A test of Gaia Data Release 1 parallaxes: implications for the local distance scale [IMA]

One of the important cosmological issues that will be addressed by GAIA (which I blogged about last week) is the local distance scale, more precisely whether some modification to the calibration of Cepheid distances may be needed. This paper looks at this question using the GAIA DR1 results, and finds that – as yet – there isn’t any evidence of major problems, but it’s early days. The “tension” between “direct” estimates of the Hubble constant and those from Planck remains unresolved.



We present a comparison of Gaia Data Release 1 (DR1) parallaxes with photometric parallaxes for a sample of 212 Galactic Cepheids at a median distance of 2~kpc, and explore their implications on the distance scale and the local value of the Hubble constant H_0. The Cepheid distances are estimated from a recent calibration of the near-infrared Period-Luminosity P-L relation. The comparison is carried out in parallax space, where the DR1 parallax errors, with a median value of half the median parallax, are expected to be well-behaved. With the exception of one outlier, the DR1 parallaxes are in remarkably good global agreement with the predictions, and the published errors may be conservatively overestimated by about 20%. The parallaxes of 9 Cepheids brighter than G = 6 may be systematically underestimated, trigonometric parallaxes measured with the HST FGS for three of these objects confirm this trend. If interpreted as an independent…

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