Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble

Abstract

We report new observations from a systematic, spectroscopic, ultraviolet absorption-line survey that maps the spatial and kinematic properties of the high-velocity gas in the Galactic Center region. We examine the hypothesis that this gas traces the biconical nuclear outflow. We use ultraviolet spectra of 47 background QSOs and halo stars projected inside and outside the northern Fermi Bubble from the Hubble Space Telescope to study the incidence of high velocity absorption around it. We use five lines of sight inside the northern Fermi Bubble to constrain the velocity and column densities of outflowing gas traced by O I, Al II, C II, C IV, Si II, Si III, Si IV and other species. All five lines of sight inside the northern Fermi Bubble exhibit blueshifted high velocity absorption components, whereas only 9 out of the 42 lines of sight outside the northern Fermi Bubble exhibit blueshifted high velocity absorption components. The observed outflow velocity profile decreases with Galactic latitude and radial distance (R) from the Galactic Center. The observed blueshifted velocities change from $v_{GSR}$=-265 km/s at R~2.3 kpc to $v_{GSR}$=-91 km/s at R~6.5 kpc. We derive the metallicity of the entrained gas along the 1H1613-097 sightline, which passes through the center of the northern Fermi Bubble, finding [O/H] $\gtrsim -0.54\pm 0.15$. A simple kinematic model tuned to match the observed absorption component velocities along the five lines of sight inside the Bubble, constrains the outflow velocities to ~1000$-$1300 km/s, and the age of the outflow to be ~ 6$-$9 Myr. We estimate a minimum mass outflow rate for the nuclear outflow to be $\gtrsim$ 0.2 ${\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. Combining the age and mass outflow rates, we determine a minimum mass of total UV absorbing cool gas entrained in the Fermi Bubbles to be $\gtrsim 2\times 10^6{\mathrm{M}}_{\odot }$.

24 pages, 9 9 figures, Accepted for Publication in ApJ

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• The MUSE Hubble Ultra Deep Field Survey: VII. FeII* Emission in Star-Forming Galaxies
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• Revealing the Ionization Properties of the Magellanic Stream using Optical Emission
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• Kinematics of the Magellanic Stream and Implications for its Ionization
• NGC 7469 as seen by MEGARA: new results from high-resolution IFU spectroscopy
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• The QuaStar Survey: Detecting Hidden Low-Velocity Gas in the Milky Way's Circumgalactic Medium
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• Characterising the turbulent multiphase halos with periodic box simulations
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• Evidence of a Cloud-Cloud Collision from Overshooting Gas in the Galactic Center
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• Virial Clumps in Central Molecular Zone Clouds
• Caught in the Act: A Metal-Rich High-Velocity Cloud in the Inner Galaxy
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• Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo
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• Modeling the Galactic center gamma-ray emission with more realistic cosmic-ray dynamics
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• Not gone with the Wind: Survival of High-Velocity Molecular Clouds in the Galactic center
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• Multi-wavelength emission from leptonic processes in ageing galaxy bubbles
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• The role of magnetic fields in shaping $\gamma$-ray emission from the Fermi bubbles
• How is Cold Gas Loaded into Galactic Nuclear Outflows?
• Diffuse Hot Plasma in the Interstellar Medium and Galactic Outflows