Highly Ionized Envelopes of High Velocity Clouds

TL;DR

This study investigates the highly ionized gas in Galactic High-Velocity Clouds, revealing their composition, distribution, and potential role in galaxy evolution through new UV spectroscopic observations.

Contribution

The paper provides new measurements of ionized gas in HVCs using FUSE and HST data, estimating their mass, infall rate, and physical properties, advancing understanding of their role in galactic gas accretion.

Findings

Detected SiIII absorption with 81% sky coverage in HVCs.

HVCs contain approximately 10^8 solar masses of low-metallicity ionized gas.

Infall rate of about 1 solar mass per year suggests significant contribution to galactic star formation.

Abstract

We present recent results on highly ionized gas in Galactic High-Velocity Clouds (HVCs), originally surveyed in OVI (Sembach et al. 2003). In a new FUSE/HST survey of SiII/III/IV (Shull et al. 2009) toward 37 AGN, we detected SiIII (lambda 1206.500 A) absorption with a sky coverage fraction 81 +/- 5% (61 HVCs along 30 of 37 high-latitude sight lines). The SiIII (lambda 1206.500 A) line is typically 4-5 times stronger than OVI (lambda 1031.926 A). The mean HVC column density of perhaps 10^19 cm^-2 of low-metallicity (0.1 - 0.2 Z_sun) ionized gas in the low halo. Recent determinations of HVC distances allow us to estimate a total reservoir of ~10^8 M_sun. Estimates of infall velocities indicate an infall rate of around 1 M_sun yr^-1, comparable to the replenishment rate for star formation in the disk. HVCs appear to be sheathed by intermediate-temperature gas (10^4.0 - 10^4.5 K) detectable in SiIII and SiIV, as well as hotter gas seen in OVI and other high ions. To prepare for HST observations of 10 HVC-selected sight lines with the Cosmic Origins Spectrograph (COS), we compile FUSE/STIS spectra of these ions, plus FeIII, CIII, CIV, and SIV. Better constraints on the physical properties of HVC envelopes and careful treatment of HVC kinematics and infall rates should come from high-quality (S/N ~ 30-40) COS data.