Has Blending Compromised Cepheid-based Determinations of the Extragalactic Distance Scale?

Abstract

We examine the suggestion that half of the galaxies observed by the Hubble Space Telescope Key Project and the Type Ia Supernova Calibration Team have had their distances systematically underestimated, by 0.1-0.3 mag in the distance modulus, because of the underappreciated influence of stellar profile blending on the Wide Field Camera chips. The signature of such an effect would be a systematic trend in (1) the Type Ia supernova-corrected peak luminosity and (2) the Tully-Fisher residuals, with increasing calibrator distance, and (3) a differential offset between Planetary Camera and Wide Field Camera distance moduli, within the same galaxy. The absence of a trend would be expected if blending were negligible (as has been inherently assumed in the analyses of the aforementioned teams). We adopt a functional form for the predicted influence of blending that is consistent with the models of Mochejska et al. and Stanek & Udalski, and we demonstrate that the expected correlation with distance predicted by these studies is not supported by the data. We conclude that the Cepheid-based extragalactic distance scale has not been severely compromised by the neglect of blending.