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

Abstract

The SH0ES collaboration Hubble constant determination is in a $\sim 5\sigma$ 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. (R20) compared the light curves amplitudes of extragalactic and MW Cepheids to constrain an unaccounted systematic blending bias, $\gamma =-0.029\pm 0.037\mathrm{mag}$, which cannot explain the required, $\gamma =0.24\pm 0.05\mathrm{mag}$, to resolve the Hubble tension. Further checks by Riess et al. 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 $P\lesssim 50\mathrm{d}$, 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 $\approx 0.04\mathrm{mag}$ uncertainty in determining $\gamma$, not included by R20. While our final estimate, $\gamma =0.013\pm 0.057\mathrm{mag}$, is consistent with the value derived by R20 and is consistent with no bias, the error is somewhat larger, and the best fitting value is shifted by $\approx 0.04\mathrm{mag}$ and closer to zero. Future observations, especially with JWST, would allow better calibration of $\gamma$.

19 pages, 10 figures. Accepted for publication in MNRAS

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