New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant

TL;DR

This paper delivers new trigonometric parallaxes for seven long-period Milky Way Cepheids using HST/WFC3 spatial scanning, achieving high-precision, geometry-based distance anchors at $D \approx 1.7$–$3.6$ kpc. By aligning MW Cepheid photometry to the same WFC3 system used in SN Ia hosts and addressing period- and photometric-system systematics, the authors test the Cepheid calibration of the Hubble constant with reduced biases. They find a scale consistent with Riess et al. (2016) and derive $H_0 = 73.48 \pm 1.66\ \mathrm{km\ s^{-1}\; Mpc^{-1}}$, preserving the tension with Planck+$\Lambda$CDM at $\sim3.7\sigma$. The results underscore the viability of spatial-scan parallax and photometry to push toward a sub-percent calibration of $H_0$, aided by future Gaia releases and expansions to ~50 MW Cepheids.

Abstract

We present new parallax measurements of 7 long-period (> 10 days) Milky Way Cepheids (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup and S Vul) using astrometry from spatial scanning of WFC3 on HST. Observations were obtained at 6 month intervals over 4 years. The distances are 1.7--3.6 kpc with a mean precision of 45 microarcseconds and a best of 29 microarcseconds (SNR = 14). The accuracy of the parallaxes is demonstrated through independent analyses of >100 reference stars. This raises to 10 the number of long-period Cepheids with significant parallax measurements, 8 obtained from this program. We also present high-precision F555W, F814W, and F160W magnitudes of these Cepheids, allowing a direct, zeropoint-independent comparison to >1800 extragalactic Cepheids in the hosts of 19 SNeIa. This sample addresses two outstanding systematic uncertainties affecting prior comparisons of Milky Way and extragalactic Cepheids used to calibrate H_0: their dissimilarity of periods and photometric systems. Comparing the new parallaxes to their predicted values derived from reversing the distance ladder gives a ratio (or independent scale for H_0) of 1.037+/-0.036, consistent with no change and inconsistent at the 3.5 sigma level with a ratio of 0.91 needed to match the value predicted by Planck+LCDM. Using these data instead to augment the Riess et al. (2016) measurement of H_0 improves the precision to 2.3%, yielding 73.48+/-1.66 km/s/Mpc, and tension with Planck+LCDM increases to 3.7 sigma. The future combination of Gaia parallaxes and HST spatial scanning photometry of 50 Milky Way Cepheids can support a < 1% calibration of H_0.