  We calculate the expected imprint of the ionized gas in the Milky-Way halo on
the Cosmic Microwave Background (CMB) through the kinetic Sunyaev-Zel'dovich
(kSZ) effect. Unlike other Galactic foregrounds, the halo kSZ signature covers
the full sky, generates anisotropies on large angular scales, is not
accompanied by spectral distortions, and could therefore be confused with
primordial CMB anisotropies. We construct theoretical models for various halo
components, including smooth diffuse gas, filaments of cold inflowing gas and
high velocity clouds. We find that the kSZ effect for all components is above
the sensitivity of the Planck satellite, over a range of angular scales.
However, the typical halo contribution is well below the cosmic variance noise
in the primordial CMB power spectrum. High velocity clouds could dominate the
halo contribution and better observational data is required to mask them out.
We derive expected kSZ maps based on existing data from tracers of the halo gas
distribution, such as 21cm maps of neutral hydrogen and H_alpha maps of
recombining gas. The cross-correlation of these maps with the WMAP5 data does
not yield any statistically significant signal.
