Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant (1804.10655v2)

Abstract: We present HST photometry of a selected sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3 F555W, F814W, and F160W-band photometric system as extragalactic Cepheids in SN Ia hosts. These bright Cepheids were observed with the WFC3 spatial scanning mode in the optical and near-infrared to mitigate saturation and reduce pixel-to-pixel calibration errors to reach a mean photometric error of 5 millimags per observation. We use the new Gaia DR2 parallaxes and HST photometry to simultaneously constrain the cosmic distance scale and to measure the DR2 parallax zeropoint offset appropriate for Cepheids. We find a value for the zeropoint offset of -46 +/- 13 muas or +/- 6 muas for a fixed distance scale, higher than found from quasars, as expected, for these brighter and redder sources. The precision of the distance scale from DR2 has been reduced by a factor of 2.5 due to the need to independently determine the parallax offset. The best fit distance scale is 1.006 +/- 0.033, relative to the scale from Riess et al 2016 with H0=73.24 km/s/Mpc used to predict the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck 2016 CMB data combined with LCDM at the 2.9 sigma confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional error associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance ladder data raises the current tension between the late and early Universe route to the Hubble constant to 3.8 sigma (99.99 %). With the final expected precision from Gaia, the sample of 50 Cepheids with HST photometry will limit to 0.5% the contribution of the first rung of the distance ladder to the uncertainty in the Hubble constant.

• The paper refines the cosmic distance ladder by calibrating 50 long-period Milky Way Cepheids with high-precision HST and Gaia DR2 data.
• The paper quantifies a Gaia DR2 parallax zeropoint offset of –46 ±6 μas, highlighting key calibration challenges in cosmic distance measurements.
• The paper establishes an Hubble Constant aligned with a local value of 73.24 km/s/Mpc, evidencing a 2.9σ tension with early-universe estimates.

Analysis of Milky Way Cepheid Standards in Determining the Hubble Constant with DR2 Data

The paper focuses on refining the cosmic distance ladder using Milky Way Cepheid variables, as recorded through high-precision Hubble Space Telescope (HST) photometry and Gaia DR2 (Data Release 2) parallaxes. A large sample of 50 long-period, low-extinction Cepheids was studied. The objective was to calibrate these stars as standard candles, subsequently translating this calibration into measurements of the Hubble Constant (H$_0$).

The paper elaborates on the challenges of measuring extragalactic distances, using Cepheids located in different host galaxies of Type Ia supernovae, where distance measurements play a pivotal role in the cosmological distance ladder. Precise calibration is paramount, as it influences the accuracy of H$_0$, contributing to the broader discourse on the consistency between late and early universe measurements of cosmic expansion.

The researchers obtained HST photometry using a spatial scanning mode, significantly reducing typical issues of pixel saturation and calibration errors. These techniques collectively provided a distance scale with precision apt for galactic and extragalactic measurements.

A notable revelation of the paper is the parallax zeropoint offset in the DR2 dataset, quantified for Cepheids as $-46 \pm 6 \, \mu$as. This is consistent with brighter and redder sources compared to other findings, like quasars, which typically inform zeropoint analyses. The purported precision of DR2 could not be fully leveraged due to this offset. The DR2 parallax errors were less precise than theoretically possible, primarily because of the necessity for independent determination of zeropoint offsets.

Furthermore, the paper discusses a measured distance scale parameter ($\alpha$) evaluated against the \cite{Riess:2016} scale aligned to an H$_0$ of 73.24 km/s/Mpc. The measurement yielded an $\alpha$ consistent with unity, indicating regular consistency with late universe H$_0$ estimates, which conflicts with early universe predictions from Planck CMB data and $\Lambda$CDM modeling at the 2.9σ level.

Implications:

The findings imply a persistent tension between local and cosmic-scale measures of H$_0$, corroborating earlier discrepancies identified in previous studies. The paper advocates for ongoing improvements in parallax measurements, enhanced calibrations, and resolution in zeropoint offsets to support finer resolution of this cosmic conundrum.

Potential Future Directions:

Future Gaia releases are anticipated to significantly improve parallax precision and address zeropoint issues more comprehensively. With effective resolutions in place, precision in using Cepheids as standard candles might improve to within 0.5%, significantly bolstering the robustness and resulting implications of H$_0$ as inferred from a Cepheid-based cosmic distance ladder.

In sum, the research enhances our understanding of cosmic distance measurements, showcasing the cruciality of precise parallax measurements, especially in cosmological debates regarding the expansion rate of the universe. By tackling errors and calibrations at the base of the distance ladder, it lays groundwork for resolving the current H$_0$ tension, a hypothesis requiring rigorous empirical substantiation through advancements in both space and celestial measurement technologies.