  We present results on the radiation drag exerted by an isotropic and
homogenous background of Lya photons on neutral gas clouds orbiting within HII
regions around Population III stars of different masses. The Doppler shift
causes a frequency difference between photons moving in the direction of the
cloud and opposite to it resulting in a net momentum loss of the cloud in the
direction of motion. We find that half of the angular momentum of gas with
v_theta <~ 20 km/s near (r <~ 3 kpc) a Population III star of 120 Msun at z=20
is lost within ~ 10^6 yr. The radiation drag is a strong function of cloud
velocity that peaks at v ~ 20 km/s reflecting the frequency dependence of the
photon cross section. Clouds moving with velocities larger than ~ 100 km/s
loose their angular momentum on time scales of ~ 10^8 yr. At lower redshifts
radiation drag becomes inefficient as the Lya photon density in HII regions
decreases by a factor (1+z)^3 and angular momentum is lost on time scales >
10^8 yr even for low velocity clouds. Our results suggest that a sweet spot
exists for the loss of angular momentum by radiation drag for gas clouds at z >
10 and with v ~ 20 km/s. Comparison to dynamical friction forces acting on
typical gas clouds suggest that radiation drag is the dominant effect impacting
the orbit. We propose that this effect can suppress the formation of extended
gas discs in the first galaxies and help gas accretion near galactic centres
and central black holes.
