Abstract
Abstract We address the spatial scale, ionization structure, mass, and metal content of gas at the Milky Way disk–halo interface detected as absorption in the foreground of seven closely spaced, high-latitude halo blue horizontal branch stars with heights z = 3–14 kpc. We detect transitions that trace multiple ionization states (e.g., Ca ii, Fe ii, Si iv, C iv) with column densities that remain constant with height from the disk, indicating that the gas most likely lies within z < 3.4 kpc. The intermediate ionization state gas traced by C iv and Si iv is strongly correlated over the full range of transverse separations probed by our sight lines, indicating large, coherent structures greater than 1 kpc in size. The low ionization state material traced by Ca ii and Fe ii does not exhibit a correlation with either N H i or transverse separation, implying cloudlets or clumpiness on scales less than 10 pc. We find that the observed ratio log(N Si iv /N C iv ), with a median value of −0.69 ± 0.04, is sensitive to the total carbon content of the ionized gas under the assumption of either photoionization or collisional ionization. The only self-consistent solution for photoionized gas requires that Si be depleted onto dust by 0.35 dex relative to the solar Si/C ratio, similar to the level of Si depletion in DLAs and in the Milky Way interstellar medium. The allowed range of values for the areal mass infall rate of warm, ionized gas at the disk−halo interface is 0.0003 < dM gas/dtdA [M ⊙ kpc−2 yr−1] <0.006. Our data support a physical scenario in which the Milky Way is fed by complex, multiphase processes at its disk−halo interface that involve kiloparsec-scale ionized envelopes or streams containing parsec-scale, cool clumps.
Highlights
The gaseous halo of the Milky Way (MW) supports a dynamic baryon cycle that provides fuel for future star formation and collects the by-products of stellar evolution (Putman et al 2012)
Using the corresponding constraint on log NC,total sin b ≈ 14.9 (Figure 12) in the collisionally ionized medium (CIM) and requiring that NH,CIM < NH,hot ionized medium (HIM), we find that the hot corona of the MW must have [M/H] > 0.3 in this physical scenario
We find that C IV and Si IV are strongly correlated over the full range of transverse separations probed by our sight lines (∼40° or 2 kpc at z = 3 kpc) and with NH I from 21 cm emission
Summary
The gaseous halo of the Milky Way (MW) supports a dynamic baryon cycle that provides fuel for future star formation and collects the by-products of stellar evolution (Putman et al 2012). This cycle is often envisioned as a Galactic fountain, in which hot, overpressurized gas is ejected from the disk into the halo and subsequently cools, recombines, and rains back down onto the disk (Shapiro & Field 1976; Fraternali & Binney 2008). Despite a rich observational history of absorption and emission studies, following the complete gas cycle of star-forming galaxies, from halo to disk, has remained a challenge.
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