Search

We measure the two-point correlation function (CF) of 1357 galaxy clusters with a mass of log10 M 200 ≥ 13.6 h −1 M⊙ and at a redshift of z ≤ 0.125. This work differs from previous analyses in that it utilizes a spectroscopic cluster catalogue, |$\tt {SDSS-GalWCat}$|⁠ , to measure the CF and detect the baryon acoustic oscillation (BAO) signal. Unlike previous studies which use statistical techniques, we compute covariance errors directly by generating a set of 1086 galaxy cluster light-cones from the GLAM N -body simulation. Fitting the CF with a power-law model of the form ξ(s) = (s / s 0)−γ, we determine the best-fitting correlation length and power-law index at three mass thresholds. We find that the correlation length increases with increasing the mass threshold while the power-law index is almost constant. For log10 M 200 ≥ 13.6 h −1 M⊙, we find s 0 = 14.54 ± 0.87 h −1 Mpc and γ = 1.97 ± 0.11. We detect the BAO signal at s  = 100 h −1 Mpc with a significance of 1.60σ. Fitting the CF with a Lambda cold dark matter model, we find |$D_\mathrm{V}(z = 0.089)\mathit{r}^{\mathrm{ fid}}_\mathrm{ d}/\mathit{r}_\mathrm{ d} = 267.62 \pm 26$|   h −1 Mpc, consistent with Planck 2015 cosmology. We present a set of 108 high-fidelity simulated galaxy cluster light-cones from the high-resolution Uchuu N -body simulation, employed for methodological validation. We find D V(z  = 0.089)/ r d = 2.666 ± 0.129, indicating that our method does not introduce any bias in the parameter estimation for this small sample of galaxy clusters. [ABSTRACT FROM AUTHOR]

Making cosmological inferences from the observed galaxy clustering requires accurate predictions for the mean clustering statistics and their covariances. Those are affected by cosmic variance – the statistical noise due to the finite number of harmonics. The cosmic variance can be suppressed by fixing the amplitudes of the harmonics instead of drawing them from a Gaussian distribution predicted by the inflation models. Initial realizations also can be generated in pairs with 180○ flipped phases to further reduce the variance. Here, we compare the consequences of using paired-and-fixed versus Gaussian initial conditions on the average dark matter clustering and covariance matrices predicted from N -body simulations. As in previous studies, we find no measurable differences between paired-and-fixed and Gaussian simulations for the average density distribution function, power spectrum, and bispectrum. Yet, the covariances from paired-and-fixed simulations are suppressed in a complicated scale- and redshift-dependent way. The situation is particularly problematic on the scales of Baryon acoustic oscillations where the covariance matrix of the power spectrum is lower by only |$\sim 20{{\ \rm per\ cent}}$| compared to the Gaussian realizations, implying that there is not much of a reduction of the cosmic variance. The non-trivial suppression, combined with the fact that paired-and-fixed covariances are noisier than from Gaussian simulations, suggests that there is no path towards obtaining accurate covariance matrices from paired-and-fixed simulations – result, that is theoretically expected and accepted in the field. Because the covariances are crucial for the observational estimates of galaxy clustering statistics and cosmological parameters, paired-and-fixed simulations, though useful for some applications, cannot be used for the production of mock galaxy catalogues. [ABSTRACT FROM AUTHOR]

