A Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z<2

TL;DR

This paper investigates the growth of supermassive black holes in elliptical galaxies since redshift 2, finding significant SMBH mass offsets that suggest a preferential growth mechanism or underestimated biases.

Contribution

It provides evidence for redshift-dependent SMBH growth in elliptical galaxies, highlighting a potential preferential growth channel for SMBHs at z<2.

Findings

SMBH mass offsets reach a factor of 7 between z~1 and z~0.

Offsets in stellar mass are small and likely due to measurement bias.

SMBH offsets may reach a factor of 20 at z~2.

Abstract

The assembly of stellar and supermassive black hole (SMBH) mass in elliptical galaxies since $z\sim1$ can help to diagnose the origins of locally-observed correlations between SMBH mass and stellar mass. We therefore construct three samples of elliptical galaxies, one at $z\sim0$ and two at $0.7\lesssim z \lesssim2.5$, and quantify their relative positions in the $M_{BH}-M_*$ plane. Using a Bayesian analysis framework, we find evidence for translational offsets in both stellar mass and SMBH mass between the local sample and both higher redshift samples. The offsets in stellar mass are small, and consistent with measurement bias, but the offsets in SMBH mass are much larger, reaching a factor of seven between $z\sim1$ and $z\sim0$. The magnitude of the SMBH offset may also depend on redshift, reaching a factor of $\sim20$ at $z\sim 2$. The result is robust against variation in the high and low redshift samples and changes in the analysis approach. The magnitude and redshift evolution of the offset are challenging to explain in terms of selection and measurement biases. We conclude that either there is a physical mechanism that preferentially grows SMBHs in elliptical galaxies at $z\lesssim 2$, or that selection and measurement biases are both underestimated, and depend on redshift.