A NEW CEPHEID DISTANCE TO THE GIANT SPIRAL M101 BASED ON IMAGE SUBTRACTION OF HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS OBSERVATIONS

We accurately determine a new Cepheid distance to M101 (NGC 5457) 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 Delta mu(LMC) = 10.63 +/- 0.04 (random) +/- 0.06 (systematic) mag. If we use the geometrically determined maser distance to NGC 4258 as our distance anchor, the distance modulus of M101 is mu(0) = 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 (+/- 0.15 mag), which should improve over the next few years. We determine a steep metallicity dependence, gamma, for our Cepheid sample through two methods, yielding gamma = -0.80 +/- 0.21 (random) +/- 0.06 (systematic) mag dex(-1) and gamma = -0.72(-0.25)(+0.22) (random) +/- 0.06 (systematic) mag dex(-1). We see marginal evidence for variations in theWesenheit 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 mu(0) = 29.05 +/- 0.06 (random) +/- 0.12 (systematic) mag.