In order to capture as much information as possible large galaxies surveys have been increasing their volume and redshift depth. To face this challenge theory has responded by making cosmological simulations of huge computational volumes with equally increasing numbers of dark matter particles and supercomputing resources. Thus, it is taken for granted that the ideal situation is when a single computational box encompasses the whole volume of the observational survey, e.g. |$\sim 50\, h^{-3}\,{\rm Gpc}^3$| for the DESI and Euclid surveys. Here we study the effects of missing long waves in a finite volume using several relevant statistics: the abundance of dark matter haloes, the probability distribution function (PDF), the correlation function and power spectrum, and covariance matrices. Finite volume effects can substantially modify the results if the computational volumes are less than |$\sim (500\mbox{$\, h^{-1}$Mpc})^3$|⁠. However, the effects become extremely small and practically can be ignored when the box size exceeds ∼1 Gpc3. We find that the average power spectra of dark matter fluctuations show remarkable lack of dependence on the computational box size with less than 0.1 per cent differences between |$1$| and |$4\mbox{$\, h^{-1}\,$Gpc}$| boxes. No measurable differences are expected for the halo mass functions for these volumes. The covariance matrices are scaled trivially with volume, and small corrections due to supersample modes can be added. We conclude that there is no need to make those extremely large simulations when a box size of |$1-1.5\mbox{$\, h^{-1}$Gpc}$| is sufficient to fulfil most of the survey science requirements. [ABSTRACT FROM AUTHOR]

The one-point probability distribution function (PDF) of the matter density field in the universe is a fundamental property that plays an essential role in cosmology for estimates such as gravitational weak lensing, non-linear clustering, massive production of mock galaxy catalogues, and testing predictions of cosmological models. Here we make a comprehensive analysis of the dark matter PDF, using a suite of ∼7000 N -body simulations that covers a wide range of numerical and cosmological parameters. We find that the PDF has a simple shape: it declines with density as a power-law P ∝ ρ−2, which is exponentially suppressed on both small and large densities. The proposed double-exponential approximation provides an accurate fit to all our N -body results for small filtering scales R < 5 h−1Mpc with rms density fluctuations σ > 1. In combination with the spherical infall model that works well for small fluctuations σ < 1, the PDF is now approximated with just few per cent errors over the range of 12 orders of magnitude – a remarkable example of precision cosmology. We find that at |${\sim } 5{-}10{{\ \rm per\ cent}}$| level the PDF explicitly depends on redshift (at fixed σ) and on cosmological density parameter Ωm. We test different existing analytical approximations and find that the often-used lognormal approximation is always 3–5 times less accurate than either the double-exponential approximation or the spherical infall model. [ABSTRACT FROM AUTHOR]

Predicting structural properties of dark matter haloes is one of the fundamental goals of modern cosmology. We use the suite of MultiDark cosmological simulations to study the evolution of dark matter halo density profiles, concentrations, and velocity anisotropies. We find that in order to understand the structure of dark matter haloes and to make 1-2 per cent accurate predictions for density profiles, one needs to realize that halo concentration is more complex than the ratio of the virial radius to the core radius in the Navarro-Frenk-White (NFW) profile. For massive haloes, the average density profile is far from the NFW shape and the concentration is defined by both the core radius and the shape parameter α in the Einasto approximation. We show that haloes progress through three stages of evolution. They start as rare density peaks and experience fast and nearly radial infall that brings mass closer to the centre, producing a highly concentrated halo. Here, the halo concentration increases with increasing halo mass and the concentration is defined by the a parameter with a nearly constant core radius. Later haloes slide into the plateau regime where the accretion becomes less radial, but frequent mergers still affect even the central region. At this stage, the concentration does not depend on halo mass. Once the rate of accretion and merging slows down, haloes move into the domain of declining concentration-mass relation because new accretion piles up mass close to the virial radius while the core radius is staying constant. Accurate analytical fits are provided. [ABSTRACT FROM AUTHOR]

