The Relation between Black Hole Mass and Host Spheroid Stellar Mass out to z~2

TL;DR

This study investigates the evolution of the black hole mass to host spheroid stellar mass relation up to redshift 2, finding significant evolution consistent with black holes forming before their host galaxies, and suggesting secular processes influence galaxy growth.

Contribution

It provides a more accurate measurement of the black hole to spheroid mass ratio evolution out to z~2 without luminosity correction uncertainties.

Findings

Black hole to spheroid mass ratio evolves as (1+z)^{1.96±0.55}.

Black hole to total stellar mass ratio evolves as (1+z)^{1.15±0.15}.

Host galaxies show little evidence of major mergers, implying secular evolution or minor mergers dominate growth.

Abstract

We combine Hubble Space Telescope images from the Great Observatories Origins Deep Survey with archival Very Large Telescope and Keck spectra of a sample of 11 X-ray selected broad-line active galactic nuclei in the redshift range 1<z<2 to study the black hole mass - stellar mass relation out to a lookback time of 10 Gyrs. Stellar masses of the spheroidal component are derived from multi-filter surface photometry. Black hole masses are estimated from the width of the broad MgII emission line and the 3000A nuclear luminosity. Comparing with a uniformly measured local sample and taking into account selection effects, we find evolution in the form M_BH/M_spheroid ~ (1+z)^(1.96+/-0.55), in agreement with our earlier studies based on spheroid luminosity. However, this result is more accurate because it does not require a correction for luminosity evolution and therefore avoids the related and dominant systematic uncertainty. We also measure total stellar masses. Combining our sample with data from the literature, we find M_BH/M_host ~ (1+z)^(1.15+/-0.15), consistent with the hypothesis that black holes (in the range M_BH ~ 10^8-9 M_sun) predate the formation of their host galaxies. Roughly one third of our objects reside in spiral galaxies; none of the host galaxies reveal signs of interaction or major merger activity. Combined with the slower evolution in host stellar masses compared to spheroid stellar masses, our results indicate that secular evolution or minor mergers play a non-negligible role in growing both BHs and spheroids.