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 $\mathord {\approx }7.6\, \, \text{Mpc}$ with percent-level accuracy, a key anchor in extragalactic distance ladder calibrations and recent determinations of the Hubble constant. Our method relies exclusively on tracking individual maser components, rather than assuming a single averaged trajectory as in previous works, offering an alternative to data averaging that may influence interpretations of the disk’s geometry and dynamics. This approach requires spatially resolved individual maser components; consequently, most observational epochs were excluded, as they lack sufficient spatial resolution to localize the masers. We track individual maser components across multiple epochs and introduce an efficient marginalization method over each maser’s azimuth and angular radius, reducing the number of free parameters from hundreds to 14. Our analysis reveals that the current observational cadence is insufficient to reliably track individual masers, given their intrinsic variability. Across a range of maser selections and model configurations, inferred distances span $\mathord {\approx }6.7$ to $\mathord {\approx }8.1\, \text{Mpc}$, demonstrating 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 conclude that the current observational data limit the achievable precision of the individual-component tracking approach, fundamentally preventing the percent-level accuracy required for a cosmological anchor. We evaluate the impact of observational cadence on tracking fidelity and distance precision, showing that high-cadence monitoring is needed to track individual masers and produce a robust distance measurement.