Dwarf galaxies and the black hole scaling relations

Andrew King,Rebecca Nealon

doi:10.1093/mnrasl/slaa200

Abstract

ABSTRACT The sample of dwarf galaxies with measured central black hole masses M and velocity dispersions σ has recently doubled, and gives a close fit to the extrapolation of the M - σ relation for more massive galaxies. We argue that this is difficult to reconcile with suggestions that the scaling relations between galaxies and their central black holes are simply a statistical consequence of assembly through repeated mergers. This predicts black hole masses significantly larger than those observed in dwarf galaxies unless the initial distribution of uncorrelated seed black hole and stellar masses is confined to much smaller masses than earlier assumed. It also predicts a noticeable flattening of the M - σ relation for dwarfs, to M ∝ σ2 compared with the observed M ∝ σ4. In contrast black hole feedback predicts that black hole masses tend towards a universal M ∝ σ4 relation in all galaxies, and correctly gives the properties of powerful outflows recently observed in dwarf galaxies. These considerations emphasize once again that the fundamental physical black hole – galaxy scaling relation is between M and σ. The relation of M to the bulge mass Mb is acausal, and depends on the quite independent connection between Mb and σ set by stellar feedback.

Highlights

It is widely accepted that the centre of every medium– or high–mass ( 1010M ) galaxy contains a supermassive black hole (SMBH)
In this paper we argue that recent observations of the central black holes in dwarf galaxies distinguish sharply between two approaches to understanding the scaling relations
One picture of these relations uses the fact that the SMBH binding energy EBH = ηM c2 is typically > 1000× the binding energy ∼ fgMbσ2 of the bulge gas of the host galaxy

Summary

INTRODUCTION

It is widely accepted that the centre of every medium– or high–mass ( 1010M ) galaxy contains a supermassive black hole (SMBH). To improve the fit to the observed M −Mb relation, Jahnke & Maccio (2011) go beyond the pure merger picture by adding in the effects of star formation, black hole accretion, and the conversion to bulge mass of a fraction of the stars formed in the disc component of each halo They do not explicitly derive an M − σ relation, but for high–mass galaxies this follows, since the FJ relation gives Mb ∝ σ4, which implies M ∝ σ4. Since the predicted low–redshift scatter scales roughly as N , and is about an order of magnitude above observations, this reduces the required initial scatter in black hole mass to a factor 100 This already makes the assembly picture considerably less attractive, but a second problem for it appears in deriving the M − σ relation at low galaxy masses. We plot the original M ∝ σ4 relation (2) for comparison

THE BLACK–HOLE VS BULGE–MASS RELATION FOR DWARF GALAXIES

OUTFLOWS FROM DWARF GALAXIES

CONCLUSION