We present a new model for computing the spectral evolution of stellar populations at ages between 100,000 yr and 20 Gyr at a resolution of 3 A across the whole wavelength range from 3200 to 9500 A for a wide range of metallicities. These predictions are based on a newly available library of observed stellar spectra. We also compute the spectral evolution across a larger wavelength range, from 91 A to 160 micron, at lower resolution. The model incorporates recent progress in stellar evolution theory and an observationally motivated prescription for thermally-pulsing stars on the asymptotic giant branch. The latter is supported by observations of surface brightness fluctuations in nearby stellar populations. We show that this model reproduces well the observed optical and near-infrared colour-magnitude diagrams of Galactic star clusters of various ages and metallicities. Stochastic fluctuations in the numbers of stars in different evolutionary phases can account for the full range of observed integrated colours of star clusters in the Magellanic Clouds. The model reproduces in detail typical galaxy spectra from the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We exemplify how this type of spectral fit can constrain physical parameters such as the star formation history, metallicity and dust content of galaxies. Our model is the first to enable accurate studies of absorption-line strengths in galaxies containing stars over the full range of ages. Using the highest-quality spectra of the SDSS EDR, we show that this model can reproduce simultaneously the observed strengths of those Lick indices that do not depend strongly on element abundance ratios [abridged].

https://hal.archives-ouvertes.fr/hal-00012948/document

Stellar population synthesis at the resolution of 2003

Statistics for Spatial Data.

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 cooling of gas in the centers of dark matter halos is expected to lead to a more concentrated dark matter distribution. The response of dark matter to the condensation of baryons is usually calculated using the model of adiabatic contraction, which assumes spherical symmetry and circular orbits. In contrast, halos in the hierarchical structure formation scenarios grow via multiple violent mergers and accretion along filaments, and particle orbits in the halos are highly eccentric. We study the effects of the cooling of gas in the inner regions of halos using high-resolution cosmological simulations which include gas dynamics, radiative cooling, and star formation. We find that the dissipation of gas indeed increases the density of dark matter and steepens its radial profile in the inner regions of halos compared to the case without cooling. For the first time, we test the adiabatic contraction model in cosmological simulations and find that the standard model systematically overpredicts the increase of dark matter density in the inner 5% of the virial radius. We show that the model can be improved by a simple modification of the assumed invariant from M(r)r to M(r_av)r, where r and r_av are the current and orbit-averaged particle positions. This modification approximately accounts for orbital eccentricities of particles and reproduces simulation profiles to within 10-20%. We present analytical fitting functions that accurately describe the transformation of the dark matter profile in the modified model and can be used for interpretation of observations.

http://export.arxiv.org/pdf/astro-ph/0406247

Response of dark matter halos to condensation of baryons: cosmological simulations and improved adiabatic contraction model

We study dynamics of bars in models of disc galaxies embedded in realistic dark matter haloes We find that disc thickness plays an important, if not dominant, role in the evolution and structure of the bars We also make extensive numerical tests of different N-body codes used to study bar dynamics Models with thick discs typically used in this type of modelling (height-to-length ratio h z l R d = 02) produce slowly rotating, and very long, bars In contrast, more realistic thin discs with the same parameters as in our Galaxy (h z /R d ≈ 01) produce bars with normal length R bar ≈ R d , which rotate quickly with the ratio of the corotation radius to the bar radius R = 12-14 compatible with observations Bars in these models do not show a tendency to slow down, and may lose as little as 2-3 per cent of their angular momentum due to dynamical friction with the dark matter over cosmological time We attribute the differences between the models to a combined effect of high phase-space density and smaller Jeans mass in the thin-disc models, which result in the formation of a dense central bulge Special attention is paid to numerical effects, such as the accuracy of orbital integration, force and mass resolution Using three N-body codes - GADGET, adaptive refinement tree (ART) and PKDGRAV - we find that numerical effects are very important and, if not carefully treated, may produce incorrect and misleading results Once the simulations are performed with sufficiently small time-steps and with adequate force and mass resolution, all the codes produce nearly the same results: we do not find any systematic deviations between the results obtained with TREE codes (GADGET and PKDGRAV) and with the adaptive mesh refinement (ART) code

Dynamics of barred galaxies: effects of disc height

JC and FP acknowledge support from the Spanish MICINNs Consolider-Ingenio 2010 Programme under grant MD CSD2009-00064, MINECO Centro de Excelencia Severo Ochoa Programme under the grants SEV-20120249, FPA2012-34694, and the projects AYA2014-60641-C2-1-P and AYA2012-31101. GY acknowledges financial support from MINECO/FEDER (Spain) under project number AYA2012-31101 and AYA2015-63810-P.

