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

TL;DR

This study reports the first detection of extended neutral hydrogen gas in a massive elliptical galaxy at z=0.4, revealing insights into its interstellar medium composition, kinematics, and metal enrichment.

Contribution

It presents the first observation of extended HI gas in a massive elliptical galaxy beyond the local universe, using gravitational lensing and UV spectroscopy.

Findings

Detected HI gas at 3.3 and 4.6 kpc from the galaxy center.

Measured HI column densities of log N(HI) = 19.6-19.7.

Estimated Fe mass in the cool ISM as 5-8 x 10^4 M_sun.

Abstract

We report the first detection of extended neutral hydrogen (HI) 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_sun) 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 (STIS), 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 HI column density is log N(HI) = 19.6-19.7 at both locations across the lens, comparable to what is seen in 21 cm images of nearby ellipticals. The HI gas kinematics are well-matched with the kinematics of the FeII absorption complex revealed in ground-based echelle data, displaying a large velocity shear of 360 km/s 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)x10^4 M_sun 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.