Cepheids in the Coma Cluster

We present preliminary results of a search for Cepheid variables in NGC 4921, a spiral galaxy located in the core of the Coma Cluster. The ultimate goal of this project is to determine the Hubble constant from a Cepheid distance in the Hubble flow, without relying on any secondary distance indicators. We imaged NGC 4921 on 12 epochs over 90 days using the Wide Field Camera 3 onboard the Hubble Space Telescope. We acquired most of the images using a long-pass filter (F350LP) that significantly reduced the exposure time required to detect the variables. Mean V and I magnitudes were obtained from 7 and 3 epochs, respectively, most of which were obtained using the Advanced Camera for Surveys in December 2006 just before the failure of that instrument. We present preliminary light curves and period-luminosity relations of candidate Cepheid variables, along with color-magnitude diagrams of all the objects detected in our images. We also present details of the extensive artificial star simulations required to correct our photometric measurements for blending and crowding. This project is supported by NASA/STScI through HST-GO programs 10842 and 12476