large velocity shear of ≈360 {\text{km s}}^-1 across the galaxy. We estimate an ISM Fe abundance of 0.3-0.4 solar at both locations. Including likely dust depletions increases the estimated Fe abundances to solar or supersolar, similar to those of the hot ISM and stars of nearby ellipticals. Assuming 100% covering fraction of this Fe-enriched gas, we infer a total Fe mass of {M}_{cool}({Fe})∼ (5{--}8)× {10}⁴ {M}_⊙ in the cool ISM of the massive elliptical lens, which is no more than 5% of the total Fe mass observed in the hot ISM.