An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way’s circumgalactic medium and the Local Group

Abstract

<i>Context. <i/>The Milky Way is surrounded by large amounts of diffuse gaseous matter that connects the stellar body of our Galaxy with its large-scale Local Group (LG) environment.<i>Aims. <i/>To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the <i>Hubble <i/>Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen.<i>Methods. <i/>Along 270 sightlines we measure metal absorption in the lines of Si ii, Si iii, C ii, and C iv and associated H i 21 cm emission in HVCs in the velocity range | <i>v<i/><sub>LSR<sub/> | = 100–500 km s<sup>-1<sup/>. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the HVC absorption characteristics with that of damped Lyman <i>α<i/> absorbers (DLAs) and constrained cosmological simulations of the LG (CLUES project).<i>Results. <i/>The overall sky-covering fraction of high-velocity absorption is 77 ± 6 percent for the most sensitive ion in our survey, Si iii, and for column densities log <i>N<i/>(Si iii)≥ 12.1. This value is ~4–5 times higher than the covering fraction of 21 cm neutral hydrogen emission at log <i>N<i/>(H i)≥ 18.7 along the same lines of sight, demonstrating that the Milky Way’s CGM is multi-phase and predominantly ionized. The measured equivalent-width ratios of Si ii, Si iii, C ii, and C iv are inhomogeneously distributed on large and small angular scales, suggesting a complex spatial distribution of multi-phase gas that surrounds the neutral 21 cm HVCs. We estimate that the total mass and accretion rate of the neutral and ionized CGM traced by HVCs is <i>M<i/><sub>HVC<sub/> ≥ 3.0 × 10<sup>9<sup/><i>M<i/><sub>⊙<sub/> and d<i>M<i/><sub>HVC<sub/>/d<i>t<i/> ≥ 6.1 <i>M<i/><sub>⊙<sub/> yr<sup>-1<sup/>, where the Magellanic Stream (MS) contributes with more than 90 percent to this mass/mass-flow. If seen from an external vantage point, the Milky Way disk plus CGM would appear as a DLA that would exhibit for most viewing angles an extraordinary large velocity spread of Δ<i>v<i/> ≈ 400–800 km s<sup>-1<sup/>, a result of the complex kinematics of the Milky Way CGM that is dominated by the presence of the MS. We detect a velocity dipole of high-velocity absorption at low/high galactic latitudes that we associate with LG gas that streams to the LG barycenter. This scenario is supported by the gas kinematics predicted from the LG simulations.<i>Conclusions. <i/>Our study confirms previous results, indicating that the Milky Way CGM contains sufficient gaseous material to feed the Milky Way disk over the next Gyr at a rate of a few solar masses per year, if the CGM gas can actually reach the MW disk. We demonstrate that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The MS clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way’s CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way.