Black Holes in Pseudobulges: demography and models

TL;DR

This paper examines the distribution and growth of black holes in late-type galaxies, comparing hierarchical galaxy formation models with observational data, and highlights the need to refine models of black hole fueling during disk instabilities.

Contribution

It evaluates how different models of black hole growth align with observed black hole demographics in late-type galaxies, emphasizing the importance of disk instability processes.

Findings

Models with tight BH-bulge growth relations produce too narrow BH mass distributions.

Looser BH growth connections better match the observed abundance of low-mass BHs.

The scatter in the BH-bulge mass relation is large in late-type galaxies, requiring model adjustments.

Abstract

There is mounting evidence that a significant fraction of Black Holes (BHs) today live in late-type galaxies, including bulge-less galaxies and those hosting pseudobulges, and are significantly undermassive with respect to the scaling relations followed by their counterpart BHs in classical bulges of similar stellar (or even bulge) mass. Here we discuss the predictions of two state-of-the-art hierarchical galaxy formation models in which BHs grow via mergers and, in one, also via disk instability. Our aim is to understand if the wealth of new data on local BH demography is consistent with standard models. We follow the merger trees of representative subsamples of BHs and compute the fractional contributions of different processes to the final BH mass. We show that the model in which BHs always closely follow the growth of their host bulges, also during late disk instabilities (i.e., bars), produces too narrow a distribution of BHs at fixed stellar mass to account for the numerous low-mass BHs now detected in later-type galaxies. Models with a looser connection between BH growth and bar instability instead predict the existence of a larger number of undermassive BHs, in better agreement with the observations. The scatter in the updated local BH-bulge mass relation (with no restriction on galaxy type) appears to be quite large when including later-type systems, but it can still be managed to be reproduced within current hierarchical models. However, the fuelling of BHs during the late bar-instability mode needs to be better quantified/improved to properly fit the data. We conclude discussing how the possibly large number of BHs in later type galaxies demands for an in-depth revision of the local BH mass function and its modelling.