Friday.
1502.07346
A primordial origin for the composition similarity between the Earth and the Moon
Mastrobuono-Battisti, Perets, Raymond
Most of the Earth-moon system can be explained by a collision between a planetary embryo and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic composition show them to be highly similar. In contrast, the compositions of other solar system bodies are significantly different than the Earth and Moon. This poses a major challenge to the giant impact scenario since the Moon-forming impactor is then thought to also have differed in composition from the proto-Earth. Track the feeding zones of growing planets in a suite of simulations of planetary accretion, in order to measure the composition of Moon-forming impactors. Find that different planets formed in the same simulation have distinct compositions, but the compositions of giant impactors are systematically more similar to the planets they impact. A significant fraction of planet-impactor pairs have virtually identical compositions. Thus, the similarity in composition between the Earth and Moon could be a natural consequence of a late giant impact.
1502.07347
Galaxy cluster mass reconstruction project: II. quantifying scatter and bias using contrasting mock catalogues
Old, ... Rozo, Rykoff, et al
An comparison of various galaxy-based cluster mass estimation techniques that utilise the positions, velocities and colors of galaxies. Aim: quantify the scatter, systematic bias and completeness of cluster masses derived from a diverse set of 25 galaxy-based methods using two contrasting mock galaxy catalogues based on a sophisticated halo occupation model and a SAM. Analyzing 968 clusters, find a wide range in the RMS errors in log M200c delivered by the different methods (0.18 to 1.08 dex, or a factor of ~1.5 to 12), with abundance matching and richness methods providing the best results, irrespective of the input model assumptions. In addition, certain methods produce a significant number of catastrophic cases where the mass is under- or over-estimated by a factor grater than 10. Given the steeply falling high-mass end of the cluster MF, recommend that richness or abundance matching-based methods are used in conjunction with these methods as sanity check for studies selecting high mass clusters. See a stronger correlation of the recovered to input number of galaxies for both catalogues in comparison with the group/cluster mass, however, this does not guarantee that the correct member galaxies are being selected. Do not observe significantly higher scatter for either mock galaxy catalogues. Results have implications for cosmo analysis that utilize the masses, richness or abundances of clusters, which have different uncertainties when different methods are used.
1502.07356
Milking the spherical cow: on aspherical dynamics in spherical coordinates
Pontzen, ... Roth et al
Galaxies and DM haloes that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealised calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analysis of stellar distribution functions, collisions relaxation, or DM core-creation? Prove a general answer to this question for a wide class of aspherical systems; specifically, consider distribution functions that are "maximally stable", i.e., that do not evolve at first order when external potentials (which arise from baryons, large scale tidal fields or inflating substructure) are applied. Show that a spherically-symmetric analysis of such systems gives rise to the case conclusion that the density of particles in phase space is ergodic (a function of energy alone). Using this idea, able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the center of spheroids; (b) numerical simulations that use an idealized spherical-symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial DM haloes (formed in collisions cosmo sims) nearly attain the maximally-stable limit, but their evolution freezes out before reaching it.
1502.07357
Baryon impact on the halo mass function: fitting formula and implications for cluster cosmology
Bocquet, Saro, Dolag, Mohr
Calibrate the halo MF accounting for halo baryons and present fitting formulae for spherical overdensity masses M500c, M200c, and M200m. Use the hydrodynamical Magneticum simulations, suited because of their high resolution nan large cosmo volumes up to ~2 Gpc^3. Baryonic effects globally decrease the masses of galaxy clusters, which at given mass, results in a decrease of their number density. This effect vanishes at high z (~2) and for high masses >5e14 Msun. Perform cosmo analyses of 3 idealized approximations to the cluster surveys by SPT, Planck, and eROSITA. For the SPT-like and the Planck-like samples, find that the impact of baryons on the cosmo results is negligible. In the eROSITA-like case, find that neglecting the baryonic impact leads to an underestimate of Omega_m by about 0.01, which is comparable to the expected uncertainty from eROSITA. Compare MF fits with the literature. In the analysis of the Planck-like sample, results obtained using MF are shifted by Delta(sigma_8)~0.05 with respect to results obtained using the Tinker+2008 fit. This shift represent a large fraction of the observed difference between the latest results from Planck clusters and CMB anisotropies, and the tension is essentially removed. Discuss biases that can be introduced through inadequate MF parameterizations that introduce false cosmo sensitivity. Additional work to calibrate the halo MF is crucial for progress in cluster cosmology.
1502.07442
ngravs: distinct gravitational interactions in GADGET-2
Croker
Discuss an extension of the massively parallel cosmo sim code GADGET-2, which enables investigations of distinct gravitational force laws between particle species. In addition to simplifying investigations of a universally modified force law, the graves extension allows state-of-the-art collisions cosmo sims of quite exotic gravitational scenarios. Briefly review the algorithms used by GADGET-2, and present extension to multiple gravitates, highlighting additional features that facilitate consideration of exotic force laws. Discuss the accuracy and performance of the graves extension, both internally and with an unaltered GADGET-2, under all relevant operational modes. The graves extension is publicly released .
1502.07696
Why are dense planetary rings only found between 8AU and 20AU?
Hedman
Recent discovery reveals that complete dense planetary rings are not only found around Saturn and Uranus, but also found small bodies orbiting in the vicinity of these giant planets. This report examines whether there could be a physical process that would make rings more likely to form or persist in this particular part of the outer Solar System. Specifically, the ring material orbiting Saturn and Uranus appears to be much weaker than the material forming the innermost moons of Jupiter and Neptune. Also, the mean surface temperatures of Saturn's, Uranus' and Chariklo's rings are all close to 70K. Thus the restricted distribution of dense rings in our SS may arise because icy materials are particularly weak around that temperature.
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