https://biblio.ugent.be/publication/685594/file/1130605.pdf

Review of Particle Physics

We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of the halo mass, the initial density fluctuation spectrum, and the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fitted over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology, the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation that reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results with previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, that allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model.

https://iopscience.iop.org/article/10.1086/304888/pdf

A Universal Density Profile from Hierarchical Clustering

We discuss the cosmological simulation code GADGET-2, a new massively parallel TreeSPH code, capable of following a collisionless fluid with the N-body method, and an ideal gas by means of smoothed particle hydrodynamics (SPH). Our implementation of SPH manifestly conserves energy and entropy in regions free of dissipation, while allowing for fully adaptive smoothing lengths. Gravitational forces are computed with a hierarchical multipole expansion, which can optionally be applied in the form of a TreePM algorithm, where only short-range forces are computed with the ‘tree’ method while long-range forces are determined with Fourier techniques. Time integration is based on a quasi-symplectic scheme where long-range and short-range forces can be integrated with different time-steps. Individual and adaptive short-range time-steps may also be employed. The domain decomposition used in the parallelization algorithm is based on a space-filling curve, resulting in high flexibility and tree force errors that do not depend on the way the domains are cut. The code is efficient in terms of memory consumption and required communication bandwidth. It has been used to compute the first cosmological N-body simulation with more than 10 10 dark matter particles, reaching a homogeneous spatial dynamic range of 10 5 per dimension in a three-dimensional box. It has also been used to carry out very large cosmological SPH simulations that account for radiative cooling and star formation, reaching total particle numbers of more than 250 million. We present the algorithms used by the code and discuss their accuracy and performance using a number of test problems. GADGET-2 is publicly released to the research community. Ke yw ords: methods: numerical ‐ galaxies: interactions ‐ dark matter.

/pdf/the-cosmological-simulation-code-gadget-2-48ba4eyb8k.pdf

The Cosmological simulation code GADGET-2

This paper describes the Seventh Data Release of the Sloan Digital Sky Survey (SDSS), marking the completion of the original goals of the SDSS and the end of the phase known as SDSS-II. It includes 11,663 deg^2 of imaging data, with most of the ~2000 deg^2 increment over the previous data release lying in regions of low Galactic latitude. The catalog contains five-band photometry for 357 million distinct objects. The survey also includes repeat photometry on a 120° long, 2°.5 wide stripe along the celestial equator in the Southern Galactic Cap, with some regions covered by as many as 90 individual imaging runs. We include a co-addition of the best of these data, going roughly 2 mag fainter than the main survey over 250 deg^2. The survey has completed spectroscopy over 9380 deg^2; the spectroscopy is now complete over a large contiguous area of the Northern Galactic Cap, closing the gap that was present in previous data releases. There are over 1.6 million spectra in total, including 930,000 galaxies, 120,000 quasars, and 460,000 stars. The data release includes improved stellar photometry at low Galactic latitude. The astrometry has all been recalibrated with the second version of the USNO CCD Astrograph Catalog, reducing the rms statistical errors at the bright end to 45 milliarcseconds per coordinate. We further quantify a systematic error in bright galaxy photometry due to poor sky determination; this problem is less severe than previously reported for the majority of galaxies. Finally, we describe a series of improvements to the spectroscopic reductions, including better flat fielding and improved wavelength calibration at the blue end, better processing of objects with extremely strong narrow emission lines, and an improved determination of stellar metallicities.

/pdf/the-seventh-data-release-of-the-sloan-digital-sky-survey-5cm9fqkek5.pdf

The Seventh Data Release of the Sloan Digital Sky Survey

The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a simulation of the growth of dark matter structure using 2,1603 particles, following them from redshift z = 127 to the present in a cube-shaped region 2.230 billion lightyears on a side. In postprocessing, we also follow the formation and evolution of the galaxies and quasars. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies.

