The Black Hole - Bulge Mass Relation in Megamaser Host Galaxies

TL;DR

This study uses high-resolution HST images of nine megamaser host galaxies to analyze their nuclear structures and examine black hole-bulge mass relations, revealing lower-than-expected black hole masses and significant scatter in spiral galaxies.

Contribution

It provides detailed photometric decompositions of megamaser host galaxy nuclei and investigates their black hole scaling relations, highlighting differences from early-type galaxies.

Findings

Black hole masses range from 10^6 to 10^8 solar masses.

Host galaxy stellar masses are around 10^11 solar masses.

Black hole masses are lower than expected from early-type galaxy relations.

Abstract

We present HST images for nine megamaser disk galaxies with the primary goal of studying photometric BH-galaxy scaling relations. The megamaser disks provide the highest-precision extragalactic BH mass measurements, while our high-resolution HST imaging affords us the opportunity to decompose the complex nuclei of their late-type hosts in detail. Based on the morphologies and shapes of the galaxy nuclei, we argue that most of these galaxies' central regions contain secularly evolving components (pseudo-bulges), and in many cases we photometrically identify co-existing "classical" bulge components as well. Using these decompositions, we draw the following conclusions: (1) The megamaser BH masses span two orders of magnitude ($10^6$ -- $10^8 M_\odot$) while the stellar mass of their spiral host galaxies are all $\sim 10^{11} M_\odot$ within a factor of three; (2) the BH masses at a given bulge mass or total stellar mass in the megamaser host spiral galaxies tend to be lower than expected, when compared to an extrapolation of the BH-bulge relation based on early-type galaxies; (3) the observed large intrinsic scatter of BH masses in the megamaser host galaxies raises the question of whether scaling relations exist in spiral galaxies.