A new cepheid distance to the giant spiral M101 based on image subtraction of hubble space telescope/advanced camera for surveys observations

Abstract

We accurately determine a new Cepheid distance to M101 (NGC5457) using archival Hubble Space Telescope (HST)/Advanced Camera for Surveys V and I time series photometry of two fields within the galaxy. We make a slight modification to the ISIS image subtraction package to obtain optimal differential light curves from HST data. We discovered 827 Cepheids with periods between 3 and 80 days, the largest extragalactic sample of Cepheids observed with HST by a factor of two. With this large Cepheid sample, we find that the relative distance of M101 from the Large Magellanic Cloud is ΔμLMC = 10.63 ± 0.04 (random) ± 0.06 (systematic)mag. If we use the geometrically determined maser distance to NGC4258 as our distance anchor, the distance modulus of M101 is μ0 = 29.04 ± 0.05 (random) ± 0.18 (systematic)mag or D = 6.4 ± 0.2 (random) ± 0.5 (systematic)Mpc. The uncertainty is dominated by the maser distance estimate (± 0.15mag), which should improve over the next few years. We determine a steep metallicity dependence, γ, for our Cepheid sample through two methods, yielding γ = -0.80 ± 0.21 (random) ± 0.06 (systematic)magdex-1 and γ = -0.72+0.22-0.25 (random) ± 0.06 (systematic)magdex-1. We see marginal evidence for variations in the Wesenheit period-luminosity relation slope as a function of deprojected galactocentric radius. We also use the tip of the red giant branch method to independently determine the distance modulus to M101 of μ0 = 29.05 ± 0.06 (random) ± 0.12 (systematic)mag. © 2011. The American Astronomical Society. All rights reserved.