Doubly Discordant SH0ES NGC4258 Cepheid Relations (HVI), and Impactful Extinction Laws

TL;DR

The paper investigates why SH0ES Cepheid-based distances anchored to NGC4258 yield discordant Wesenheit relations, highlighting two main issues: photometric inhomogeneities in the H-VI and I-VI bands and uncertainties in extinction laws characterized by $R_{H-VI}$ and $R_{I-VI}$. By comparing SH0ES 2016-2022 data, it reports a Wesenheit offset $ΔW_{0,H-VI} = -0.13 ± 0.02$ mag and a prior $ΔW_{0,I-VI} ≈ -0.30$ mag, and shows that photometric contamination and extinction-ratio ambiguities can drive distance moduli by several percent. The analysis finds that plausible $R_{H-VI}$ and $R_{I-VI}$ ranges produce median distance shifts of a few percent, with reddened subsamples potentially biasing $H_0$ by order unity factors, and identifies a spurious SMC Cepheid photometry issue in the 2022 SH0ES dataset. The work underscores substantial systematic uncertainties in local distance calibrators and argues for independent cross-checks (e.g., TRGB/JAGB) and more robust extinction characterization to stabilize the inferred $H_0$.

Abstract

S$H_0$ES 2016-2022 $HVI$ data for classical Cepheids in the keystone galaxy NGC4258 yield doubly discordant Wesenheit Leavitt functions:~$ΔW_{0,H-VI} = -0.13\pm0^{m}.02$ ($-0^{m}.17$ unweighted) and that is paired with a previously noted $ΔW_{0,I-VI}\simeq-0^{m}.3$, which in concert with complimentary evidence suggest the 2016 S$H_0$ES NGC4258-anchored $H_0 \pm σ_{H_0}$ warrants scrutiny (i.e., $σ_{H_0}/{H_0}\gtrsim 6$\%). Cepheid distance uncertainties are further exacerbated by extinction law ambiguities endemic to such Leavitt relations (e.g., NGC4258), particularly for comparatively obscured variables (e.g., $Δd \gtrsim 4$\%, reddened Cepheid subsamples in the Milky Way, M31, NGC2442, NGC4424, NGC5643, NGC7250). Lastly, during the analysis it was identified that the 2022 S$H_0$ES database relays incorrect SMC Cepheid photometry.