Measuring the Hubble constant with observations of water-vapor megamasers

Abstract To constrain models of dark energy, a precise measurement of the Hubble constant, H0, provides a powerful complement to observations of the cosmic microwave background. Recent, precise measurements of H0 have been based on the ‘extragalactic distance ladder,’ primarily using observations of Cepheid variables and Type Ia supernovae as standard candles. In the past, these methods have been limited by systematic errors, so independent methods of measuring H0 are of high value. Direct geometric distance measurements to circumnuclear H2O megamasers in the Hubble flow provide a promising new method to determine H0. The Megamaser Cosmology Project (MCP) is a systematic effort to discover suitable H2O megamasers and determine their distances, with the aim of measuring H0 to a few percent. Based on observations of megamasers in UGC 3789 and NGC 6264, and preliminary results from Mrk 1419, the MCP has so far measured H0 = 68.0 ± 4.8 km s−1 Mpc−1. This measurement will improve as distances to additional galaxies are incorporated. With the Green Bank Telescope, we recently discovered three more excellent candidates for distance measurements, and we are currently acquiring data to measure their distances.