Growth problems of stellar black holes in early galaxies

TL;DR

This study models the growth of stellar black hole seeds in early galaxies and finds they cannot grow enough to explain high-redshift supermassive black holes, while also predicting their X-ray signatures for future detection.

Contribution

It provides a detailed simulation of stellar black hole accretion in early galaxies, showing growth limitations and potential observational signatures.

Findings

Black hole seeds grow less than 30% in 300 Myr.

Accretion on light seeds is insufficient for high-z SMBH formation.

X-ray emission from early black holes may affect the 21 cm line signal.

Abstract

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.