The Supermassive Black Hole in the Nearby Spiral Galaxy M81: A Robust Mass from JWST/NIRSpec Stellar Dynamics

TL;DR

This paper presents a precise measurement of the supermassive black hole in galaxy M81 using JWST stellar dynamics, resolving previous uncertainties and establishing a reliable mass estimate for galaxy scaling relations.

Contribution

First robust stellar-dynamical SMBH mass measurement in M81 using JWST/NIRSpec data with comprehensive Bayesian modeling.

Findings

SMBH mass in M81 is approximately 4.78 x 10^7 solar masses.

High-resolution JWST data penetrates dust and separates stellar light from AGN continuum.

Modeling confirms a central dark mass drives the velocity dispersion rise.

Abstract

Despite its proximity, the mass of the supermassive black hole (SMBH) in the spiral galaxy M81 (NGC~3031) has remained uncertain, with previous dynamical measurements being unreliable. We present the first robust stellar-dynamical measurement of its mass using high-resolution, two-dimensional kinematics from JWST/NIRSpec observations of the central $3''\times3''$. By tracing stellar motions in the near-infrared, our data penetrate the obscuring nuclear dust and allow for the separation of stellar light from the non-thermal AGN continuum. We modeled the kinematics using JAM within a Bayesian framework, exploring a comprehensive suite of models that systematically account for uncertainties in the point-spread function, orbital anisotropy, and stellar mass-to-light ratio. This ensemble modeling approach demonstrates that a central dark mass unambiguously drives the central rise in velocity dispersion. The models yield a robust SMBH mass of $M_{\rm BH} = (4.78^{+0.07}_{-0.10})\times10^7$ M$_\odot$. This result resolves a long-standing uncertainty in the mass of M81's black hole and provides a crucial, reliable anchor point for SMBH-galaxy scaling relations.