A recent Green Bank Telescope (GBT) survey for 21cm emission from neutral clouds entrained in the Fermi Bubbles is altering our understanding of the kinematic structure of the outflow that has created the Bubbles. Studies based on previous data for several hundred HI clouds had indicated that the neutral cloud population could be characterized by an outflow filling a bi-conical volume having an opening angle of approximately 145 degrees centered on the Galactic center region (DiTeodorio et al. 2018, ApJ, 855, 33). The flow accelerates from ~175 km/s near the Galactic center to its maximum velocity of around 330 km/s over a distance ~3.5 kpc (Lockman et al. 2020, ApJ, 888,51). Now, new measurements with the GBT in regions of the sky not previously observed with high sensitivity, have detected HI clouds with a VLSR of -340 km/s and +425 km/s at longitudes within a few degrees of zero, and latitudes of 10 to 13 degrees. The LSR velocity extremes of the cloud population require an outflow velocity that is at least 400 km/s at a distance of 3.5 kpc from the Galactic Center. The newly-discovered clouds lie at least 1 kpc and possibly as much as 3 kpc from the Galactic plane, depending on their exact position within the outflow.
The location of the newly-discovered clouds is consistent with the basic model proposed earlier, but it is clear that we have not yet found the vertical limit of the neutral cloud population, nor, likely, its maximum outflow velocity. This poster will describe results from the ongoing GBT survey for high-velocity HI emission associated with the Fermi Bubbles, with implications for the energetics and lifetime of the nuclear wind.
The Green Bank Observatory is a Facility of the National Science Foundation, Operated by Associated Universities, Inc.