1602.05345
Bayesian hierarchical modeling of weak lensing - the golden goal
Heavens, et al
To accomplish correct Bayesian inference from WL shear data requires a complete statistical description of the data. Starting with a catalogue of shear estimates in tomographic bins, build a model that allows to sample simultaneously from the underlying tomographic shear fields and the relevant power spectra (E-mode, B-mode, and E-B, for auto- and cross-power spectra). The procedure deals easily with masked data and intrinsic alignments. Using Gibbs sampling and messenger fields, show with simulated data that the large (>67000-)dimensional parameter space can be efficiently sampled and the full joint posterior probability density function for the parameters can feasibly be obtained. The method correctly recovers the underlying shear fields and all of the power spectra, including at levels well below the shot noise.
1602.05580
Pixel color magnitude diagrams for semi-resolved stellar populations: the star formation history of regions within the disk and bulge of M31
Conroy, van Dokkum
The analysis of stellar populations has been developed for two limiting cases: spatially-resolved stellar populations in the color-magnitude diagram, and integrated light observations of distant systems. In between these two extremes lies the semi-resoled regime, which encompasses a rich and relatively unexplored realm of observational phenomena. Develop the concept of pixel color magnitude diagrams (pCMDs) as a powerful technique for analyzing stellar populations in the semi-resolved regime. pCMDs show the distribution of imaging data in the plane of pixel luminosity vs. pixel color. A key feature of pCMDs is that they are sensitive to all stars, including both the evolved giants and the unevolved main sequence stars. An important variable in this regime is the mean number of stars per pixel, N_pix. Simulated pCMDs demonstrate a strong sensitivity to the star formation history (SFH) and allow one to break degeneracies between age, metallicity and dust based on two filter data for values of N_pix up to at least 1e4. Extract pCMDs from HST optical imaging of M31 and derive non-parametric SFHs from 1e6 yr to 1e10 yr for both the crowded disk and bulge regions (where N_pix~30-1e3). From analyzing a small region of the disk, find a non-parametric SFH that is smooth and consistent with an exponential decay timescale of 4Gyr. The bulge SFH is also smooth and consistent with a 2 Gyr decay timescale. pCMDs will likely play an important role in maximizing the science returns from next generation ground and space-base facilities.
1602.05836
RadioLensfit: Bayesian weak lensing measurement in the visibility domain
Rivi, Miller, Makhathini, Abdalla
New gen radio telescopes such as SKA will provide a density of detected galaxies that is comparable to that found at optical wavelengths, and with signifiant source shape measurements to make large area radio surveys competitive for WL studies. This will lead WL to become one of the primary science drivers in radio surveys, too, with the advantage that they will access the largest scales in the Universe going beyond optical surveys, like LSST and Euclid, in terms of redshifts that are probed. RadioLensfit is an adaptation to radio data of "lensfit", a model-fitting approach for galaxy shear measurement, originally developed for optical WL surveys. Its key advantage is working directly in the visibility domain, which is the natural approach to adopt with radio data, avoiding systematics due to the imaging process. Present results on galaxy shear measurements, including investigation of sensitivity to instrumental parameters such as the visibilities gridding size, based on simulations of individual galaxy visibilities performed by using SKA1-MID baseline configuration. Get an amplitude of the shear bias in the method comparable with SKA1 requirements for a population of galaxies with realistic flux and scale length distributions estimated from the VLA SWIRE catalog.
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