We present newmeasurements of the abundance of galaxies with a given circular velocity in the Local Volume: a region centred on the MilkyWay Galaxy and extending to distance ~10 Mpc. The sample of ~750 mostly dwarf galaxies provides a unique opportunity to study the abundance and properties of galaxies down to absolute magnitudes MB ≈ -10 and virial masses Mvir = 109M⊙. We find that the standard Λ cold dark matter (ΛCDM) model gives remarkably accurate estimates for the velocity function of galaxies with circular velocities V #8819; 70 kms-1 and corresponding virial masses Mvir ≳ 5 × 1010M⊙, but it badly fails by overpredicting ~5 times the abundance of large dwarfs with velocities V = 30-40 kms-1. The warm dark matter (WDM) models cannot explain the data either, regardless of mass of WDM particle. Just as in previous observational studies, we find a shallow asymptotic slope dN/dlog V ∝ Vα, α ≈ -1 of the velocity function, which is inconsistent with the standard ΛCDMmodel that predicts the slope α=-3. Though reminiscent to the known overabundance of satellite problem, the overabundance of field galaxies is a much more difficult problem. For the standard CDM model to survive, in the 10 Mpc radius of the Milky Way there should be 1000 not yet detected galaxies with virial mass Mvir ≈ 1010M⊙, extremely low surface brightness and no detectable H I gas. So far none of this type of galaxies have been discovered. [ABSTRACT FROM AUTHOR]

Accurate predictions of the abundance and clustering of dark matter haloes play a key role in testing the standard cosmological model. Here, we investigate the accuracy of one of the leading methods of connecting the simulated dark matter haloes with observed galaxies– the halo abundance matching (HAM) technique. We show how to choose the optimal values of the mass and force resolution in large volume N-body simulations so that they provide accurate estimates for correlation functions and circular velocities for haloes and their subhaloes – crucial ingredients of the HAM method. At the 10 per cent accuracy, results converge for ∼50 particles for haloes and ∼150 particles for progenitors of subhaloes. In order to achieve this level of accuracy a number of conditions should be satisfied. The force resolution for the smallest resolved (sub)haloes should be in the range (0.1–0.3)rs, where rs is the scale radius of (sub)haloes. The number of particles for progenitors of subhaloes should be ∼150. We also demonstrate that the two-body scattering plays a minor role for the accuracy of N-body simulations thanks to the relatively small number of crossing-times of dark matter in haloes, and the limited force resolution of cosmological simulations. [ABSTRACT FROM AUTHOR]

Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today's galaxies. This inefficiency is necessary for reproducing the low stellar-to-virial mass fractions, suggested by current abundance matching models. A possible driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive stars. The effect of radiation pressure in cosmological, zoom-in galaxy formation simulations is modelled as a non-thermal pressure that acts only in dense and optically thick star-forming regions. We also include photoionization and photoheating by massive stars. The full photoionization of hydrogen reduces the radiative cooling in the 104-4.5 K regime. The main effect of radiation pressure is to regulate and limit the high values of gas density and the amount of gas available for star formation. This maintains a low star formation rate of ∼1 M⊙ yr−1 in haloes with masses about 1011 M⊙ at z ≃ 3. Infrared trapping and photoionization/photoheating processes are secondary effects in this mass range. The galaxies residing in these low-mass haloes contain only ∼0.6 per cent of the total virial mass in stars, roughly consistent with abundance matching. Radiative feedback maintains an extended galaxy with a rising circular velocity profile. [ABSTRACT FROM AUTHOR]

We present the public data release of the Uchuu-UM galaxy catalogues by applying the UniverseMachine algorithm to assign galaxies to the dark matter haloes in the Uchuu N -body cosmological simulation. It includes a variety of baryonic properties for all galaxies down to ∼5 × 108 M⊙ with haloes in a mass range of 1010 < M halo/M⊙ < 5 × 1015 up to redshift z = 10. Uchuu-UM includes more than 104 cluster-size haloes in a volume of 8(h −1Gpc)3, reproducing observed stellar mass functions across the redshift range of z = 0−7, galaxy quenched fractions, and clustering statistics at low redshifts. Compared to the previous largest UM catalogue, the Uchuu-UM catalogue includes significantly more massive galaxies hosted by large-mass dark matter haloes. Overall, the number density profile of galaxies in dark matter haloes follows the dark matter profile, with the profile becoming steeper around the splashback radius and flattening at larger radii. The number density profile of galaxies tends to be steeper for larger stellar masses and depends on the colour of galaxies, with red galaxies having steeper slopes at all radii than blue galaxies. The quenched fraction exhibits a strong dependence on the stellar mass and increases towards the inner regions of clusters. The publicly available Uchuu-UM galaxy catalogue presented here can serve to model ongoing and upcoming large galaxy surveys. [ABSTRACT FROM AUTHOR]

