KwangHo Park, Massimo Ricotti
In this paper, the third of a series, we study the growth rate and luminosity of black holes (BHs) in motion with respect to their surrounding medium by running a large set of 2D axis-symmetric radiation-hydrodynamic simulations. Contrary to the case without radiation feedback, we find that the accretion rate increases with increasing BH velocity v reaching a maximum value at v = 2c_s ~ 50 km/s, where c_s is the sound speed inside the "cometary-shaped" HII region around the BH, before decreasing as v^{-3}. The increase of the accretion rate with v is produced by the formation of a D-type (density) ionization front (I-front) preceded by a standing bow-shock that reduces the downstream gas velocity to transonic values. Since the I-front is beyond the classical Bondi radius for the hot ionized gas, the accretion flow in the BH frame of reference is similar to the stationary case. Interestingly, there is a range of densities and velocities in which the dense shell downstream of the bow-shock is unstable; its central part is destroyed and reformed periodically, producing a periodic accretion rate with peak values about 10 times the mean. This effect can significantly increase the detectability of accreting intermediate mass BHs from the ISM in nearby galaxies. For v>2c_s, the central part of the bow-shock is not able to regenerate, the I-front becomes R-type and the accretion rate approaches the classical Bondi-Hoyle-Lyttleton solution. We find that the maximum accretion rate for a moving BH is larger than that of a stationary BH of the same mass, accreting from the same medium if the medium temperature is T<10^4 K. This result could have an important impact on our understanding of the growth of seed BHs in the multi-phase medium of the first galaxies and for building and early X-ray background that may affect the formation of the first galaxies and the reionization process.
View original:
http://arxiv.org/abs/1211.0542
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