Observation of Acceleration of HI Clouds Within the Fermi Bubbles

TL;DR

This study provides evidence that HI clouds within the Fermi Bubbles are accelerating as they move away from the Galactic Center, with new observations supporting this dynamic behavior.

Contribution

It presents new Green Bank Telescope data confirming cloud acceleration and correlates this with UV absorption line kinematics, advancing understanding of Fermi Bubble dynamics.

Findings

HI clouds accelerate from ~150-200 km/s to ~330 km/s

Cloud lifetime estimated at 4-10 million years

Results consistent with UV absorption line observations

Abstract

The ~200 HI clouds observed to be entrained in the Fermi Bubble wind show a trend of increasing maximum |VLSR| with Galactic latitude. We analyze previous observations and present new data from the Green Bank Telescope that rule out systematic effects as the source of this phenomenon. Instead, it is likely evidence for acceleration of the clouds. The data suggest that clouds in the lower 2 kpc of the Fermi Bubbles, within the Bubble boundaries established from X-ray studies, have an outflow velocity that rises from ~150 - 200 km/s close to the Galactic Center and reaches ~330 km/s at a distance of 2.5 - 3.5 kpc. These parameters are also consistent with the kinematics of UV absorption lines from highly ionized species observed against two targets behind the Fermi Bubbles at $b = -6.6^{\circ}$, and $b = +11.2^{\circ}$. The implied neutral cloud lifetime is 4 - 10 Myr.