The Quest For 3%: Strategies to Improve H₀ Precision with the Megamaser Cosmology Project

The Megamaser Cosmology Project (MCP) aims to measure the Hubble Constant (H₀) using observations of water megamasers in the disks of nearby active galaxies. The current MCP measurement of H₀ is 73.9 ± 3.0 km s^-1 Mpc^-1 (Pesce et al. 2020; 4.1% uncertainty). A 1%-level measurement using megamasers will be possible later with the advent of the ngVLA. Here we assess several strategies for improving the MCP precision using existing observational facilities. We investigate three approaches, supplementing the measurements of six galaxies used by Pesce et al. These approaches are: (1) including the maser measurements for Mrk 1419, which requires refining the modeling of the thin, warped maser disk in this galaxy, (2) including new measurements of the three megamaser galaxies IC 2560, ESO 558-G009, and J0437+2456, for which the observational data exists but modeling is incomplete, and (3) reobserving galaxies already used in the Pesce et al. analysis. We consider the value of new, more sensitive observations of NGC 6323 and NGC 6264, and new observations (providing an independent measurement) with comparable sensitivity of NGC 5765b, which already had exceptional precision. We find that the most meaningful improvements would result from new observations of the megamaser systems in NGC 6323 and NGC 5765b, and the combined overall improvement would result in an overall uncertainty of 3.1%.