Angular momentum loss of primordial gas in Ly alpha radiation field

作者：Yajima Hidenobu^*; Khochfar Sadegh

来源：Monthly Notices of the Royal Astronomical Society, 2014, 441(1): 769-775.

DOI：10.1093/mnras/stu505

• radiative transfer
• galaxies: evolution
• galaxies: formation
• dark ages, reionization, first stars

We present results on the radiation drag exerted by an isotropic and homogeneous background of Ly alpha photons on neutral gas clouds orbiting within H-Pi 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) less than or similar to 20 km s(-1) near (r less than or similar to 3 kpc) a Population III star of 120M(circle dot) at z = 20 is lost within similar to 10(6) yr. The radiation drag is a strong function of cloud velocity that peaks at v similar to 20 km s(-1) reflecting the frequency dependence of the photon cross-section. Clouds moving with velocities larger than similar to 100 km s(-1) lose their angular momentum on time-scales of similar to 10(8) yr. At lower redshifts radiation drag becomes inefficient as the Ly alpha photon density in H-Pi regions decreases by a factor (1 + z)(3) and angular momentum is lost on time-scales greater than or similar to 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 similar to 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.

• 出版日期2014-6