A LARGE RESERVOIR OF IONIZED GAS IN THE GALACTIC HALO: IONIZED SILICON IN HIGH-VELOCITY AND INTERMEDIATE-VELOCITY CLOUDS

Abstract

The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M☉ yr−1) cooling inflow to replenish star formation in the disk. Using absorption spectra from the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer toward 37 active galactic nuclei at high latitude, we detect widespread interstellar Si iii λ1206.50 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at ℓ = 180°, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81% ± 5% for 30 out of 37 directions) with Si iii optical depth typically 4–5 times that of O vi λ1031.926. The mean HVC column density per sight line, 〈log NSi III〉 = 13.42 ± 0.21, corresponds to total column density NHII ≈ (6 × 1018 cm−2)(ZSi/0.2Z☉)−1 of ionized low-metallicity gas, similar to that inferred in O vi. This reservoir could total 108M☉ and produce a mass infall rate ∼1 M☉ yr−1. By modeling Si ii, Si iii, Si iv, and H i in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, 〈log U〉 = −3.0+0.3−0.4, approximately 10 times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [Si ii/H i], are 10%–30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. These formally lower matallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The HVC and IVC metallicities may be compared with the median photometric metallicity, [Fe/H] = −1.46 ± 0.30, for ∼200,000 halo F/G stars in the Sloan Digital Sky Survey.