Dwarf spheroidal galaxies (dSphs) are extremely gas-poor, dark matter-dominated galaxies, which make them ideal to test the predictions of the cold dark matter (CDM) model. We argue that the removal of the baryonic component from gas-rich dwarf irregular galaxies, the progenitors of dSphs, can substantially reduce their central density. Thus, it may play an important role in alleviating one of the problems of the CDM model related to the structure of relatively massive satellite galaxies of the Milky Way (MW). Traditionally, collisionless cosmological N-body simulations are used when confronting theoretical predictions with observations. However, these simulations assume that the baryon fraction everywhere in the Universe is equal to the cosmic mean, an assumption which is incorrect for dSphs. We find that the combination of (i) the lower baryon fraction in dSphs compared to the cosmic mean and (ii) the concentration of baryons in the inner part of the MW halo can go a long way towards explaining the observed circular velocity profiles of dSphs. We perform controlled numerical simulations that mimic the effects of baryons. From these we find that the blowing away of baryons by ram pressure, when the dwarfs fall into larger galaxies, decreases the circular velocity profile of the satellite and reduces the density in the central ∼200–500 pc by a factor of $(1- f_{\rm b})^4\approx 0.5$, where $f_{\rm b}$ is the cosmological fraction of baryons. Additionally, the enhanced baryonic mass in the central regions of the parent galaxy generates tidal forces, which are larger than those experienced by subhaloes in traditional N-body simulations. Increased tidal forces substantially alter circular velocity profiles for satellites with pericentres less than 50 kpc. We show that these two effects are strong enough to bring the predictions of subhaloes from CDM simulations into agreement with the observed structure of MW dSphs, regardless of the details of the baryonic processes. [ABSTRACT FROM AUTHOR]

ABSTRACT We study the concentration of dark matter haloes and its evolution in N-body simulations of the standard Λ cold dark matter (ΛCDM) cosmology. The results presented in this paper are based on four large N-body simulations with ∼10 billion particles each: the Millennium-I and -II, Bolshoi and MultiDark simulations. The MultiDark (or BigBolshoi) simulation is introduced in this paper. This suite of simulations with high mass resolution over a large volume allows us to compute with unprecedented accuracy the concentration over a large range of scales (about six orders of magnitude in mass), which constitutes the state of the art of our current knowledge on this basic property of dark matter haloes in the ΛCDM cosmology. We find that there is consistency among the different simulation data sets, despite the different codes, numerical algorithms and halo/subhalo finders used in our analysis. We confirm a novel feature for halo concentrations at high redshifts: a flattening and upturn with increasing mass. The concentration c( M, z) as a function of mass and the redshift and for different cosmological parameters shows a remarkably complex pattern. However, when expressed in terms of the linear rms fluctuation of the density field σ( M, z), the halo concentration c(σ) shows a nearly universal simple U-shaped behaviour with a minimum at a well-defined scale at σ∼ 0.71. Yet, some small dependences with redshift and cosmology still remain. At the high-mass end (σ < 1), the median halo kinematic profiles show large signatures of infall and highly radial orbits. This c-σ( M, z) relation can be accurately parametrized and provides an analytical model for the dependence of concentration on halo mass. When applied to galaxy clusters, our estimates of concentrations are substantially larger - by a factor up to 1.5 - than previous results from smaller simulations, and are in much better agreement with results of observations. [ABSTRACT FROM AUTHOR]

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 ) produce slowly rotating, and very long, bars. In contrast, more realistic thin discs with the same parameters as in our Galaxy produce bars with normal length , which rotate quickly with the ratio of the corotation radius to the bar radius 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) andpkdgrav – 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 withtree codes (gadget andpkdgrav) and with the adaptive mesh refinement (art) code. [ABSTRACT FROM AUTHOR]

