Probing the Outflowing Multiphase Gas ~1 kpc Below the Galactic Center

TL;DR

This study investigates the complex, multiphase gas outflowing approximately 1 kpc below the Galactic Center, revealing a substantial reservoir of plasma and high ions driven by hot gas outflows, with implications for understanding galactic nuclear winds.

Contribution

It provides the first detailed analysis of the multiphase outflowing gas at ~1 kpc below the Galactic Center using UV absorption spectra, highlighting the presence of extensive high-ion absorption and physical conditions.

Findings

Detection of complex high-ion absorption spanning -290 to +94 km/s.

Identification of a large reservoir of plasma with high thermal pressure.

Evidence supporting non-equilibrium cooling and turbulent mixing in the outflow.

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 deg and b = -6.63 deg, 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, while absorption at d > 7 kpc, ~1 kpc below the galactic plane, is complex and spans -290 to + 94 km/ s. 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 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 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.