The parallax zero-point offset from Gaia EDR3 data

Abstract

The second data release of Gaia revealed a parallax zero-point offset of −0.029 mas based on quasars. The value depended on the position on the sky, and also likely on magnitude and colour. The offset and its dependence on other parameters inhibited improvement in the local distance scale using for example the Cepheid and RR Lyrae period–luminosity relations. Analysis of the recent Gaia Early Data Release 3 (EDR3) reveals a mean parallax zero-point offset of −0.021 mas based on quasars. The Gaia team addresses the parallax zero-point offset in detail and proposes a recipe to correct for it based on ecliptic latitude, G-band magnitude, and colour information. This paper presents a completely independent investigation into this issue focusing on the spatial dependence of the correction based on quasars and the magnitude dependence based on wide binaries. The spatial and magnitude corrections are connected to each other in the overlap region in the range 17 < G < 19. The spatial correction is presented at several spatial resolutions based on the HEALPix formalism. The colour dependence of the parallax offset is unclear and in any case secondary to the spatial and magnitude dependence. The spatial and magnitude corrections are applied to two samples of brighter sources, namely a sample of approximately 100 stars with independent trigonometric parallax measurements from Hubble Space Telescope data, and a sample of 75 classical cepheids using photometric parallaxes. The mean offset between the observed GEDR3 parallax and the independent trigonometric parallax (excluding outliers) is about −39 μas, and after applying the correction it is consistent with being zero. For the classical cepheid sample the analysis presented here suggests that the photometric parallaxes may be underestimated by about 5%.