We study the luminosity function, the peculiar velocities and the sizes of voids in the Local Volume (LV) in observational samples of galaxies which contain galaxies down to and to within the distance 4–8 Mpc. When we compare the results with the predictions of the standard cosmological λ cold dark matter (ΛCDM) model, we find that the theory faces a sever problem: it predicts a factor of 10 more dwarf haloes as compared with the observed number of dwarf galaxies. In the LV, we identify voids with sizes ranging from 1 to 4.5 Mpc and compare the observational distribution of void sizes with the voids in very high-resolution simulations of the ΛCDM model with Wilkinson Microwave Anisotropy Probe 1( WMAP1) and WMAP3 parameters. The theoretical void function matches the observations remarkably well only if we use haloes with circular velocities Vc larger than 40–45 km s−1[ ] for models with and [ ] for . We exclude the possibility that in the ΛCDM model haloes with circular velocities can host galaxies as bright as : there are too many small haloes in the ΛCDM model resulting in voids being too small as compared with the observations. The problem is that many of the observed dwarf galaxies have H i rotational velocities below 25 km s−1 that strictly contradicts the ΛCDM predictions. Thus, the ΛCDM model faces the same overabundance problem, which it had with the number of satellites in the Local Group (LG). We also estimate the rms deviations from the Hubble flow σH for galaxies at different distances from the LG and find that in most of our model LV candidates the rms peculiar velocities are consistent with observational values: for distances less than 3 Mpc and for distances less than 8 Mpc. At distances 4 (8) Mpc, the observed overdensities of galaxies are 3.5–5.5 (1.3–1.6) – significantly larger than typically assumed. [ABSTRACT FROM AUTHOR]

Using high-resolution N-body simulations of stellar discs embedded in cosmologically motivated dark matter haloes, we study the evolution of bars and the transfer of angular momentum between haloes and bars. We find that dynamical friction results in some transfer of angular momentum to the halo, but the effect is much smaller than previously found in low-resolution simulations and is incompatible with early analytical estimates. After 5 Gyr of evolution the stellar component loses only 5–7 per cent of its initial angular momentum. Mass and force resolutions are crucial for the modelling of bar dynamics. In low-resolution (300–500 pc) simulations we find that the bar slows down and angular momentum is lost relatively fast. In simulations with millions of particles reaching a resolution of 20–40 pc, the pattern speed may not change over billions of years. Our high-resolution models produce bars that are fast rotators, where the ratio of the corotation radius to the bar semi-major axis lies in the range , marginally compatible with observational results. In contrast to many previous simulations, we find that bars are relatively short. As in many observed cases, the bar semi-major axis is close to the exponential length of the disc. The transfer of angular momentum between inner and outer parts of the disc plays a very important role in the secular evolution of the disc and the bar. The bar formation increases the exponential length of the disc by a factor of 1.2–1.5. The transfer substantially increases the stellar mass in the centre of the galaxy and decreases the dark matter to baryon ratio. As the result, the central 2-kpc region is always strongly dominated by the baryonic component. At intermediate (3–10 kpc) scales the disc is sub-dominant. These models demonstrate that the efficiency of angular momentum transfer to the dark matter has been greatly overestimated. More realistic models produce bar structure in striking agreement with observational results. [ABSTRACT FROM AUTHOR]

