New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant (1801.01120v2)

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.

• The paper introduces a spatial scanning technique using HST’s WFC3 to measure Cepheid parallaxes with sub-milliarcsecond precision.
• The new measurements yield a H₀ value of 73.48 km/s/Mpc with 2.3% precision, intensifying the tension with Planck CMB estimates.
• The study validates multiple independent calibration anchors, reinforcing the potential for uncovering new physics in cosmic expansion.

The Impact of Precise Galactic Cepheid Parallaxes on the Determination of the Hubble Constant

The paper by Riess et al. introduces a comprehensive paper that presents new parallax measurements for seven long-period Milky Way Cepheid variables using astrometric data obtained via spatial scanning with the Hubble Space Telescope's (HST) Wide-Field Camera 3 (WFC3). These measurements are pivotal due to their implications for the calibration of the Hubble Constant (H₀), an essential parameter in cosmology that quantifies the rate of expansion of the Universe.

Methodology and Measurements

The authors employed a novel astrometric method—spatial scanning with WFC3—to acquire unprecedented precision in measuring the parallaxes of distant Galactic Cepheids located at distances ranging from 1.7 to 3.6 kpc. The spatial scanning technique affords significant advantages over traditional methods by achieving sub-milliarcsecond measurement precision, crucial for resolving the parallax of long-period Cepheids typically found beyond 2 kpc.

The measured parallaxes exhibit a mean precision of 45 μas, with the highest precision reaching 29 μas. This accuracy stems not only from the spatial scanning technique but also from a rigorous astrometric modeling process that involves comparison against a well-calibrated set of reference stars across multiple epochs.

Implications for Cosmology

The paper corroborates the prevailing tension between the locally measured H₀ and the value derived from Planck CMB data under the ΛCDM model. Specifically, the new parallaxes yield a H₀ value of 73.48 km s⁻¹ Mpc⁻¹ with a precision of 2.3%, thereby increasing the divergence with CMB-based measurements to a statistical significance of 3.7σ.

The authors argue that such a tension is unlikely attributable to systematic errors, as demonstrated by the consistency of their results across multiple independent geometric calibration anchors—namely, LMC detached eclipsing binaries, NGC 4258 masers, and Milky Way parallax measures by HST's Fine Guidance Sensors. This persistent discrepancy may hint at new physics or modifications to the standard cosmological model.

Future Directions

The authors highlight the prospects of forthcoming Gaia data releases, which are expected to enhance parallax precision for numerous Cepheids, and the ongoing accumulation of a comprehensive photometric dataset for 50 Milky Way Cepheids in HST's photometric system. Together, these efforts aim to achieve a less than 1% calibration of H₀—an ambitious target that promises to provide an extraordinarily robust test of the Universe's expansion parameters when contrasted with early-Universe observations.

Ultimately, the advancements in precision parallax measurements for Milky Way Cepheids, as chronicled in this paper, serve as a pivotal step in the unrelenting pursuit of a more refined understanding of cosmological distance scales, fortifying our grasp of the fundamental parameters that govern the Universe. These efforts will continue to inform both theoretical and practical developments in astrophysics, possibly unveiling paths to novel insights into the fabric of the cosmos.