/pdf/accurate-mass-and-velocity-functions-of-dark-matter-haloes-181a9k3b1r.pdf

Accurate mass and velocity functions of dark matter haloes

Using extensive N-body simulations we estimate redshift space power spectra of clusters of galaxies for different cosmological models (SCDM, TCDM, CHDM, Lambda-CDM, OCDM, BSI, tau-CDM) and compare the results with observational data for Abell-ACO clusters. Our mock samples of galaxy clusters have the same geometry and selection functions as the observational sample which contains 417 clusters of galaxies in a double cone of galactic latitude |b| > 30 degrees up to a depth of 240 Mpc/h. 
The power spectrum has been estimated for wave numbers k in the range 0.03 k_max ~ 0.05 h/Mpc the power spectrum of the Abell-ACO clusters has a power-law shape, P(k)\propto k^n, with n ~ -1.9, while it changes sharply to a positive slope at k < k_max. By comparison with the mock catalogues SCDM, TCDM (n=0.9), and also OCDM with Omega_0 = 0.35 are rejected. Better agreement with observation can be found for the Lambda-CDM model with Omega_0 = 0.35 and h = 0.7 and the CHDM model with two degenerate neutrinos and Omega_HDM = 0.2 as well as for a CDM model with broken scale invariance (BSI) and the tau-CDM model. As for the peak in the Abell-ACO cluster power spectrum, we find that it does not represent a very unusual finding within the set of mock samples extracted from our simulations.

Constraining cosmological models with cluster power spectra

We make detailed theoretical predictions for the assembly properties of the Local Group (LG) in the standard LambdaCDM cosmological model We use three cosmological N-body dark matter simulations from the CLUES project, which are designed to reproduce the main dynamical features of the matter distribution down to the scale of a few Mpc around the LG Additionally, we use the results of an unconstrained simulation with a sixty times larger volume to calibrate the influence of cosmic variance We characterize the Mass Aggregation History (MAH) for each halo by three characteristic times, the formation, assembly and last major merger times A major merger is defined by a minimal mass ratio of 10:1 We find that the three LGs share a similar MAH with formation and last major merger epochs placed on average \approx 10 - 12 Gyr ago Between 12% and 17% of the halos in the mass range 5 x 10^11 Msol/h < M_h < 5 x 10^12 Msol/h have a similar MAH In a set of pairs of halos within the same mass range, a fraction of 1% to 3% share similar formation properties as both halos in the simulated LG An unsolved question posed by our results is the dynamical origin of the MAH of the LGs The isolation criteria commonly used to define LG-like halos in unconstrained simulations do not narrow down the halo population into a set with quiet MAHs, nor does a further constraint to reside in a low density environment The quiet MAH of the LGs provides a favorable environment for the formation of disk galaxies like the Milky Way and M31 The timing for the beginning of the last major merger in the Milky Way dark matter halo matches with the gas rich merger origin for the thick component in the galactic disk Our results support the view that the specific large and mid scale environment around the Local Group play a critical role in shaping its MAH and hence its baryonic structure at present

/pdf/the-dark-matter-assembly-of-the-local-group-in-constrained-4vw4saollt.pdf

Anatoly Klypin

Papers

Response of dark matter halos to condensation of baryons: cosmological simulations and improved adiabatic contraction model

Dynamics of barred galaxies: effects of disc height

Accurate mass and velocity functions of dark matter haloes

Constraining cosmological models with cluster power spectra

The dark matter assembly of the Local Group in constrained cosmological simulations of a LambdaCDM universe