We introduce the Uchuu suite of large high-resolution cosmological N -body simulations. The largest simulation, named Uchuu, consists of 2.1 trillion (12 8003) dark matter particles in a box of side-length 2.0 |$\, h^{-1} \, \rm Gpc$|⁠ , with particle mass of 3.27 × 108 |$\, h^{-1}\, \rm M_{\odot }$|⁠. The highest resolution simulation, Shin-Uchuu, consists of 262 billion (64003) particles in a box of side-length 140 |$\, h^{-1} \, \rm Mpc$|⁠ , with particle mass of 8.97 × 105 |$\, h^{-1}\, \rm M_{\odot }$|⁠. Combining these simulations, we can follow the evolution of dark matter haloes and subhaloes spanning those hosting dwarf galaxies to massive galaxy clusters across an unprecedented volume. In this first paper, we present basic statistics, dark matter power spectra, and the halo and subhalo mass functions, which demonstrate the wide dynamic range and superb statistics of the Uchuu suite. From an analysis of the evolution of the power spectra, we conclude that our simulations remain accurate from the baryon acoustic oscillation scale down to the very small. We also provide parameters of a mass–concentration model, which describes the evolution of halo concentration and reproduces our simulation data to within 5 per cent for haloes with masses spanning nearly eight orders of magnitude at redshift 0 ≤ z ≤ 14. There is an upturn in the mass–concentration relation for the population of all haloes and of relaxed haloes at z ≳ 0.5, whereas no upturn is detected at z < 0.5. We make publicly available various N -body products as part of Uchuu Data Release 1 on the Skies & Universes site. 1 Future releases will include gravitational lensing maps and mock galaxy, X-ray cluster, and active galactic nucleus catalogues. [ABSTRACT FROM AUTHOR]

Measurements of the luminosity function of active galactic nuclei (AGN) at high redshift (z ≳ 6) are expected to suffer from field-to-field variance, including cosmic and Poisson variances. Future surveys, such as those from the Euclid telescope and JWST, will also be affected by field variance. We use the Uchuu simulation, a state-of-the-art cosmological N -body simulation with 2.1 trillion particles in a volume of 25.7 Gpc3, combined with a semi-analytic galaxy and AGN formation model, to generate the Uchuu–ν2GC catalogue, publicly available, that allows us to investigate the field-to-field variance of the luminosity function of AGN. With this Uchuu–ν2GC model, we quantify the cosmic variance as a function of survey area, AGN luminosity, and redshift. In general, cosmic variance decreases with increasing survey area and decreasing redshift. We find that at z ∼ 6 − 7, the cosmic variance depends weakly on AGN luminosity. This is because the typical mass of dark matter haloes in which AGN reside does not significantly depend on luminosity. Due to the rarity of AGN, Poisson variance dominates the total field-to-field variance, especially for bright AGN. We also examine the effect of parameters related to galaxy formation physics on the field variance. We discuss uncertainties present in the estimation of the faint-end of the AGN luminosity function from recent observations, and extend this to make predictions for the expected number of AGN and their variance for upcoming observations with Euclid , JWST, and the Legacy Survey of Space and Time (LSST). [ABSTRACT FROM AUTHOR]

We present 2000 mock galaxy catalogues for the analysis of baryon acoustic oscillations (BAOs) in the Emission Line Galaxy (ELG) sample of the extended Baryon Oscillation Spectroscopic Survey Data Release 16 (eBOSS DR16). Each mock catalogue has a number density of |$6.7 \times 10^{-4} h^3 \rm Mpc^{-3}$|⁠ , covering a redshift range from 0.6 to 1.1. The mocks are calibrated to small-scale eBOSS ELG clustering measurements at scales of |$\lesssim 30\, h^{-1}$| Mpc. The mock catalogues are generated using a combination of GaLAxy Mocks (GLAM) simulations and the quick particle-mesh (QPM) method. GLAM simulations are used to generate the density field, which is then assigned dark matter haloes using the QPM method. Haloes are populated with galaxies using a halo occupation distribution. The resulting mocks match the survey geometry and selection function of the data, and have slightly higher number density that allows room for systematic analysis. The large-scale clustering of mocks at the BAO scale is consistent with data and we present the correlation matrix of the mocks. [ABSTRACT FROM AUTHOR]

