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 (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 ΔμLMC = 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 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 (±0.15 mag), 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) mag dex-1 and γ = -0.72+0.22 - 0.25 (random) ± 0.06 (systematic) mag dex-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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2011
- DOI:
- 10.1088/0004-637X/733/2/124
- arXiv:
- arXiv:1012.3747
- Bibcode:
- 2011ApJ...733..124S
- Keywords:
-
- distance scale;
- galaxies: distances and redshifts;
- galaxies: individual: M101;
- stars: variables: Cepheids;
- stars: variables: general;
- techniques: image processing;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 63 pages, 30 figures, 14 tables, submitted to The Astrophysical Journal, full resolution version is available at http://www.astronomy.ohio-state.edu/~shappee/M101.pdf