Probing the Southern Fermi Bubble in Ultraviolet Absorption using Distant AGNs

TL;DR

This study uses UV absorption spectroscopy from Hubble's COS instrument to investigate the gas kinematics and ionization in the southern Fermi Bubble, revealing evidence of a decelerating outflow and ionization variations.

Contribution

It provides new insights into the southern Fermi Bubble's gas dynamics and ionization state using UV absorption-line data, complementing previous studies of the northern Bubble.

Findings

High-velocity clouds are detected in the Bubble with decreasing velocities at higher latitudes.

Evidence of a decelerating outflow within the Bubble based on velocity measurements.

Ionization ratios vary with galactic latitude, indicating changing physical conditions.

Abstract

The Fermi Bubbles are two giant gamma-ray emitting lobes extending 55$^{\circ}$ above and below the Galactic Center. While the Northern Bubble has been extensively studied in ultraviolet (UV) absorption, little is known about the gas kinematics of the southern Bubble. We use UV absorption-line spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to probe the southern Fermi Bubble using a sample of 17 background AGN projected behind or near the Bubble. We measure the incidence of high-velocity clouds (HVC), finding that four out of six sightlines passing through the Bubble show HVC absorption, versus six out of eleven passing outside. We find strong evidence that the maximum absolute LSR velocity of the HVC components decreases as a function of galactic latitude within the Bubble, for both blueshifted and redshifted components, as expected for a decelerating outflow. We explore whether the column-density ratios SiIV/SiIII, SiIV/SiII and SiIII/SiII correlate with the absolute galactic latitude within the Bubble. These results demonstrate the use of UV absorption-line spectroscopy to characterize the kinematics and ionization conditions of embedded clouds in the Galactic Center outflow.