Angular momentum loss of primordial gas in Lyα radiation field

We present results on the radiation drag exerted by an isotropic and homogeneous background of Lyα photons on neutral gas clouds orbiting within H II 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 vθ ≲ 20 km s-1 near (r ≲ 3 kpc) a Population III star of 120 M⊙ at z = 20 is lost within ˜106 yr. The radiation drag is a strong function of cloud velocity that peaks at v ˜ 20 km s-1 reflecting the frequency dependence of the photon cross-section. Clouds moving with velocities larger than ˜100 km s-1 lose their angular momentum on time-scales of ˜108 yr. At lower redshifts radiation drag becomes inefficient as the Lyα photon density in H II regions decreases by a factor (1 + z)3 and angular momentum is lost on time-scales ≳ 108 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-1. 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.