Day 1154

Monday.

1609.04822
A free-form lensing model of A370 revealing stellar mass dominated BCGs, in Hubble Frontier Fields images
Diego, et al

Derive a free-form mass distribution for the unrelaxed cluster A370 (z=0.375), using the latest HFF images and GLASS spectroscopy.  Staring from a reliable set of 10 multiply lensed systems, produce a free-form model that identifies ~80 multiple-images.  Good consistency is found between models using independent subsamples and these lensed systems, with detailed agreement for the well resolved arcs.  The mass distribution has two very similar concentrations centered on the two prominent BCGs, with mass profiles that are accurately constrained by a uniquely useful system of long radially lensed images centered on both BCGs.  Show that the lensing mass profiles of these BCGs are mainly accounted for by their stellar mass profiles, with a modest contribution from dark matter within r<100 kpc of each BCG.  This conclusion may favor a cooled cluster gas origin for BCGs, rather than via mergers of normal galaxies for which DM should dominate over stars.  Growth via merging between BCGs is, however, consistent with this finding, so that stars still dominated over dark matter.


1609.05175
A test of Gaia Data Release 1 parallaxes: implications for the local distance scale
Casertano, Riess, Bucciarelli, Lattanzi

Present a comparison of Gaia DR1 parallaxes with photometric parallaxes for a sample of 212 Galactic Cepheids at a median distance of 2 kpc, and explore their implications on the distance scale and the local value of the Hubble constant H0.  The Cepheid distances are estimated from a recent calibration of the NIR Period-Luminosity P-L relation.  The comparison is carried out in parallax space, where the DR1 parallax errors, with a median value of half the median parallax, are expected to be well-behaved.  With the exception of one outlier, the DR1 parallaxes are in remarkably good global agreement with the reductions, and the published errors may be conservatively overestimated by about 20%.  The parallaxes of 9 Cepheids brighter than G=6 may be systematically underestimated, trigonometric parallaxes measured with the HST FGS for three of these objects confirm this trend.  If interpreted as an independent calibration of the Cepheid luminosities and assumed to be otherwise free of systematic uncertainties, DR1 parallaxes would imply a decrease of 0.3% in the current estimate of the local Hubble constant, well within their statistical uncertainty, and corresponding to a value 2.5 sigma (3.5 sigma if the errors are scaled) higher than the value inferred from Planck CMB data used in conjunction with LCDM.  Also test for zero point error in Gaia parallaxes and find none to a precision of ~20 mas.  Caution however that with this early release, the complete systematic properties of the measurements may not be fully understood at the statistical level of the Cepheid sample mean, a level an order of magnitude below the individual uncertainties.  The early results from DR1 demonstrate again the enormous impact that the full mission with likely have on fundamental questions in astrophysics and cosmology.