Nature15569
Resonant interactions and chaotic rotation of Pluto's small moons
Showalter and Hamilton
With NASA's New Horizons probe fast approaching Pluto for its flyby on 14 July, the authors present new results from the Pluto system and offer some predictions about what the spacecraft might observe. Pluto's four small moons -- Styx Nix, Kerberos and Hydra -- follow near-circular, near-equatorial orbits around the central 'binary planet' comprising Pluto and its large moon, Charon. In an analysis of HST images, the authors demonstrate that Styx, Nix and Hydra are tied together by a 3-body resonance. Perturbations by the other bodies inject chaos into this otherwise stable configuration so that Nix and Hydra rotate chaotically, driven by the large torques of the Pluto-Charon binary. Nix and Hydra have bright surfaces similar to that of Charon. Kerberos may be much darker.
1506.01019
Signatures of dark matter expansion in galaxy populations
Brook, Cintio
Dark matter cores within galaxy haloes can be formed by energy feedback from star forming regions: an energy balance suggests that the maximum core formation efficiency arises in galaxies with M*~1e8.6Msun. Show that a model population of galaxies, in which the density profile has been modified by such baryonic feedback, is able to explain the observed galaxy velocity function and TF relations significantly better than a model in which a universal cusp density profile is assumed. Alternative models, namely warm or self-interacting DM, also provide a better match to these observed relations than a universal profile model does, but make different predictions for how halo density profiles vary with mass compared to the baryonic feedback case. Propose that different core formation mechanisms may be distinguished based on the imprint they leave on galaxy populations over a wide range of mass. Within the current observational data, find evidence of the expected signatures of the mass dependence of core formation generated by baryonic feedback.
1506.01076
A new model to predict weak-lensing peak counts II. Parameter constraint strategies
Lin, Kilbinger
Peak counts have been shown to be an excellent tool to extract the non-Gaussian part of the WL signal. Recently developed a fast stochastic forward model to predict WL peak counts. The model is able to reconstruct the underlying distribution of observables for analyses. In this work, explore and compare various strategies for constraining parameter using the model, focusing on the matter density Omega_m and the density fluctuation amplitude sigma_8. First, examine the impact from the cosmo dependency of covariances (CDC). Second, perform the analysis with the copula likelihood, a technique which makes a weaker assumption compared to the Gaussian likelihood. Third, direct, non-analytic parameter estimations are applied using the full information of the distribution. Fourth, obtain constraints with approximate Bayesian computation (ABC), an efficient, robust, and likelihood-free algorithm based on accept-reject sampling. Find that neglecting the CDC effect enlarges parameter contours by 22%, and that the covariance-varying copula likelihood is a very good approximation to the true likelihood. The direct techniques work well in spite of noisier contours. Concerning ABC, the iterative process converges quickly to a posterior distribution that is in an excellent agreement with results from other analyses. The time cost for ABC is reduced by two orders of magnitude. The stochastic nature of the WL peak count model allow various techniques to be used that approach the true underlying probability distribution of observables, without making simplifying assumptions. This work can be generalized to other observables where forward simulations provide samples of the underlying distribution.
1506.01100
Fingers-of-Got effect of infalling satellite galaxies
Hikage, Yamamoto
Understanding the nonlinear property of redshift-space distortion, i.e., Figners-of-God (FoG) effect, is important for the redshift-space distortion studies to test gravity models. FoG effect has been usually attributed to the random motion of galaxies inside the clusters. In this paper, demonstrate the importance of the coherent infalling motion of satellite galaxies toward the cluster center. Analytically derive the satellite velocity distribution due to the infall motion together with the random motion and show that the velocity distribution becomes far from Maxwellian when the infalling motion is dominant. Use simulated sub halo catalogs to find that the contribution of infall motion is important for massive subhalos and that their velocity distribution has top-hat like shape as expected from the analytic model. Also study the FoG effect due to infall motion on the z-space PS. Using the simulated sub halo catalogs based on the halo occupation distribution of luminous red galaxies, find that the z-space PS significantly differs form the expectations when the infall motion is ignored. Discuss the impacts on the estimation of the satellite velocity bias.
1506.01369
Measuring the lensing potential with galaxy clustering
Montanari, Durrer
Investigate how the lensing potential can be constrained with future galaxy surveys using their number counts. Such a measurement is an independent test of the standard LCDM framework and can be used to discern modified gravity models. Perform a Fisher matrix forecast based on galaxy angular PS, assuming specifications consistent with future photometric Euclid-like surveys, for which a computation and fitting formula of magnification bias is given, and spectroscopic SKA-like surveys. This analysis suggests that the amplitude of the lensing potential can be constrained at the same level as other standard LCDM cosmo parameters.
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