The DIRECT Project: Influence of Blending on the Cepheid Distance Scale. I. Cepheids in M31

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  • 27 August 1999
Abstract
We investigate the influence of blending on the Cepheid distance scale. Blending leads to systematically low distances to galaxies observed with HST, and therefore to systematically high estimates of H_0. High-resolution HST images are compared to our ground-based data, obtained as part of the DIRECT project, for a sample of 22 Cepheids in the M31 galaxy. The average (median) V-band flux contribution from luminous companions which are not resolved on the ground-based images is about 19% (12%) of the flux of the Cepheid. Our ground-based resolution in M31 corresponds to the HST resolution at about 10 Mpc. When we use our data to estimate the blending effects for a galaxy at ~25 Mpc observed by HST, we find that 50% of our Cepheids would have blended companions contributing more than 40% of the flux of the Cepheid. This has strong and direct implications for the Cepheid distances to galaxies observed by the HST Key Project on the Extragalactic Distance Scale and other teams. We also discuss the use of well-sampled light curves, in at least two optical bands, to find which Cepheids are blended.

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