Assigned ID: 68 ----- Attempting Submission 1 (Wed Mar 05 13:57:25 GMT 2008) ----- The local value of the Hubble Constant remains one of the most important constraints in cosmology, but improving on the 10% accuracy of the HST Key Project is challenging. No improvements will be convincing until the metallicity dependence is well constrained and blending effects are fully understood. M81 and its dwarf companion Holmberg IX are superb laboratories for studying Cepheid systematics because they contain large numbers of bright Cepheids with a good spread in metallicity lying at a common, relatively close distance. We have identified 180 12<P< 70 day Cepheids in these two galaxies using the Large Binocular Telescope (compared to 30 in total by the KP), and will expand the sample further in 2008-2009. We will use archival BVI ACS images of M81 and Holmberg IX to flux calibrate these Cepheids and then measure or limit the metallicity dependence. Three band photometry allows us to flux calibrate, estimate extinction, measure metallicity effects and check for additional systematic effects, particularly blending. A separate proposal aims to expand the study to H-band. Our M81 sample is three times larger than the next best sample, that of NGC4258, and suffers less from blending because M81 is at half the distance, so it is an excellent laboratory for studying Cepheid systematics even if it lacks as precise a geometric distance as NGC4258.

COSMOLOGY 12 79000 false 0.0 0.0 false false false false false SUPPORTED