A Cepheid Distance to the Coma Cluster: Initial Progress Report

We have begun a 180 orbit project using the Advanced Camera for Surveys to search for and phase Cepheid variables in two spiral galaxies in the core of the Coma cluster. A direct application of this canonical primary distance indicator at Coma's recession velocity of 7000 km/s will measure the far-field Hubble constant free of many of the systematic uncertainties which beset current determinations relying on secondary indicators. Establishing the far-field H0 with Cepheids will provide one of the strongest links in the extragalactic distance scale. Reducing the uncertainty in the far-field H0 will help constrain the equation of state of the dark energy. We will directly calibrate the fiducial fundamental plane of elliptical galaxies in Coma, as well as the Thomson optical depth of the cluster for comparison with the 30 to 300 Ghz Sunyaev-Zeldovich spectrum.A S/N=5-10 or better can be reached for Cepheids with periods of 40d to 70d at mean light in 5 orbits with the F606W filter if H0 is approximately 70 km/s/Mpc. Each of the two target galaxies will be observed 12 times, for a duration of 5 orbits, optimally spaced for periods of 40-70d, providing robust detection and phasing information. An additional 6 observational visits using the F814W filter are planned. We will present a progress report of the first few epochs of data which will be in hand by the time of the January 2007 AAS meeting.NRC publication: Ye