The Distance to NGC 4258 from Individual Maser Component Tracking

Abstract

We present a reanalysis of the water maser system in NGC 4258 to reassess its geometric distance, commonly reported as approximately 7.6 Mpc with percent-level accuracy, a key anchor in extragalactic distance ladder calibrations and recent determinations of the Hubble constant. We introduce a method that relies exclusively on tracking individual maser components, rather than assuming a single averaged trajectory as in previous works, thereby avoiding arbitrary data averaging that can bias interpretations of the disk's geometry and dynamics. This approach requires spatially resolved individual maser components; consequently, the majority of observational epochs were excluded, as they lack sufficient spatial resolution to localize the maser position. We track individual maser components across multiple epochs and introduce an efficient marginalization method over nuisance parameters (angular radius and azimuth of each maser), reducing the number of free parameters from hundreds to 14. Our analysis reveals that the current observational cadence is insufficient to reliably track the individual masers, which our method relies on, given their intrinsic variability. Across a range of maser selections and model configurations, inferred distances span approximately between 6.7 to 8.1 Mpc, demonstrating significant sensitivity to how our method selects individual masers. Even visually and statistically robust fits can differ by several standard deviations, reflecting ambiguity in component identification across sparsely sampled epochs. We evaluate the impact of observational cadence on tracking fidelity and distance precision, and show that high-cadence monitoring is needed for our method to track individual masers and produce a robust anchor for cosmology.