When the Shadow Meets Its Measure: Assessing the Feasibility of Submillimeter Black Hole Shadow Imaging in Megamaser-disk Active Galactic Nuclei

Abstract Well-modeled water megamaser disks offer a path to black hole shadow (BHS) tests of strong-field gravity beyond Sgr A* and M87*: their very long baseline interferometry (VLBI) angular dynamics precisely constrains M • / D , the maser-disk orientation, and the dynamical center within the maser reference frame, thereby predicting the fiducial Schwarzschild BHS angular size without an externally adopted distance. We assess 21 megamaser galaxies as BHS targets using published maser measurements to calculate BHS angular sizes and baseline requirements. We combine new Submillimeter Array (SMA) continuum observations with archival Atacama Large Millimeter/submillimeter Array (ALMA) and Very Large Array (VLA) measurements to estimate arcsecond-scale 230 GHz flux densities and identify the strongest nuclear-region sources for higher-resolution follow-up. We also estimate the maximum dust and extended-jet contributions, examine limited variability measurements, and, for the leading candidate NGC 4258, evaluate the astrometry required to detect a spin-dependent offset between the BHS center and the maser dynamical center. Only NGC 4258 has an adopted BHS angular size resolvable with an Earth–L2 baseline; the other adopted sizes require longer baselines, while an Earth–L4/L5 separation would be sufficient for all. Five sources reach S 230 ≳ 10 mJy. However, whether the observed 230 GHz emission arises on BHS scales and is sufficiently optically thin to reveal a BHS remains unknown throughout the sample. NGC 4258’s submillimeter excess is consistent with a thin disk transitioning to an advection-dominated flow; its spin-offset test remains infeasible with present maser astrometry.