Probing the Outflowing Multiphase Gas ∼1 kpc below the Galactic Center

Abstract

Comparison of ISM absorption in the UV spectrum of LS 4825, a B1 Ib-II star d=21±5 kpc from the Sun toward l = 1.67° and b = -6.63°, with ISM absorption toward an aligned foreground star at d < 7.0±1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph (STIS) show low ion absorption out to d < 7 kpc ( e.g., O I, C II, Mg II, Si II, Fe II, S II) only between 0 and 40 km s -1 , while absorption at d > 7 kpc, ~1 kpc below the galactic plane, is complex and spans -290 to + 94 km s -1 . The intermediate and high ions Si III, C IV, Si IV and N V show extremely strong absorption with multiple components from -283 to 107 km s -1 implying that the ISM ~1 kpc below the galactic center has a substantial reservoir of plasma, and more gas containing C IV and N V than in the Carina OB1 association at z = 0 kpc. Abundances and physical conditions are presented for many absorption components. The high ion absorption traces cooling transition temperature plasma probably driven by the outflowing hot gas, while the extraordinary large thermal pressure, p/k ~ 10 5 cm -3 K -1 in an absorption component at -114 km s -1 probably arises from the ram pressure of the outflowing hot gas. The observations are consistent with a flow whose ionization structure in the high ions can be understood through a combination of non-equilibrium radiative cooling and turbulent mixing.

Observation of Acceleration of H i Clouds within the Fermi Bubbles

“…These absorption velocities are consistent with the velocity range of the Hi clouds at a similar latitude. Savage et al (2017) note that the interstellar absorption toward a foreground star nearly aligned with LS 4825, but at a distance of only < 7 ± 1.7 kpc, is completely consistent with normal Galactic rotation, confirming that the high non-circular velocities arise in the Fermi Bubble and are not a general feature of the interstellar medium in the direction of the Galactic Center.…”

“…• 67, −6. • 63) shows discrete absorption features between velocities −283 km s −1 and +107 km s −1 over a range of species and ionization states, including Si III, C IV, Si IV, and N V (Savage et al 2017). Above the plane, absorption spectra toward the AGN PDS 456 at ( , b) = (10.…”

HST/COS Observations of the Warm Ionized Gaseous Halo of NGC 891

“…Another way to estimate the HVC mass of NGC 891 is by assuming the HVC has a covering factor of 50% within a radius of 10 kpc or 19 kpc. Then, the total mass of the HVC is log M = 8.1 or log M = 8.7 with the assumed radii, which is also comparable with the MW (log M = 7.9; Putman et al 2012) In the MW, studies of sightlines towards the galactic center discovered that HVCs are common (Fox et al 2015;Savage et al 2017). The metallicity measurements indicate the disk origin of these gases with [S/H] = 0.02 or [S/H] = 1.38 (Savage et al 2017), which are consistent with the outflow from the disk.…”

“…Then, the total mass of the HVC is log M = 8.1 or log M = 8.7 with the assumed radii, which is also comparable with the MW (log M = 7.9; Putman et al 2012) In the MW, studies of sightlines towards the galactic center discovered that HVCs are common (Fox et al 2015;Savage et al 2017). The metallicity measurements indicate the disk origin of these gases with [S/H] = 0.02 or [S/H] = 1.38 (Savage et al 2017), which are consistent with the outflow from the disk. In the case of NGC 891, we found the HVC in LQAC 035+042 003 cannot be an outflow due to the low metallicity, but it is not clear whether this system is close to the galactic center.…”

“…In the outflow model, the geometry has to be biconical to have the observed radial velocity, since outflows perpendicular to the disk will have a zero radial velocity for edge-on galaxies. Then, the line widths of different ions are mainly determined by the gas density distribution along the sightline (Fox et al 2015;Savage et al 2017).…”