The Optical Gravitational Lensing Experiment. Investigating the Influence of Blending on the Cepheid Distance Scale with Cepheids in the Large Magellanic Cloud

TL;DR

This study examines how blending of Cepheids with nearby stars biases distance measurements, revealing that it causes underestimation of galaxy distances and overestimation of the Hubble constant, especially at larger distances.

Contribution

It models the blending effect on Cepheid distances using LMC Cepheids and quantifies the bias introduced at various distances, impacting extragalactic distance estimates.

Findings

Blending causes underestimation of distances by up to 0.35 mag at 25 Mpc.

Half of the HST Key Project sample likely has blending bias >0.1 mag.

Blending significantly affects the accuracy of the Cepheid distance scale.

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 the Hubble constant H_0. We select a sample of 43 long-period, large-amplitude Cepheids in the Large Magellanic Cloud, from the catalog recently released by the Optical Gravitational Lensing Experiment. We then model the effects of blending, as observed by HST at large distances, by adding the V,I-bands contributions from nearby bright stars. We find that the derived distance modulus would be too short, compared to the true value, by ~0.07 mag at distance of 12.5 Mpc and by \~0.35 mag at distance of 25 Mpc. This has direct and important implications for the Cepheid distances to galaxies observed by the HST Key Project on the Extragalactic Distance Scale and other teams: half of the KP sample is likely to exhibit a blending bias greater than 0.1 mag.