Measuring the Central Dark Mass in NGC 4258 with JWST/NIRSpec Stellar Kinematics

Abstract

We present a new stellar dynamical measurement of the supermassive black hole (SMBH) mass in the nearby spiral galaxy NGC 4258, a critical benchmark for extragalactic mass measurements. We use archival JWST/NIRSpec IFU data (G235H/F170LP grating) to extract high-resolution two-dimensional stellar kinematics from the CO bandhead absorption features within the central $3'' \times 3''$. We extract the stellar kinematics after correcting for instrumental artifacts and separating the stellar light from the non-thermal AGN continuum. We employ Jeans Anisotropic Models (JAM) to fit the observed kinematics, exploring a grid of 12 models to systematically test the impact of different assumptions for the point-spread function, stellar mass-to-light ratio ($M/L$) profile, and orbital anisotropy. All 12 models provide broadly acceptable fits, albeit with minor differences. The ensemble median and 68% (1$\sigma$) bootstrap confidence intervals of our 12 models yield a black hole mass of $M_{\rm BH} = (4.08^{+0.19}_{-0.33}) \times 10^7$ M$_\odot$. This paper showcases the utility of using the full model ensemble to robustly account for systematic uncertainties, rather than relying on formal errors from a single preferred model, as has been common practice. Our result is just 5% larger than, and consistent with, the benchmark SMBH mass derived from water maser dynamics, validating the use of NIRSpec stellar kinematics for robust SMBH mass determination. Our analysis demonstrates JWST's capability to resolve the SMBH's sphere of influence and deliver precise dynamical masses, even in the presence of significant AGN continuum emission.