H0 Revisited

TL;DR

This study reanalyzes the Riess et al. Cepheid data using the revised NGC 4258 maser distance and tests multiple distance anchors (NGC 4258, LMC, MW) while scrutinizing outlier rejection and metallicity effects. It finds H0 ≈ 70.6 ± 3.3 km s^-1 Mpc^-1 when anchored to NGC 4258, compatible with Planck within 1σ; incorporating LMC and MW anchors with metallicity priors yields H0 ≈ 72.5 ± 2.5 with reduced sensitivity to outliers. The work uncovers sub-luminous, low-metallicity Cepheids that bias P-L fits and demonstrates that metallicity priors significantly affect H0, underscoring the need for robust, independent distance anchors. Gaia-era astrometry is anticipated to resolve remaining tensions by expanding Milky Way Cepheid parallaxes and clarifying anchor consistency. Overall, the Planck and direct measurements remain broadly consistent within uncertainties, with systematics in Cepheid calibration dominating current discrepancies.

Abstract

I reanalyse the Riess et al. (2011, hereafter R11) Cepheid data using the revised geometric maser distance to NGC 4258 of Humphreys et al. (2013). I explore different outlier rejection criteria designed to give a reduced chi-squared of unity and compare the results with the R11 rejection algorithm, which produces a reduced chi-squared that is substantially less than unity and, in some cases, to underestimates of the errors on parameters. I show that there are sub-luminous low metallicity Cepheids in the R11 sample that skew the global fits of the period-luminosity relation. This has a small but non-negligible impact on the global fits using NGC 4258 as a distance scale anchor, but adds a poorly constrained source of systematic error when using the Large Magellanic Cloud (LMC) as an anchor. I also show that the small Milky Way (MW) Cepheid sample with accurate parallax measurements leads to a distance to NGC 4258 that is in tension with the maser distance. I conclude that H0 based on the NGC 4258 maser distance is H0 = 70.6 +/- 3.3 km/s/Mpc compatible within 1 sigma with the recent determination from Planck for the base six-parameter LCDM cosmology. If the H-band period-luminosity relation is assumed to be independent of metallicity and the three distance anchors are combined, I find H0 = 72.5 +/- 2.5 km/s/Mpc, which differs by 1.9 sigma from the Planck value. The differences between the Planck results and these estimates of H0 are not large enough to provide compelling evidence for new physics at this stage.

Figures (6)

• Figure 1: Period-luminosity relation for the LMC Cepheids. The line shows the best fit of equation (4a). The vertical dotted lines show the range of periods used in the fit of equation (\ref{['LMC3b']}).
• Figure 2: P-L magnitude residuals relative to global fit 5 of Table 2: Residuals for NCG4258 Cepheids are shown on the left and the SNe host galaxy Cepheids are shown on the right. Filled (red) dots show residuals for Cepheids that are accepted by both the R11 and the $T=2.25$ rejection criterion. Filled (blue) squares are rejected by both algorithms. Open (red) squares are rejected by R11 but accepted by the $T=2.25$ rejection criterion and the single open (blue) circle is rejected by the $T=2.25$ criterion but accepted by R11. The errors show the H-band magnitude errors as listed in R11. The dotted lines show offsets that would produce changes of $\pm 2 \ {\rm km}\ {\rm s}^{-1}{\rm Mpc}^{-1}$ in the value of $H_0$ (increasing $H_0$ is shown by the direction of the arrow in each plot).
• Figure 3: P-L relations for the data used in fit 5. The symbols and colour coding of the points are as in Figure 3. In these plots, the F160W magnitudes have not been corrected for a metallicity dependence. The figure to the left shows high metallicity Cepheids and the figure to the right shows low metallicity Cepheids. The lines show the best fit P-L relation for the entire sample.
• Figure 4: Period-luminosity relation for 13 MW Cepheids with parallax measurements (from van Leeuwen et al. 2007). The line shows the best fit of (\ref{['MW1']})
• Figure 5: Period-luminosity fit used to determine the distance modulus to NGC 4258 using the MW Cepheids as a distance anchor. The MW Cepheids are shown by the (green) filled stars. The rest of the points (red and blue) show the R11 Cepheids. As in Figure 2, filled (red) circles are accepted by both the R11 and $T=2.25$ rejection criteria while filled (blue) sqaures are rejected by both criteria. Open (red) squares are rejected by R11 but accepted by the $T=2.25$ criterion. The line shows the best fit P-L relation.