/pdf/simulations-of-the-formation-evolution-and-clustering-of-1yahvmv5qk.pdf

Simulations of the formation, evolution and clustering of galaxies and quasars

A Photographic Search for Satellites of Neptune

Collisional fragmentation is shown to not be a barrier to rocky planet formation at small distances from the host star. Simple analytic arguments demonstrate that rocky planet formation via collisions of homogeneous gravity-dominated bodies is possible down to distances of order the Roche radius ($r_\mathrm{Roche}$). Extensive N-body simulations with initial bodies ${\gtrsim}1700$ km that include plausible models for fragmentation and merging of gravity-dominated bodies confirm this conclusion and demonstrate that rocky planet formation is possible down to ${\sim}$1.1 $r_\mathrm{Roche}$. At smaller distances, tidal effects cause collisions to be too fragmenting to allow mass build-up to a final, dynamically stable planetary system. We argue that even differentiated bodies can accumulate to form planets at distances that are not much larger than $r_\mathrm{Roche}$.

Collisional Fragmentation Is Not a Barrier to Close-in Planet Formation

Semi-analytic treatments of the evolution of orbits of weakly interacting massive particles (WIMPs) in the solar system suggest that the WIMPs bound to the solar system may enhance the direct detection rate relative to that of the unbound pop ulation by up to a factor of order unity, and boost the flux of neutrinos from WIMP annihilation in the Earth by up to two orders of magnitude. To test these important but uncertain results, we perform a suite of numerical orbit integrations to explore the properties of the bound WIMP population as a function of the WIMP mass and the scattering cross section with baryonic matter. For regions of WIMP parameter space presently allowed by experiments, we find that (i) the bound W IMP population enhances the direct detection rate by at most ∼ 1% relative to the rate from unbound halo WIMPs; (ii) it is unlikely that planned km 3 -scale neutrino telescopes will detect neutrinos from WIMP annihilation in the Earth; (iii) the event rate from neutrinos produced by WIMP annihilation in the Sun may be much smaller than implied by the usual calculations, which assume that WIMPs scattered onto bound orbits are rapidly thermalized in the Sun.

/pdf/dynamics-of-wimps-in-the-solar-system-and-implications-for-5cwvpke3lb.pdf

Dynamics of WIMPs in the Solar System and Implications for Direct and Indirect Detection

We have examined the effect of slow growth of a central black hole on spherical galaxies that obey Sersic or R-1/m surface-brightness profiles. During such growth the actions of each stellar orbit are conserved, which allows us to compute the final distribution function (DF) if we assume that the initial DF is isotropic. We find that black hole growth leads to a central cusp or ``excess light,'' in which the surface brightness varies with radius as R-1.3 (with a weak dependence on Sersic index m), the line-of-sight velocity dispersion varies as R-1/2, and the velocity anisotropy is beta similar or equal to -0.24 to -0.28 depending on m. The excess stellar mass in the cusp scales approximately linearly with the black hole mass, and is typically 0.5-0.85 times the black hole mass. This process may strongly influence the structure of nuclear star clusters in spheroidal galaxies if they contain black holes.

https://iopscience.iop.org/article/10.3847/0004-6256/151/5/119/pdf

Adiabatic black hole growth in sérsic models of elliptical galaxies

Black holes, an extreme consequence of the mathematics of General Relativity, have long been suspected of being the prime movers of quasars, which emit more energy than any other objects in the Universe. Recent evidence indicates that supermassive black holes, which are probably quasar remnants, reside at the centers of most galaxies. As our knowledge of the demographics of these relics of a violent earlier Universe improve, we see tantalizing clues that they participated intimately in the formation of galaxies and have strongly influenced their present-day structure.

Scott Tremaine

Papers

A Photographic Search for Satellites of Neptune

Collisional Fragmentation Is Not a Barrier to Close-in Planet Formation

Dynamics of WIMPs in the Solar System and Implications for Direct and Indirect Detection

Adiabatic black hole growth in sérsic models of elliptical galaxies

Supermassive Black Holes and the Evolution of Galaxies