A test of<i>Gaia</i>Data Release 1 parallaxes: implications for the local distance scale

Abstract

We present a comparison of Gaia Data Release 1 (DR1) parallaxes with\nphotometric parallaxes for a sample of 212 Galactic Cepheids at a median\ndistance of 2~kpc, and explore their implications on the distance scale and the\nlocal value of the Hubble constant H_0. The Cepheid distances are estimated\nfrom a recent calibration of the near-infrared Period-Luminosity P-L relation.\nThe comparison is carried out in parallax space, where the DR1 parallax errors,\nwith a median value of half the median parallax, are expected to be\nwell-behaved. With the exception of one outlier, the DR1 parallaxes are in\nremarkably good global agreement with the predictions, and the published errors\nmay be conservatively overestimated by about 20%. The parallaxes of 9 Cepheids\nbrighter than G = 6 may be systematically underestimated, trigonometric\nparallaxes measured with the HST FGS for three of these objects confirm this\ntrend. If interpreted as an independent calibration of the Cepheid luminosities\nand assumed to be otherwise free of systematic uncertainties, DR1 parallaxes\nwould imply a decrease of 0.3% in the current estimate of the local Hubble\nconstant, well within their statistical uncertainty, and corresponding to a\nvalue 2.5 sigma (3.5 sigma if the errors are scaled) higher than the value\ninferred from Planck CMB data used in conjunction with Lambda-CDM. We also test\nfor a zeropoint error in Gaia parallaxes and find none to a precision of ~20\nmuas. We caution however that with this early release, the complete systematic\nproperties of the measurements may not be fully understood at the statistical\nlevel of the Cepheid sample mean, a level an order of magnitude below the\nindividual uncertainties. The early results from DR1 demonstrate again the\nenormous impact that the full mission will likely have on fundamental questions\nin astrophysics and cosmology.\n

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Affiliated Institutions

• Osservatorio Astrofisico di Torino IT
• Johns Hopkins University US
• Space Telescope Science Institute US
• Université Bourgogne Franche-Comté FR

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