Supernova

Cepheid variable

Cosmic distance ladder

Quasar

Stars

Galaxy

Hubble's law

Astronomy

Astrophysics

Quantum mechanics

Anisotropy

Parallax

Milky Way

Photometry (optics)

Cosmic microwave background

Redshift

Physics

The Astrophysical Journal

We present Hubble Space Telescope (HST) photometry of a selected sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3 F 555 W -, F 814 W -, and F 160 W -band photometric system as extragalactic Cepheids in Type Ia supernova host galaxies. 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 mmag 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 the latter to be −46 ± 13 μ as or ±6 μ as 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 because of the need to independently determine the parallax offset. The best-fit distance sca...

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