A population of weak metal-line absorbers surrounding the Milky Way

TL;DR

This paper reports the discovery of a population of weak, narrow metal-line absorbers in the Milky Way's halo, likely representing isolated, partly neutral gas clumps below current detection thresholds, analogous to weak MgII systems around other galaxies.

Contribution

It presents high-resolution UV spectra revealing a new population of low-column-density, high-velocity absorbers in the Milky Way's halo, previously undetected by 21cm surveys.

Findings

Absorbers are Lyman-Limit Systems with HI < 3x10^18 cm^-2.

They are not associated with known high-column HVC complexes.

These structures are isolated, partly neutral gas clumps in the halo or intergalactic environment.

Abstract

We report on the detection of a population of weak metal-line absorbers in the halo or nearby intergalactic environment of the Milky Way. Using high-resolution ultraviolet absorption-line spectra of bright QSOs obtained with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we have observed unsaturated, narrow absorption in OI and SiII together with mildly saturated CII absorption at high radial velocities (|v_LSR|=100-320 km/s). The measured OI column densities are small, implying that these structures represent Lyman-Limit Systems and sub-Lyman-Limit System with HI column densities < 3x10^18 cm^-2, thus below the detection limits of current 21cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently are not directly associated with any of the large high-column density HVC complexes, but rather represent isolated, partly neutral gas clumps embedded in a more tenuous, ionized gaseous medium situated in the halo or nearby intergalactic environment of the Galaxy. We speculate that this absorber population represents the local analog of weak MgII systems that are commonly observed in the circumgalactic environment of low- and high-redshift galaxies.