The Impact of the Uncertainty in Single-Epoch Virial Black Hole Mass Estimates on the Observed Evolution of the Black Hole - Bulge Scaling Relations

TL;DR

This paper investigates how uncertainties in single-epoch virial black hole mass estimates and the shape of the black hole mass function bias observed evolution in black hole-bulge relations, suggesting these biases can explain the apparent high-redshift black hole mass excess.

Contribution

It highlights a previously under-appreciated statistical bias from virial mass uncertainties and the black hole mass function shape, which can account for observed high-redshift black hole mass excesses.

Findings

Virial mass bias can significantly contribute to observed black hole mass excess.

Both virial mass bias and Lauer bias can fully explain the high-redshift black hole mass evolution.

Biases are independent and can compound to affect observed relations.

Abstract

Recent observations of the black hole (BH) - bulge scaling relations usually report positive redshift evolution, with higher redshift galaxies harboring more massive BHs than expected from the local relations. All of these studies focus on broad line quasars with BH mass estimated from virial estimators based on single-epoch spectra. Since the sample selection is largely based on quasar luminosity, the cosmic scatter in the BH-bulge relation introduces a statistical bias leading to on average more massive BHs given galaxy properties at high redshift (Lauer et al. 2007). We here emphasize a previously under-appreciated statistical bias resulting from the uncertainty of single-epoch virial BH mass estimators and the shape of the underlying (true) BH mass function, which leads to on average overestimation of the true BH masses at the high-mass end (Shen et al. 2008). We demonstrate that the latter virial mass bias can contribute a substantial amount to the observed excess in BH mass at fixed bulge properties, comparable to the Lauer et al. bias. The virial mass bias is independent of the Lauer et al. bias, hence if both biases are at work, they can largely (or even fully) account for the observed BH mass excess at high redshift.