When the Shadow Meets Its Measure: Assessing the Feasibility of Submillimeter Black Hole Shadow Imaging in Megamaser Disk AGN

TL;DR

This paper evaluates the feasibility of imaging supermassive black hole shadows in megamaser disk AGN using submillimeter interferometry, highlighting NGC4258 as the most promising candidate for future space-based VLBI observations.

Contribution

It maps predicted black hole shadow sizes, estimates flux densities, and assesses astrometric requirements for detecting black hole spin effects in megamaser disk AGN.

Findings

NGC4258 is detectable with Earth-L2 baselines in submm regime.

Most megamaser AGN require baselines approaching Earth-L4/L5 for detection.

NGC4258 shows a submillimeter excess indicating a thin accretion disk down to small radii.

Abstract

Active galactic nuclei (AGN) hosting water megamaser disks enable exceptionally precise geometric determinations of black hole mass, distance, inclination, and dynamical center. In anticipation of upcoming space-based very long baseline interferometry (SVLBI) missions, megamaser disk AGN offer a uniquely valuable probe of strong-gravity regimes through black hole shadow (BHS) imaging beyond SgrA* and M87*. In this work, we (1) map the predicted BHS diameters of twenty-one of the most precisely characterized megamaser disk AGN to submillimeter-millimeter (submm-mm) interferometric baseline requirements, (2) estimate their respective AGN-core flux densities at submm-mm wavelengths, accounting for thermal-dust contamination, extended-jet emission, and intrinsic variability, and (3) determine the astrometric precision required to detect spin-dependent positional offsets between the BHS and the megamaser disk dynamical center. NGC4258 stands out as the only megamaser disk AGN detectable on Earth-L2 baselines in the submm-mm regime, while other megamaser disk AGN in the sample would require baselines approaching Earth-L4/L5 distances; moreover, only a handful exhibit flux densities above $\sim$10mJy. Our results further indicate a submillimeter excess in NGC4258, suggesting that the accretion disk remains thin down to a transitional radius of $\lesssim 100$Schwarzschild radii, within which the flow becomes advection dominated. For a maximally spinning supermassive black hole in NGC4258, we show that the astrometric precision of the BHS centroid necessary to detect the BHS-dynamical center offset could, in principle, be achieved with Earth-Moon baselines; however, it would also demand astrometric precision of the water maser dynamical center roughly fifty times better than what is currently attainable.