Impact of Accretion Flow Dynamics on Gas-dynamical Black Hole Mass Estimates

TL;DR

This paper introduces a non-Keplerian gas disk model that explains discrepancies in SMBH mass estimates from gas dynamics versus stellar kinematics, emphasizing the importance of velocity dispersion measurements.

Contribution

It presents a simple non-Keplerian gas disk model that can reconcile differences in SMBH mass estimates and improve the accuracy of gas-dynamical measurements.

Findings

Non-Keplerian models produce higher, narrower velocity dispersions.

Velocity curves of non-Keplerian models differ from Keplerian ones away from the center.

The model can resolve discrepancies between gas and stellar kinematic SMBH mass estimates.

Abstract

At low redshift, the majority of supermassive black hole (SMBH) mass estimates are obtained from modeling stellar kinematics or ionized gas dynamics in the vicinity of the galaxy nucleus. For large early type galaxies, stellar kinematics models predict higher masses than gas-dynamical models. In the case of M87, this discrepancy is larger than 2 $\sigma$. Critical to gas-dynamical modeling is the assumed underlying dynamical state of the gas: that it lies on circular Keplerian orbits, potentially with some additional turbulent pressure support. This is inconsistent with models of the gas flow about low-accretion-rate SMBHs and at odds with observations of the Galactic Center. We present a simple model for non-Keplerian gas disks and explore their implications for SMBH mass measurements. We show that a larger central black hole with gas experiencing small amounts of sub-Keplerian motion can produce velocity curves similar to models that just contain circular Keplerian motions and a lower black hole mass. However, these non-Keplerian models are distinguishable from low-mass Keplerian models primarily through measurements of the velocity dispersion, wherein non-Keplerian models produce higher and narrower peak dispersions. Away from the galaxy center, but still within the circumnuclear gas disk, non-Keplerian models also become distinguishable from Keplerian models via a shift in the velocity curve. The velocity model presented in this paper is capable of resolving the discrepancy between the ionized gas dynamics and stellar kinematics mass estimates, and is applicable to gas-dynamical mass estimates of SMBHs in general.