Probing the Fermi Bubbles in Ultraviolet Absorption: A Spectroscopic Signature of the Milky Way's Biconical Nuclear Outflow

TL;DR

This study uses ultraviolet absorption spectroscopy to detect high-velocity gas in the Fermi Bubbles, revealing a biconical outflow from the Galactic Center with velocities around 900 km/s, indicating recent energetic activity.

Contribution

It provides the first UV absorption-line evidence of a biconical outflow from the Galactic Center, constraining its velocity and geometry, and linking it to the Fermi Bubbles.

Findings

Detected high-velocity absorption components at -235 and +250 km/s.

Modeled the outflow as a biconical structure with ~900 km/s velocity.

Estimated the outflow activity occurred 2.5-4 million years ago.

Abstract

Giant lobes of plasma extend 55 degrees above and below the Galactic Center, glowing in emission from gamma rays (the Fermi Bubbles) to microwaves (the WMAP haze) and polarized radio waves. We use ultraviolet absorption-line spectra from the Hubble Space Telescope to constrain the velocity of the outflowing gas within these regions, targeting the quasar PDS 456 (Galactic coordinates l,b=10.4, +11.2 degrees). This sightline passes through a clear biconical structure seen in hard X-ray and gamma-ray emission near the base of the northern Fermi Bubble. We report two high-velocity metal absorption components, at v_LSR=-235 and +250 km/s, which cannot be explained by co-rotating gas in the Galactic disk or halo. Their velocities are suggestive of an origin on the front and back side of an expanding biconical outflow emanating from the Galactic Center. We develop simple kinematic biconical outflow models that can explain these observed profiles with an outflow velocity of ~900 km/s and a full opening angle of ~110 degrees (matching the X-ray bicone). This indicates Galactic Center activity over the last ~2.5-4.0 Myr, in line with age estimates of the Fermi Bubbles. The observations illustrate the use of UV absorption-line spectroscopy to probe the properties of swept-up gas venting into the Fermi Bubbles.