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Reassessing the Constraints from SH0ES Extragalactic Cepheid Amplitudes on Systematic Blending Bias

, , , , and .
(2023)cite arxiv:2305.14435Comment: 20 pages, 12 figures. Submitted to MNRAS.

Abstract

The SH0ES collaboration Hubble constant determination is in a $\sim5\sigma$ difference with the Planck value, known as the Hubble tension. The accuracy of the Hubble constant measured with extragalactic Cepheids depends on robust stellar-crowding background estimation. Riess et al. 2020 (R20) compared the light curves amplitudes of extragalactic and MW Cepheids to constrain an unaccounted systematic blending bias, $\gamma=-0.029\pm0.037\,mag$, which cannot explain the required, $\gamma=0.24\pm0.05\,mag$, to resolve the Hubble tension. Further checks by Riess et al. 2022 demonstrate that a possible blending is not likely related to the size of the crowding correction. We repeat the R20 analysis, with the following main differences: 1. We limit the extragalactic and MW Cepheids comparison to periods $Płesssim50\,d$, since the number of MW Cepheids with longer periods is minimal; 2. We use publicly available data to recalibrate amplitude ratios of MW Cepheids in standard passbands; 3. We remeasure the amplitudes of Cepheids in NGC 5584 and NGC 4258 in two HST filters (F555W and F350LP) to improve the empirical constraint on their amplitude ratio $A^555/A^350$. We show that the filter transformations introduce an $\approx0.04\,mag$ uncertainty in determining $\gamma$, not included by R20. While our final estimate, $\gamma=0.013\pm0.057\,mag$, is consistent with the value derived by R20, the error is somewhat larger, and the best-fit value is shifted by $\approx0.04\,mag$. Although the obtained $\gamma$ for this crowding test is consistent with zero, folding (in quadratures) it is $\approx3.0\sigma$ away from aligning with Planck. Future observations, especially with JWST, would allow better calibration of $\gamma$.

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