Do Nuclear Star Clusters and Supermassive Black Holes Follow the Same Host-Galaxy Correlations?

Peter Erwin,Dimitri Alexei Gadotti

doi:10.1155/2012/946368

Abstract

Studies have suggested that there is a strong correlation between the masses of nuclear star clusters (NSCs) and their host galaxies, a correlation which is said to be an extension of the well-known correlations between supermassive black holes (SMBHs) and their host galaxies. But careful analysis of disk galaxies—including 2D bulge/disk/bar decompositions—shows that while SMBHs correlate with the stellar mass of thebulgecomponent of galaxies, the masses of NSCs correlate much better with thetotalgalaxy stellar mass. In addition, the mass ratioMNSC/M⋆, totfor NSCs in spirals (at least those with Hubble types Sc and later) is typically an order of magnitude smaller than the mass ratioMBH/M⋆, bulof SMBHs. The absence of a universal “central massive object” correlation argues against common formation and growth mechanisms for both SMBHs and NSCs. We also discuss evidence for a break in the NSC-host galaxy correlation, galaxies with Hubble types earlier than Sbc appear to host systematically more massive NSCs than do types Sc and later.

Highlights

As far as we can tell, all massive galaxies in the local universe harbor supermassive black holes (SMBHs, with masses MBH ∼ 106–109M )
Studies have suggested that there is a strong correlation between the masses of nuclear star clusters (NSCs) and their host galaxies, a correlation which is said to be an extension of the well-known correlations between supermassive black holes (SMBHs) and their host galaxies
Careful analysis of disk galaxies—including 2D bulge/disk/bar decompositions—shows that while SMBHs correlate with the stellar mass of the bulge component of galaxies, the masses of NSCs correlate much better with the total galaxy stellar mass

Summary

Introduction

As far as we can tell, all massive galaxies in the local universe harbor supermassive black holes (SMBHs, with masses MBH ∼ 106–109M ). The NSCs we focus on are taken primarily from the sample of Walcher et al [21], with additional data from Ho and Filippenko [23], Boker et al [24], Kormendy and Bender [25], Matthews et al [26] and Gebhardt et al [27], Barth et al [28], Seth et al [29], and Kormendy et al [30]; we use the estimate of Launhardt et al [31] for the Milky Way’s NSC This gives us a total of 18 galaxies with dynamically determined NSC masses. We use 2D image decompositions via the BUDDA software package to determine the B/T ratios, and the bulge stellar masses, for SMBH host galaxies and for barred NSC host galaxies. A 1D decomposition for this galaxy gives a B/T value almost twice as large (0.030); similar results were found for four other barred galaxies in the sample, with mean B/T values a factor of 2.1 times larger when the bar was omitted; see [37]

Comparing Black Holes and Nuclear Star Clusters

Findings

Trends with Hubble Type