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

The discovery by Kormendy of a M• 109 M☉ massive dark object (MDO) in NGC 4594 is confirmed with higher resolution spectroscopy from the Canada-France-Hawaii Telescope (CFHT) and the Hubble Space Telescope (HST). CFHT measurements with the Subarcsecond Imaging Spectrograph improve the resolution from σ* = 040 to 027 Gaussian dispersion radius of the point-spread function (PSF). The apparent central velocity dispersion rises from σ = 250 ± 7 km s-1 to σ = 286 ± 7 km s-1. As observed with the COSTAR-corrected HST, the Faint Object Spectrograph, and a 021 aperture, σ = 321 ± 7 km s-1 is still higher, and the central rotation curve is very steep. The highest-M• published dynamical model fits the new observations reasonably well when "observed" at HST resolution. The spatial resolution has now improved by a factor of ~5 since the discovery measurements, and the case for a black hole (BH) has strengthened correspondingly. We confirm that NGC 4594 has a Seyfert spectrum; Hα is ~5200 km s-1 wide at zero intensity. However, gas velocities are lower than the circular velocities implied by the stars, so they cannot be used to test the BH case in NGC 4594. The gas may be in a ring, or it may be associated with patchy dust. HST images with the Wide Field and Planetary Camera 2 show dust at some aperture positions. NGC 4594 appears to have a bright point nucleus. However, the central absorption-line strengths are low, consistent with dilution by enough nonthermal light to explain the "nucleus." There is no evidence for a distinct nuclear star cluster. NGC 4594 is similar to M87, which also has a nonthermal nuclear source, and not to M31 and NGC 3115, which have quiescent BHs and nuclear star clusters.

http://iopscience.iop.org/article/10.1086/310399/pdf

Hubble space telescope spectroscopic evidence for A 1 × 109 M⊙ black hole in NGC 4594

A test of a statistical model for the luminosities of bright cluster galaxies

It is suggested that the mass of the individual unseen 'disk things' which furnish unseen mass satisfies an inequality that is based on the demonstrated existence of a large number of wide binary stars in the Galaxy whose projected separations are as large as 0.1 pc. Since these binaries are weakly bound, with orbital velocities of 0.3 km/sec and periods of the order of 1 million years, they can be disrupted by passing stars or other pointlike objects. This implies that the mass of the unseen disk objects, which provide about half the local mass density, must be less than 2 solar masses.

Maximum mass of objects that constitute unseen disk material

Elliptical, lenticular, and early-type spiral galaxies show a remarkably tight power-law correlation between the mass M • of their central supermassive black hole (SMBH) and the number N GC of globular clusters (GCs): M • = m •/ × N 1.08±0.04 GC with m •/ = 1.7 × 105 M ☉. Thus, to a good approximation the SMBH mass is the same as the total mass of the GCs. Based on a limited sample of 13 galaxies, this relation appears to be a better predictor of SMBH mass (rms scatter 0.2 dex) than the M •-σ relation between SMBH mass and velocity dispersion σ. The small scatter reflects the fact that galaxies with high GC specific frequency SN tend to harbor SMBHs that are more massive than expected from the M •-σ relation.

A correlation between central supermassive black holes and the globular cluster systems of early-type galaxies

We investigate the distribution of mass M and orbital period P of extra-solar planets, taking account of selection effects due to the limited velocity precision and duration of existing surveys. We fit the data on 63 planets to a power-law distribution of the form dn=CM^{-alpha}P^{-beta}(dM/M)(dP/P), and find alpha=0.12+-0.10, beta=-0.26+-0.06 for M<10M_J, where M_J is the Jupiter mass. The correlation coefficient between these two exponents is -0.32, indicating that uncertainties in the two distributions are coupled. We estimate that 3% of solar-type stars have companions in the range 1M_J<M<10M_J, 2d<P<10yr.

Scott Tremaine

Papers

Hubble space telescope spectroscopic evidence for A 1 × 109 M⊙ black hole in NGC 4594

A test of a statistical model for the luminosities of bright cluster galaxies

Maximum mass of objects that constitute unseen disk material

A correlation between central supermassive black holes and the globular cluster systems of early-type galaxies

Maximum likelihood method for estimating the mass and period distributions of extra-solar planets