Interpreting the possible break in the Black Hole - Bulge mass relation

TL;DR

This paper investigates the break in the black hole - bulge mass relation at lower masses, attributing it mainly to stellar feedback effects in star-forming bulges, supported by semi-analytic galaxy formation models.

Contribution

It demonstrates that stellar feedback is the primary driver of the break in the black hole-bulge mass relation, challenging previous emphasis on AGN feedback and other mechanisms.

Findings

The break is mainly caused by stellar feedback in star-forming bulges.

The relation's shape is insensitive to BH accretion details.

Most stars form in disks and are transferred to bulges via mergers and instabilities.

Abstract

Recent inspections of local available data suggest that the almost linear relation between the stellar mass of spheroids ($M_{\rm sph}$) and the mass of the super massive Black Holes (BHs) residing at their centres, shows a break below $M_{\rm sph} \sim 10^{10}\ {\rm M}_\odot$, with a steeper, about quadratic relation at smaller masses. We investigate the physical mechanisms responsible for the change in slope of this relation, by comparing data with the results of the semi-analytic model of galaxy formation MORGANA, which already predicted such a break in its original formulation. We find that the change of slope is mostly induced by effective stellar feedback in star-forming bulges. The shape of the relation is instead quite insensitive to other physical mechanisms connected to BH accretion such as disc instabilities, galaxy mergers, Active Galactic Nucleus (AGN) feedback, or even the exact modelling of accretion onto the BH, direct or through a reservoir of low angular momentum gas. Our results support a scenario where most stars form in the disc component of galaxies and are carried to bulges through mergers and disc instabilities, while accretion onto BHs is connected to star formation in the spheroidal component. Therefore, a model of stellar feedback that produces stronger outflows in star-forming bulges than in discs will naturally produce a break in the scaling relation. Our results point to a form of co-evolution especially at lower masses, below the putative break, mainly driven by stellar feedback rather than AGN feedback.