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

TL;DR

This study evaluates the feasibility of submillimeter VLBI imaging of black hole shadows in 21 megamaser-disk AGN by mapping predicted BHS sizes, estimating submm-core fluxes, and assessing the astrometric precision needed to detect spin-dependent offsets between the BHS and the maser-determined dynamical center. Using $\theta_{\rm BHS}=\sqrt{27}\frac{R_S}{D_{\mathrm{SMBH}}}$ with $R_S=\frac{2GM}{c^2}$, the authors compare angular BHS scales to proposed SVLBI baselines at $230\,\mathrm{GHz}$ across Earth-diameter, Earth–Moon, Earth–L2, and Earth–L4/L5 configurations. They find NGC 4258 is the only megamaser-disk AGN likely resolvable on Earth–L2 baselines, while others require much longer baselines or higher flux densities; several sources exhibit $\gtrsim10\ \mathrm{mJy}$ cores, potentially accessible to future SVLBI concepts. The work also quantifies the spin observable via the BHS–maser dynamical-center offset, concluding that achieving the required astrometric precision ($\sim0.1\ \mu\mathrm{as}$) hinges on substantial improvements in water maser center localization and careful management of systematic uncertainties, with SVLBI baselines beyond Earth–L2 likely needed to fully exploit most targets.

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.

Figures (7)

• Figure 1: Resolution and sensitivity requirements for resolving BHS of water megamaser SMBHs using VLBI. Each point shows an approximate estimate of angular BHS size and flux density at submm-mm for a given water megamaser AGN. $\bullet$ symbols represent detections, and $\blacktriangledown$ symbols represent upper limits. $\bullet$ symbols represent M87, Sgr A* for comparison. Vertical lines correspond to the angular resolution at 230 GHz for baselines equal to (1) the Earth diameter, (2) Earth--Moon, (3) Earth--L2, and (4) Earth--L4/L5 distances. Horizontal lines mark representative VLBI sensitivity thresholds.
• Figure 2: Left Panel: SED of NGC 4258. Data points correspond to continuum observations from the SMA, SCUBA-JCMT, VLA, and NMA. Due to its northern declination, NGC 4258 is outside the observable range of ALMA. The inset shows the angular to linear scale, and the color bar denotes FWHM synthesized beam size. Right Panel: Variability of NGC 4258. The horizontal axis indicates the time difference between observations, while the vertical axis shows the percentage difference in flux density between the two observations.
• Figure 3: SED of NGC 1194. Data points correspond to continuum observations from the SMA, ALMA, and VLA. The inset shows the angular to linear scale, and the color bar denotes FWHM synthesized beam size.
• Figure 4: Left Panel: SED of NGC 3079. Data points correspond to continuum observations from the SMA and VLA. Due to its northern declination, NGC 3079 is outside the observable range of ALMA. The inset shows the angular to linear scale, and the color bar denotes FWHM synthesized beam size. Right Panel: Variability of NGC 3079. The horizontal axis indicates the time difference between observations, while the vertical axis shows the percentage difference in flux density between the two observations.
• Figure 5: Left Panel: SED of NGC 4945. Data points correspond to continuum observations from ALMA. Due to its southern declination, NGC 4945 is outside the observable range of the SMA and VLA. The inset shows the angular to linear scale, and the color bar denotes FWHM synthesized beam size. Right Panel: Variability of NGC 4945. The horizontal axis indicates the time difference between observations, while the vertical axis shows the percentage difference in flux density between the two observations.