Theoretical reevaluations of the black hole mass - bulge mass relation - I. Effect of the seed black hole mass

TL;DR

This study investigates how the initial seed black hole mass influences the black hole mass - bulge mass relation in galaxies at z ~ 0, using semi-analytic models and cosmological simulations, highlighting the importance of seed mass assumptions.

Contribution

It demonstrates that seed black hole masses of 10^3 M_sun align better with observed relations than larger seeds, refining models of galaxy evolution.

Findings

Seed black holes of 10^5 M_sun produce inconsistent relations with observations.

Seed black holes of 10^3 M_sun or within 10^(3-5) M_sun range match observed data.

Observations of low-mass bulges at z ~ 0 provide stronger constraints on seed black hole mass.

Abstract

We explore the effect of varying the mass of the seed black hole on the resulting black hole mass - bulge mass relation at z ~ 0, using a semi-analytic model of galaxy formation combined with large cosmological N-body simulations. We constrain our model by requiring the observed properties of galaxies at z ~ 0 are reproduced. In keeping with previous semi-analytic models, we place a seed black hole immediately after a galaxy forms. When the mass of the seed is set at 10^5 M_sun, we find that the model results become inconsistent with recent observational results of the black hole mass - bulge mass relation for dwarf galaxies. In particular, the model predicts that bulges with ~ 10^9 M_sun harbour larger black holes than observed. On the other hand, when we employ seed black holes with 10^3 M_sun, or randomly select their mass within a 10^(3-5) M_sun range, the resulting relation is consistent with observation estimates, including the observed dispersion. We find that to obtain stronger constraints on the mass of seed black holes, observations of less massive bulges at z ~ 0 are a more powerful comparison than the relations at higher redshifts.