We present Hubble Space Telescope (HST) photometry of a selected sample of 50 long-period, lowextinction Milky Way Cepheids measured on the same WFC3 F 555W -, F 814W -, and F 160W -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 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 the latter to be −46 ± 13 µas or ± 6 µas 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 scale is 1.006 ± 0.033 , relative to the scale from Riess et al. (2016) with H 0 = 73.24 km s −1 Mpc −1 used to predict the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck 2016 CMB data combined with ΛCDM at the 2.9σ 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 3.8σ (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 H 0 .2 Riess et al.

We present new measurements of the parallax of 7 long-period (≥ 10 days) Milky Way Cepheid variables (SS CMa, XY Car, VY Car, VX Per, WZ Sgr, X Pup and S Vul) using one-dimensional astrometric measurements from spatial scanning of Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The observations were obtained at ∼ 6 month intervals over 4 years. The distances are 1.7-3.6 kpc with a mean precision of 45 µas [signal-to-noise ratio (SNR) ≈ 10] and a best precision of 29 µas (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 mean F 555W , F 814W , and F 160W magnitudes of these Cepheids, allowing a direct, zeropointindependent comparison to > 1800 extragalactic Cepheids in the hosts of 19 Type Ia supernovae. This sample addresses two outstanding systematic uncertainties affecting prior comparisons of Milky Way and extragalactic Cepheids used to calibrate the Hubble constant (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 3.5σ level with a ratio of 0.91 needed to match the value predicted by Planck CMB data in concert with ΛCDM. 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 −1 Mpc −1 , and the tension with Planck + ΛCDM increases to 3.7σ. The future combination of Gaia parallaxes and HST spatial scanning photometry of 50 Milky Way Cepheids can support a < 1% calibration of H 0 .

What are the faintest distant galaxies we can see with the Hubble Space Telescope (HST) now, before the launch of the James Webb Space Telescope? This is the challenge taken up by the Frontier Fields, a Director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combines the power of HST and Spitzer with the natural gravitational telescopes of massive highmagnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters-Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370-have been targeted by the HST ACS/WFC and WFC3/IR cameras with coordinated parallel fields for over 840 HST orbits. The parallel fields are the second-deepest observations thus far by HST with 5σ point-source depths of ∼29th ABmag. Galaxies behind the clusters experience typical magnification factors of a few, with small regions magnified by factors of 10-100. Therefore, the Frontier Field cluster HST images achieve intrinsic depths of ∼30-33 mag over very small volumes. Spitzer has obtained over 1000 hr of Director's discretionary imaging of the Frontier Field cluster and parallels in IRAC 3.6 and 4.5 μm bands to 5σ point-source depths of ∼26.5, 26.0 ABmag. We demonstrate the exceptional sensitivity of the HST Frontier Field images to faint high-redshift galaxies, and review the initial results related to the primary science goals.

We report observations from the Hubble Space Telescope (HST) of Cepheid variables in the host galaxies of 42 Type Ia supernovae (SNe Ia) used to calibrate the Hubble constant (H 0 ). These include the complete sample of all suitable SNe Ia discovered in the last four decades at z ≤ 0.01, collected and calibrated from ≥ 1000 HST orbits, more than doubling the sample whose size limits the precision of the direct determination of H 0 . The Cepheids are calibrated geometrically from Gaia EDR3 parallaxes, masers in NGC 4258 (here tripling that sample of Cepheids), and detached eclipsing binaries in the Large Magellanic Cloud. All Cepheids in these anchors and SN Ia hosts were measured with the same instrument (WFC3) and filters (F555W, F814W, F160W) to negate zeropoint errors.We present multiple verifications of Cepheid photometry and six tests of background determinations that show Cepheid measurements are accurate in the presence of crowding. The SNe Ia in these hosts calibrate the magnitude-redshift relation from the revised Pantheon+ compilation, accounting here for covariance between all SNe data and with host properties and SN surveys matched throughout to negate systematics. We decrease the uncertainty in the local determination of H 0 to 1 km s −1 Mpc −1 including systematics. We present results for a comprehensive set of nearly 70 analysis variants to explore the sensitivity of H 0 to selections of anchors, SN surveys, redshift ranges, the treatment of Cepheid dust, metallicity, form of the period-luminosity relation, SN color, peculiar-velocity corrections, sample bifurcations, and simultaneous measurement of the expansion history.Our baseline result from the Cepheid-SN Ia sample is H 0 = 73.04 ± 1.04 km s −1 Mpc −1 , which includes systematic uncertainties and lies near the median of all analysis variants. We demonstrate consistency with measures from HST of the TRGB between SN Ia hosts and NGC 4258, and include them simultaneously to yield 72.53 ± 0.99 km s −1 Mpc −1 . The inclusion of high-redshift SNe Ia yields H 0 = 73.30 ± 1.04 km s −1 Mpc −1 and q 0 = −0.51 ± 0.024. We find a 5σ difference with the prediction of H 0 from Planck CMB observations under ΛCDM, with no indication that the discrepancy arises from measurement uncertainties or analysis variations considered to date. The source of this now long-standing discrepancy between direct and cosmological routes to determining the Hubble constant remains unknown.

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on 2MASS photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CObandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjuction with evolutionary models, we argue that the cluster has an initial mass of ∼40,000M , and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue-to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge. arXiv:0708.0821v3 [astro-ph]