Selection Bias in Observing the Cosmological Evolution of the Mbh-sigma and Mbh-L Relationships

TL;DR

This paper discusses how selection biases in observing black hole-galaxy property relations at different redshifts can lead to false signals of evolution, especially for the most massive black holes, due to cosmic scatter effects.

Contribution

It identifies and quantifies the selection bias in high-redshift versus local galaxy samples, highlighting its impact on observed Mbh-sigma and Mbh-L relations.

Findings

Bias can cause overestimation of black hole masses at high redshift.

The bias can reach a factor of 3 for Mbh-sigma and 9 for Mbh-L relations.

Correcting for this bias requires consistent object selection across redshifts.

Abstract

Programs to observe evolution in the Mbh-sigma or Mbh-L relations typically compare black-hole masses, Mbh, in high-redshift galaxies selected by nuclear activity to Mbh in local galaxies selected by luminosity L, or stellar velocity dispersion sigma. Because AGN luminosity is likely to depend on Mbh, selection effects are different for high-redshift and local samples, potentially producing a false signal of evolution. This bias arises because cosmic scatter in the Mbh-sigma and Mbh-L relations means that the mean log(L) or log(sigma) among galaxies that host a black hole of given Mbh, may be substantially different than the log(L) or log(sigma) obtained from inverting the Mbh-L or Mbh-sigma relations for the same nominal Mbh. The bias is particularly strong at high Mbh, where the luminosity and dispersion functions of galaxies are falling rapidly. The most massive black holes occur more often as rare outliers in galaxies of modest mass than in the even rarer high-mass galaxies, which would otherwise be the sole location of such black holes in the absence of cosmic scatter. Because of this bias, Mbh will typically appear to be too large in the distant sample for a given L or sigma. For the largest black holes and the largest plausible cosmic scatter, the bias can reach a factor of 3 in Mbh for the Mbh-sigma relation and a factor of 9 for the Mbh-L relation. Unfortunately, the actual cosmic scatter is not known well enough to correct for the bias. Measuring evolution of the Mbh and galaxy property relations requires object selection to be precisely defined and exactly the same at all redshifts.