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arXiv · 1801.01120

New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant

Adam G. Riess·Stefano Casertano·Wenlong Yuan·Lucas Macri·Jay Anderson·John W. Mackenty·J. Bradley Bowers·Kelsey I. Clubb·Alexei V. Filippenko·David O. Jones·Brad E. Tucker
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Abstract

We present new parallax measurements of 7 long-period (> 10 days) Milky Way Cepheids (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup and S Vul) using astrometry from spatial scanning of WFC3 on HST. Observations were obtained at 6 month intervals over 4 years. The distances are 1.7--3.6 kpc with a mean precision of 45 microarcseconds and a best of 29 microarcseconds (SNR = 14). The accuracy of the parallaxes is demonstrated through independent analyses of >100 reference stars. This raises to 10 the number of long-period Cepheids with significant parallax measurements, 8 obtained from this program. We also present high-precision F555W, F814W, and F160W magnitudes of these Cepheids, allowing a direct, zeropoint-independent comparison to >1800 extragalactic Cepheids in the hosts of 19 SNeIa. This sample addresses two outstanding systematic uncertainties affecting prior comparisons of Milky Way and extragalactic Cepheids used to calibrate H_0: their dissimilarity of periods and photometric systems. Comparing the new parallaxes to their predicted values derived from reversing the distance ladder gives a ratio (or independent scale for H_0) of 1.037+/-0.036, consistent with no change and inconsistent at the 3.5 sigma level with a ratio of 0.91 needed to match the value predicted by Planck+LCDM. Using these data instead to augment the Riess et al. (2016) measurement of H_0 improves the precision to 2.3%, yielding 73.48+/-1.66 km/s/Mpc, and tension with Planck+LCDM increases to 3.7 sigma. The future combination of Gaia parallaxes and HST spatial scanning photometry of 50 Milky Way Cepheids can support a < 1% calibration of H_0.

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Adam G. Riess, Stefano Casertano, Wenlong Yuan, Lucas Macri, Jay Anderson, John W. Mackenty, J. Bradley Bowers, Kelsey I. Clubb, Alexei V. Filippenko, David O. Jones, Brad E. Tucker. 2018-01-03. New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant. https://doi.org/10.3847/1538-4357/aaadb7

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