Measuring the Hubble constant with observations of water-vapor megamasers

Abstract

To constrain models of dark energy, a precise measurement of the Hubble constant, H 0 , provides a powerful complement to observations of the cosmic microwave background. Recent, precise measurements of H 0 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 H 0 are of high value. Direct geometric distance measurements to circumnuclear H 2 O megamasers in the Hubble flow provide a promising new method to determine H 0 . The Megamaser Cosmology Project (MCP) is a systematic effort to discover suitable H 2 O megamasers and determine their distances, with the aim of measuring H 0 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 H 0 = 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.