The direct project: Influence of blending on the cepheid distance scale. I. Cepheids in M31

We investigate the influence of blending on the Cepheid distance scale. Blending is the close association of a Cepheid with one or more intrinsically luminous stars. High-resolution HST images are compared with 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 that are not resolved on the ground-based images is about 19% (12%) of the flux of the Cepheid. This is a large effect-at the 10% level for distances. The current Cepheid distance estimates to M31 are all ground based and are thus affected (underestimated). We discuss indirect methods to find which Cepheids are blended, e.g., by the use of well-sampled light curves in at least two optical bands. More generally, our ground-based resolution in M31 corresponds to the HST resolution at about 10 Mpc. Blending leads to systematically low distances in the observed galaxies and therefore to systematically high estimates of Ho; we discuss the issue and the implications.