Megamaser Disks Reveal a Broad Distribution of Black Hole Mass in Spiral Galaxies

TL;DR

This study uses precise water megamaser measurements to explore the relationship between supermassive black hole masses and galaxy properties in spiral galaxies, revealing a broad distribution of black hole masses with weak correlations.

Contribution

It provides new black hole mass measurements from megamasers and analyzes their relation to galaxy properties, highlighting the diversity and weak correlations in spiral galaxies.

Findings

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

No strong correlation between black hole mass and galaxy properties.

Offset in black hole mass for spiral galaxies compared to early-types.

Abstract

We use new precision measurements of black hole masses from water megamaser disks to investigate scaling relations between macroscopic galaxy properties and supermassive black hole (BH) mass. The megamaser-derived BH masses span 10^6-10^8 M_sun, while all the galaxy properties that we examine (including stellar mass, central mass density, central velocity dispersion) lie within a narrow range. Thus, no galaxy property correlates tightly with M_BH in ~L* spiral galaxies. Of them all, stellar velocity dispersion provides the tightest relation, but at fixed sigma* the mean megamaser M_BH are offset by -0.6+/-0.1 dex relative to early-type galaxies. Spiral galaxies with non-maser dynamical BH masses do not show this offset. At low mass, we do not yet know the full distribution of BH mass at fixed galaxy property; the non-maser dynamical measurements may miss the low-mass end of the BH distribution due to inability to resolve the spheres of influence and/or megamasers may preferentially occur in lower-mass BHs.