Science with the ngVLA: H$_2$O Megamaser Cosmology

TL;DR

The paper discusses how the next-generation Very Large Array (ngVLA) can significantly improve measurements of the Hubble Constant using water megamasers, providing a direct, independent, and precise cosmological probe.

Contribution

It proposes that ngVLA's enhanced sensitivity will enable the discovery of more megamaser systems, allowing for a percent-level measurement of H$_0$ independent of traditional methods.

Findings

Current megamaser measurements achieve ~4% accuracy.

ngVLA can discover and measure additional megamasers.

Potential to reach ~1% precision in H$_0$ measurement.

Abstract

In combination with observations of the Cosmic Microwave Background, a measurement of the Hubble Constant provides a direct test of the standard $\Lambda$CDM cosmological model and a powerful constraint on the equation of state of dark energy. Observations of circumnuclear water vapor megamasers at 22 GHz in nearby active galaxies can be used to measure their distances, geometrically, and thereby provide a direct, one step measurement of the Hubble Constant. The measurement is independent of distance ladders and standard candles. With present-day instrumentation, the Megamaser Cosmology Project is expected to reach a $\sim$4% measurement using the megamaser technique, based on distances to fewer than 10 megamaser systems. A goal of the observational cosmology community is to reach a percent-level measurement of H$_0$ from several independent astrophysical measurements to minimize the systematics. The ngVLA will provide the sensitivity at 22 GHz required to discover and measure the additional megamaser disks that will enable an H$_0$ measurement at the $\sim$1% level.