Growth problems of stellar black holes in early galaxies
  The nature of the seeds of the observed high-z super-massive black holes
(SMBH) is unknown. Although different options have been proposed, involving
e.g. intermediate mass direct collapse black holes, BH remnants of massive
stars remain the most natural explanation. To identify the most favorable
conditions (if any) for their rapid growth, we study the accretion rate of a
M_BH = 100M_sun BH formed in a typical z = 10 galaxy under different conditions
(e.g. galaxy structure, BH initial position and velocity). We model the galaxy
baryonic content and follow the BH orbit and accretion history for 300 Myr (the
time span in 10 > z > 7), assuming the radiation-regulated accretion model by
Park & Ricotti (2013). We find that, within the limits of our model, BH seeds
cannot grow by more than 30 percent, suggesting that accretion on light-seed
models are inadequate to explain high-z SMBH. We also compute the X-ray
emission from such accreting stellar BH population in the (0.5, 8) keV band and
find it comparable to the one produced by high-mass X-ray binaries. This study
suggests that early BHs, by X-ray pre-heating of the intergalactic medium at
cosmic dawn, might leave a specific signature on the HI 21 cm line power
spectrum potentially detectable with SKA.
