High-frequency water megamasers as precision probes of black holes, galaxies and cosmology: A case for a next-generation sub-mm/mm interferometer
Year: 2026
Type: preprint
Abstract: Water megamasers in the circumnuclear disks of active galactic nuclei are among the most precise probes of supermassive black holes. VLBI imaging of 22 GHz disk masers has provided geometric distances to independently constrain $H_0$, accurate black hole mass measurements, and sub-parsec views of accretion disk geometry. ALMA has opened a new window into water maser transitions in the sub-mm/mm regime, which trace temperatures, densities and disk regions that are inaccessible at 22 GHz, and can be observed at higher spatial resolution and with less free-free absorption. However, current detections are limited to a few bright extragalactic masers in nearby galaxies. We present the case for a next-generation sub-mm/mm interferometer - ALMA2040 - that would open a new parameter space for maser studies. We show that a 10-fold increase in line sensitivity and three times longer baselines compared to ALMA would transform sub-mm maser studies from a few bright detections into a population-level probe, delivering black hole masses and geometric distances from sub-mm masers alone, independent of and complementary to 22 GHz measurements. Integrated into a VLBI array, ALMA2040 would resolve maser disks out to $>$160 Mpc, extend maser black hole masses into the largely untested intermediate-mass regime, reveal magnetic field structure in accretion disks through polarization studies, and test systematics in $H_0$ measurements.
Source: arXiv (Cornell University)
Institutions: New York University Abu Dhabi, Netherlands Institute for Radio Astronomy, Harvard University, Center for Astrophysics Harvard & Smithsonian, European Southern Observatory, University of Geneva, National Sun Yat-sen University, Bucknell University, Toyo University, Institute of Astronomy and Astrophysics, Academia Sinica, National Radio Astronomy Observatory
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: The Black Hole Initiative, Sharpening the View of Black Holes, G4759703258, The Black Hole Initiative: Towards a Center for Interdisciplinary Research, Enabling the Next-Generation Event Horizon Telescope: Towards Movies of Black Holes, G5507044963, G5586996422, Mid-scale RI-1 (M1:DP): Next Generation Event Horizon Telescope Design, The Event Horizon Telescope: Resolving Black Holes in Time and Space, G7854666520, Black Hole Initiative Phase 3, Frontier Science with the Event Horizon Telescope
OpenAlex ID: W7222738073