X-ray AGN in Bo\"otes: The lack of growth of the most massive black holes since z=4

TL;DR

This study investigates the growth history of supermassive black holes in massive galaxies using X-ray data from the Boötes field, revealing that the most massive black holes have not grown significantly since redshift 4, suggesting early rapid formation.

Contribution

It provides new insights into black hole growth by analyzing AGN activity across redshifts and stellar masses, highlighting the early formation of the most massive black holes before redshift 4.

Findings

Most black hole mass accumulated before z=4 for lower mass BHs.

Most massive black holes show little to no growth since z=4.

AGN fraction shows weak dependence on stellar mass.

Abstract

Supermassive Black Holes (BHs) are known to efficiently grow through gas accretion, but even sustained and intense mass build-up through this mechanism struggles to explain the assembly of the most massive BHs observed in the local Universe. Using the Chandra Deep-Wide Field Survey (CDFWS) in the Bo\"otes field, we measure BH--galaxy assembly in massive galaxies ($M_\star\gtrsim10^{10}\rm M_\odot$) through the AGN fraction and specific Black Hole accretion rate (sBHAR) distribution as a function of redshift and stellar mass. We determine stellar masses and star formation rates for a parent sample of optically selected galaxies as well as those with X-ray detections indicating the presence of an AGN through Spectral Energy Distribution (SED) fitting. We derive a redshift-dependent mass completeness limit and extract X-ray information for every galaxy as to provide a comprehensive picture of the AGN population in massive galaxies. While X-ray AGN samples are dominated by moderately massive host galaxies of $M_{\star} \geqslant 10^{10}\rm M_{\odot}$, we do not find a strong stellar mass dependence in AGN fraction (to limits in sBHAR), indicating a bias towards massive galaxies in the observed samples. We derive BH-galaxy growth tracks over time, which reveal that while most BH mass has been accumulated since $z=4$ for lower mass BHs, the assembly of the most massive BHs is more complex, with little to no relative mass gain since $z=4$, implying that rapid and intense growth episodes prior to $z=4$ were necessary to form these massive BHs.