1508.02381
Digging deeper into the Southern skies: a compact Milky-Way companion discovered in first-year Dark Energy Survey data
Lique, et al
DES is a 5k sq deg survey in the southern hemisphere, which is rapidly reducing the existing north-south asymmetry in the census of MW satellites and other stellar substructure. Use the Y1 DES data down to previously unprobed photometric depths to search for stellar systems in the Galactic halo, therefore complementing the previous analysis of the same data carried out by the group earlier this year. The search is based on a matched filter algorithm that produces stellar density maps consistent with stellar population models of various ages, metallicities, and distances over the survey area. The most conspicuous density peaks in these maps have been identified automatically and ranked according to their significance and recurrence for different input models. Report the discovery of one additional stellar system besides those previously found by several authors using the same fist-year DES data. The object is compact, and consistent with being dominated by an old and metal-poor population. DES J0034-4902 is found at high significance and appears in the DES images as a compact concentration of faint blue point sources at ~87kpc. Its half-light radius of r_h=0.88±4.31 pc and total luminosity of M_V~=3.05±0.5 are consistent with it being a low mass halo cluster. It is also found to have a very elongated shape. In addition, the deeper probe of DES 1st year data confirms the recently reported satellite galaxy candidate Horologium II as a significant stellar overdensity. Also infer its structural properties and compare them to those reported in the literature.
1508.02469
Geometrical constraint on curvature with BAO experiments
Takada, Dore
The spatial curvature (K or Omega_K) is one of the most fundamental parameters of isotropic and homogeneous universe and has a close link to the physics of the early universe. Combining the radial and angular diameter distances measured via the baryon acoustic oscillation experiments allows us to unambiguously constrain the curvature. The method is primarily based on the metric theory, but not much on the theory of structure formation other than the existence of BAO scale and is free of any model of DE. In this paper, estimate a best-achievable accuracy of constraining the curvature with the BAO experiments. Show that an all-sky, cosmic-variance-limited galaxy survey covering the universe up to z>4 enables a precise determination of the curvature to an accuracy of sigma(Omega_K)~1e-3. When a model of dark energy is assumed, either the cosmo constraint or the (w0,wa)-model, it can achieve a precision of sigma(Omega_K)~a few 1e-4. These forecasts require a high sampling density of galaxies, and are degraded by up to a factor of a few for a survey with a finite number density of galaxies.
1508.02484
Exploring photometric redshifts as an optimization problem: an ensemble MCMC and simulated annealing-driven template-fitting approach
Speagle, Capak, Eisenstein, Masters, Steinhardt
Using a grid of ~2 million elements (Delta z = 0.005) adapted from COSMOS photo-z searches, investigate the general properties of template based photo-z likelihood surfaces. Find these surfaces are filled with numerous local minima and large degeneracies that generally confound rapid but "greedy" optimization schemes, even with additional stochastic sampling methods. In order to robustly and efficiently explore these surfaces, develop BAD-Z [Brisk Annealing-Driven Redshifts (Z)], which combines ensemble MCMC sampling with simulated annealing to sample arbitrarily large, pre-generated grids in approximately constant time. Using a mock catalog of 384k objects, show BAD-Z samples ~40 times more efficiently compared to a brute-force counterpart while maintaining similar levels of accuracy. The results represent first steps toward designing template-fitting photo-z approaches limited mainly by memory constraints rather than computation time.
1508.02654
The Mira-Titan Universe: Precision predictions for Dark Energy Surveys
Heitmann, et al
Ground band space-based sky surveys enable powerful cosmological probes based on measurements of galaxy properties an the distribution of galaxies in the Universe. These probes include WL, BAO, galaxy cluster abundance, and RSD; they are essential to improving our knowledge of the nature of DE. On the theory and modeling front, large-scale simulations of cosmic structure formation pay an important role in interpreting the observations and in the challenging task of extracting cosmological physics at the needed precision. These simulations must cover a parameter range beyond the standard six cosmological parameters and need to be run at high mass and force resolution. One key simulation-based task is the generation of accurate theoretical predictions for observables, via the method of emulation. Using a new sampling technique, explore an 8-D parameter space including massive neutrinos and variable DE EoS. Construct trial emulators using two surrogate models (the linear PS and an approximate halo mass function). The new sampling method allows us to build precision emulators from just 26 cosmo models and to increase the emulator accuracy by adding new sets of simulations in a prescribed way. This allows emulator fidelity to be systematically improved as new observational data becomes available and higher accuracy is required. Finally, using one LCDM cosmo as an example, study the demands imposed on a simulation campaign to achieve the required statistics and accuracy when building emulators for DE investigations.
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