High Velocity Neutral Gas in the Fermi Bubbles: New Kinematic Limits and Spatial Structure

Abstract

We have detected hundreds of neutral clouds entrained in the Milky Way's nuclear wind using HI data from new surveys made with the Green Bank Telescope that cover about 500 sq-degrees around the Galactic center (GC). Galactic winds are common throughout the Universe, and these data at 9.1' angular resolution (22 pc at the GC) provide the most detailed analysis of the vertical profile of a neutral nuclear wind in any galaxy. A set of 228 of these Fermi Bubble clouds with the largest values of |VLSR| has been analyzed to examine the distribution and kinematics of the outflowing gas. The clouds span -335 km/s $\leq$ VLSR $\leq$ +438 km/s, the largest positive LSR velocities ever reported for neutral HI associated with the Milky Way disk. The highest velocities are found furthest from the GC, suggesting that clouds are accelerated from a low velocity near the nucleus to at least 500 km/s at a radial distance of $\lesssim 4$ kpc. Clouds appear disrupted as they are accelerated: their line brightness and NHI decreases steadily with distance from the GC, and the population becomes more uniform. There is an abrupt cutoff in the neutral clouds at a vertical distance of $\approx2$ kpc from the Galactic plane. Kinematic models of an outflowing cloud population that fills the FB volume are used to identify structure in the gas. The kinematics of the highest velocity, highest latitude clouds imply a past azimuthal asymmetry in the outflow.