Blowing in the Milky Way wind: neutral hydrogen clouds tracing the
  Galactic nuclear outflow
  We present the results of a new sensitive survey of neutral hydrogen above
and below the Galactic Center with the Green Bank Telescope. The observations
extend up to Galactic latitude | b | < 10 deg with an effective angular
resolution of 9.5' and an average rms brightness temperature noise of 40 mK in
a 1 km/s channel. The survey reveals the existence of a population of anomalous
high-velocity clouds extending up to heights of about 1.5 kpc from the Galactic
Plane and showing no signature of Galactic rotation. These clouds have local
standard of rest velocities | Vlsr | < 360 km/s and, assuming a Galactic Center
origin, they have sizes of a few tens of parsecs and neutral hydrogen masses
spanning $10-10^5 \, M_\odot$. Accounting for selection effects, the cloud
population is symmetric in longitude, latitude, and Vlsr. We model the cloud
kinematics in terms of an outflow expanding from the Galactic Center and find
the population consistent with being material moving with radial velocity Vw ~
330 km/s distributed throughout a bi-cone with opening angle $\alpha>140$ deg.
This simple model implies an outflow luminosity $Lw > 3 \times 10^{40}$ erg/s
over the past 10 Myr, consistent with star formation feedback in the inner
region of the Milky Way, with a cold gas mass-loss rate $\lesssim 0.1 \,
M_\odot$/yr. These clouds may represent the cold gas component accelerated in
the nuclear wind driven by our Galaxy, although some of the derived properties
challenge current theoretical models of the entrainment process.
