Day 753

Sunday.  Monday.

1409.1924
The re-distribution of matter in the cores of galaxy clusters
Laporte, White

Present cosmological N-body re-sims of the assembly of BCGs in rich clusters.  At z=2 populate DM sub haloes with self-gravitating stellar systems whose abundance and structure match observed high-z galaxies.  By z=0, mergers have built much larger galaxies at cluster center.  Their DM density profiles are shallower than in corresponding DM only simulations, but their total mass density profiles (stars + DM) are quite similar.  Differences are found only at radii where the effects of central BHs may be significant.  DM density slopes shallower than gamma=1.0 occur for r/r_200 < 0.015, close to the half-light radii of the BCGs.  Experiments support earlier suggestions that NFW-like profiles are an attractor for the hierarchical growth of structure in collision less systems -- total mass density profiles asymptote to the solution found in DM-only sims over the radial range where mergers produce significant mixing between stars and DM.  Simulated DM fractions are substantially higher in BCGs than in field ellipticals, reaching 80% within the HLR.  Also estimate that SMBH mergers should create BCG cores as large as r_c~3 kpc.  The good agreement of all these properties with recent observational studies of BCG structure suggests that dissipation processes have not played a dominant role in the assembly of the observed systems.

1409.7389
Modeling the redshift-space three-point correlation function in SDSS-III
Guo et al

Present z-space 3PCF for z~0.5 CMASS LRGs in BOSS DR11.  The 3PCF measurements are interpreted within the HOD framework using high-res N-body sims, and the model successfully reproduces the 3PCF on scales larger than 1 Mpc/h.  As with the case for the z-space 2PCF, find that the z-space 3PCF measurements also require the inclusion of galaxy velocity bias in the model.  In particular, the central galaxy in a halo on average is in motion wrt the core of the halo.  Discuss the potential of the small-scale 3PCF to tighten the constraints on the relation between galaxies and DM haloes and on the phase-space distribution of galaxies.

1409.7395
GIZMO: a new class of accurate, mesh-free hydrodynamic simulation methods
Hopkins

Present and study 2 new Langrangian numerical methods for solving the equations of hydrodynamics, in a systematic comparison with moving-mesh, SPH, and non-moving grid methods.  The new methods are designed to capture many advantages of both SPH and grid-based AMR schemes.  They are based on a kernel discretization of the volume coupled to a high-order matrix gradient estimator and a Riemann solver acting over the volume 'overlap.'  Implement and test a parallel, second-order version of the method with coupled self-gravity & cosmological integration, in the code GIZMO: this maintains exact mass, energy and momentum conservation; exhibits superior angular momentum conservation compared to all other methods studied; does not require 'artificial diffusion' terms; and allows fluid elements to move with the flow so resolution is automatically adaptive.  Consider a large suite of test problems, and find that on all problems the new methods appear competitive with moving-mesh schemes, with some advantages (particularly in angular momentum conservation), at the cost of enhanced noise.  The new methods have many advantages vs SPH: proper convergence, good capturing of fluid-mixing instabilities, dramatically reduced 'particle noise' & numerical viscosity, more accurate sub-sonic flow evolution, & numerical diffusion, velocity-independence of errors, accurate coupling to N-body gravity solvers, good angular momentum conservation, and elimination of 'grid alignment' effects.  Can, for example, follow hundreds of orbits of gaseous disks, while AMR and SPH methods break down in a few orbits.  All of these differences are important for a wide range of astrophysical problems.

1409.7398
The ancient heritage of water ice in the solar system
Cleeves et al

Identifying the source of Earth's water is central to understanding the origins of life-fostering environments and to assessing the prevalence of such environments in space.  Water throughout the solar system exhibits D to H enrichments, a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent molecular cloud or (ii) the solar nebular protoplanetary disk.  Utilizing a comprehensive treatment of disk ionization, find that ion-driven D pathways are inefficient, curtailing the disk's D-water formation and its viability as the sole source for the SS's water.  This finding implies that if the SS's formation was typical, abundant interstellar ices are available to all nascent planetary systems.

1409.7583
Representations of Time Coordinates in FITS
Rots et al

Time on all scales and precisions known in astronomical datasets is to be described in an unambiguous, complete, and self-consistent manner.  Employing the well-established WCS framework, and maintaining compatibility with the FITS conventions that are currently in use to specify time, the standard is extended to describe rigorously the time coordinate.  World coordinate functions are defined for temporal axes sampled linearly and as specified by a lookup table.  The resulting standard is consistent with the existing FITS WCS standards and specifies a metadata set that achieves the aims enunciated above.