Black Hole Mass Scaling Relations for Spiral Galaxies. I. $M_{\rm BH}$-$M_{\rm *,sph}$

TL;DR

This study refines the $M_{BH}$-$M_{*,sph}$ relation for spiral galaxies using detailed multicomponent decompositions, revealing a nonlinear, near-quadratic relation that challenges previous merger-based models and includes new correlations with Sersic index and spiral-arm pitch angle.

Contribution

It provides the most comprehensive analysis to date of the $M_{BH}$-$M_{*,sph}$ relation in spiral galaxies, incorporating detailed galaxy decompositions and new correlations with galaxy structural parameters.

Findings

The $M_{BH}$-$M_{*,sph}$ relation is nonlinear with a near-quadratic slope.

Bulges with AGNs follow the same relation as inactive bulges.

New correlations found between $M_{BH}$ and Sersic index, and $M_{*,sph}$ and spiral-arm pitch angle.

Abstract

The (supermassive black hole mass, $M_\text{BH}$)-(bulge stellar mass, $M_{\rm*,sph}$) relation is, obviously, derived using two quantities. We endeavor to provide accurate values for the latter via detailed multicomponent galaxy decompositions for the current full sample of 43 spiral galaxies having directly measured $M_\text{BH}$ values; 35 of these galaxies have been alleged to contain pseudobulges, 21 have water maser measurements, and three appear bulgeless. This more than doubles the previous sample size of spiral galaxies with a finessed image analysis. We have analyzed near-infrared images, accounting for not only the bulge, disk (exponential, truncated, or inclined), and bar but also for spiral arms and rings and additional central components (active galactic nuclei (AGNs), etc.). A symmetric Bayesian analysis finds $\log\left(M_\text{BH}/M_{\odot}\right)=\left(2.44_{-0.31}^{+0.35}\right)\log\left\{M_{\rm*,sph}/[\upsilon(1.15\times10^{10}\,M_{\odot})]\right\}+(7.24\pm0.12)$, with $\upsilon$ a stellar mass-to-light ratio term. The level of scatter equals that about the $M_{\rm BH}$-$\sigma_*$ relation. The nonlinear slope rules out the idea that many mergers, coupled with the central limit theorem, produced this scaling relation, and it corroborates previous observational studies and simulations, which have reported a near-quadratic slope at the low-mass end of the $M_\text{BH}$-$M_{\rm*,sph}$ diagram. Furthermore, bulges with AGNs follow this relation; they are not offset by an order of magnitude, and models that have invoked AGN feedback to establish a linear $M_{\rm BH}$-$M_{\rm*,sph}$ relation need revisiting. We additionally present an updated $M_\text{BH}$-(S\'ersic index, $n_\text{sph}$) relation for spiral galaxy bulges with a comparable level of scatter and a new $M_{\rm*,sph}$-(spiral-arm pitch angle, $\phi$) relation.