http://arxiv.org/abs/1405.5218
We present the largest M31 near-infrared (F110W, F160W) Cepheid sample so far. The sample consists of 374 Cepheids with photometry obtained from the HST PHAT program. The sample of 322 fundamental mode Cepheids, 16 first overtone Cepheids and 36 type II Cepheids, was derived using the median absolute deviation (MAD) outlier rejection method we develop here. This method does not rely on priors and allows us to obtain this clean Cepheid sample without rejecting a large fraction of Cepheids. The obtained Period-Luminosity relations (PLRs) have a very small dispersion, i.e. 0.156 mag in F160W, despite using random phased observations. This remarkably small dispersion allows us to determine that the PLRs are significantly better described by a broken slope at ten days than a linear slope. The Hubble constant based on our improved PLRs is $69.5 \pm 2.4~\mathrm{km}~\mathrm{s}^{-1}~\mathrm{Mpc}^{-1}$ which is significantly smaller than the Riess et al. (2012) value and consistent with the Planck value (Planck Collaboration 2013).
M. Kodric, A. Riffeser, S. Seitz, et. al.
Thu, 22 May 14
4/48
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