Mapping Outflowing Gas in the Fermi Bubbles: a UV Absorption Survey of the Galactic Nuclear Wind

TL;DR

This study uses UV spectra from Hubble to analyze cool gas outflows in the Fermi Bubbles, revealing acceleration patterns and high-velocity clouds that inform models of galactic nuclear winds.

Contribution

First comprehensive UV absorption survey of the Fermi Bubbles' outflowing gas, combining new and existing data to analyze kinematics and cloud acceleration.

Findings

Outflowing clouds decrease in number with latitude.

UV clouds are flat or accelerating with distance from the Galactic center.

Presence of high-velocity clouds outside the Fermi Bubbles.

Abstract

Using new ultraviolet (UV) spectra of five background quasars from the Cosmic Origins Spectrograph on the Hubble Space Telescope, we analyze the low-latitude (|b|=20-30 degree) regions of the Fermi Bubbles, the giant gamma-ray emitting lobes at the Galactic Center. We combine these data with previous UV and atomic hydrogen (HI) datasets to build a comprehensive picture of the kinematics and metal column densities of the cool outflowing clouds entrained in the Fermi Bubbles. We find that the number of UV absorption components per sightline decreases as a function of increasing latitude, suggesting that the outflowing clouds become less common with increasing latitude. The Fermi Bubble HI clouds are accelerated up to b~7 degree, whereas when we model the UV Fermi Bubbles clouds deprojected flow velocities, we find that they are flat or even accelerating with distance from the Galactic center. This trend, which holds in both the northern and southern hemispheres, indicates that the nuclear outflow accelerates clouds throughout the Fermi Bubbles or has an acceleration phase followed by a coasting phase. Finally, we note the existence of several blueshifted high-velocity clouds at latitudes exceeding ~30 degree, whose velocities cannot be explained by gas clouds confined to the inside of the gamma-ray defined Fermi Bubbles. These anomalous velocity clouds are likely in front of the Fermi Bubbles and could be remnants from past nuclear outflows. Overall, these observations form a valuable set of empirical data on the properties of cool gas in nuclear winds from star-forming galaxies.