Reassessing the constraints from SH0ES extragalactic Cepheid amplitudes on systematic blending bias

The SH0ES collaboration Hubble constant determination is in a ∼5σ 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-curve amplitudes of extragalactic and MW Cepheids to constrain an unaccounted systematic blending bias, γ = −0.029 ± 0.037 mag, which cannot explain the required, γ = 0.24 ± 0.05 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 ≲50 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 Hubble Space Telescope filters (F555W and F350LP) to improve the empirical constraint on their amplitude ratio A⁵⁵⁵/A³⁵⁰. We show that the filter transformations introduce an ≈0.04 mag uncertainty in determining γ, not included by R20. While our final estimate, γ = 0.013 ± 0.057 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 ≈0.04 mag and closer to zero. Future observations, especially with JWST, would allow better calibration of γ.