MOIRCS Deep Survey III: Active Galactic Nuclei in Massive Galaxies at z=2-4

TL;DR

This study examines the X-ray properties of massive, high-redshift galaxies (z=2-4) revealing a high AGN activity rate among the most massive galaxies, supporting the idea of early black hole growth consistent with local universe ratios.

Contribution

It provides new insights into the AGN activity in massive galaxies at high redshift and suggests a large active phase duration in their evolution.

Findings

90% of X-ray sources are in galaxies with stellar mass > 10^{10.5} Msun

33% of galaxies with stellar mass > 10^{11} Msun host active AGNs

Black hole accretion rates are 1-50% of the Eddington limit

Abstract

We investigate the X-ray properties of the K-band-selected galaxies at redshift 2 < z < 4 by using our deep near-infrared images obtained in the MOIRCS Deep Survey project and the published Chandra X-ray source catalog. 61 X-ray sources with the 2-10 keV luminosity L_X = 10^{42}-10^{44} erg/s are identified with the K-selected galaxies and we found that they are exclusively (90%) associated with the massive objects with stellar mass larger than 10^{10.5} Msun. Our results are consistent with the idea that the M_BH/M_str ratio of the galaxies at z=2-4 is similar to the present-day value. On the other hand, the AGN detection rate among the very massive galaxies with the stellar mass larger than 10^{11} Msun is high, 33% (26/78). They are active objects in the sense that the black-hole mass accretion rate is ~ 1-50% of the Eddington limit if they indeed have similar M_BH/M_str ratio with those observed in the local universe. The active duration in the AGN duty cycle of the high-redshift massive galaxies seems large.