Using three-dimensional (3D) spectroscopy with a scanning Fabry-Perot interferometer, we study the kinematics of ionized gas in 59 nearby dwarf galaxies. Combining our results with data from the literature, we provide a global relation between the gas velocity dispersion σ and the star formation rate (SFR) and Hα luminosity for galaxies with a very broad range of star formation rates: SFR=0.001-300 M☉ yr-1.We find that the SFR-σ relation for the combined sample of dwarf galaxies, star-forming, local luminous and ultraluminous infrared galaxies can be fitted in the form σ ∝SFR5.3 ± 0.2. This implies that the slope of the L-σ relation inferred from a sample of rotation-supported disc galaxies (including mergers) is similar to the L-σ relation of individual giant H II regions. We present arguments that the velocity dispersion of the ionized gas does not reflect virial motions in the gravitational potential of dwarf galaxies and instead is determined mainly by the energy injected into the interstellar medium by ongoing star formation. [ABSTRACT FROM AUTHOR]

We present a method for computing the six-dimensional coarse-grained phase-space density f( x, v) in an N-body system, and derive its distribution function v( f). The method is based on Delaunay tessellation, where v( f) is obtained with an effective fixed smoothing window over a wide f range. The errors are estimated, and v( f) is found to be insensitive to the sampling resolution or the simulation technique. We find that in gravitationally relaxed haloes built by hierarchical clustering, v( f) is well approximated by a robust power law, , over more than four decades in f, from its virial level to the numerical resolution limit. This is tested to be valid in the Λ cold dark matter cosmology for haloes with masses , indicating insensitivity to the slope of the initial fluctuation power spectrum. By mapping the phase-space density in position space, we find that the high- f end of v( f) is dominated by the ‘cold’ subhaloes rather than the parent-halo central region and its global spherical profile. The value of f in subhaloes near the virial radius is typically >100 times higher than its value at the halo centre, and it decreases gradually from the outside in toward its value at the halo centre. This seems to reflect phase mixing due to mergers and tidal effects involving puffing up and heating. The phase-space density can thus provide a sensitive tool for studying the evolution of subhaloes during the hierarchical build-up of haloes. It remains to be understood why the evolved substructure adds up to the actual universal power law of . It seems that this behaviour results from the hierarchical clustering process and is not a general result of violent relaxation. [ABSTRACT FROM AUTHOR]

Addresses the emptiness of giant diameter voids found in the distribution of bright galaxies. Prediction of small dark matter haloes by cosmological models; Abundance of dwarf galaxies inside voids; Similarity of the structural elements in the large-scale structure of the galactic distribution.

Upcoming surveys will use a variety of galaxy selections to map the large-scale structure of the Universe. It is important to make accurate predictions for the properties and clustering of such galaxies, including the errors on these statistics. Here, we describe a novel technique which uses the semi-analytical model of galaxy formation galform , embedded in the high-resolution N -body Planck-Millennium simulation, to populate a thousand halo catalogues generated using the Parallel-PM N -body glam code. Our hybrid scheme allows us to make clustering predictions on scales that cannot be modelled in the original N -body simulation. We focus on luminous red galaxies (LRGs) selected in the redshift range z = 0.6 − 1 from the galform output using similar colour-magnitude cuts in the r, z , and W 1 bands to those that will be applied in the Dark Energy Spectroscopic Instrument (DESI) survey, and call this illustrative sample 'DESI-like' LRGs. We find that the LRG-halo connection is non-trivial, leading to the prediction of a non-standard halo occupation distribution; in particular, the occupation of central galaxies does not reach unity for the most massive haloes, and drops with increasing mass. The glam catalogues reproduce the abundance and clustering of the LRGs predicted by galform. We use the glam mocks to compute the covariance matrices for the two-point correlation function and power spectrum of the LRGs and their background dark matter density field, revealing important differences. We also make predictions for the linear-growth rate and the baryon acoustic oscillations distances at z = 0.6, 0.74, and 0.93. All 'DESI-like' LRG catalogues are made publicly available. [ABSTRACT FROM AUTHOR]

Halo bias is the main link between the matter distribution and dark matter haloes. In its simplest form, halo bias is determined by halo mass, but there are known additional dependencies on other halo properties which are of consequence for accurate modelling of galaxy clustering. Here, we present the most precise measurement of these secondary-bias dependencies on halo age, concentration, and spin, for a wide range of halo masses spanning from 1010.7 to 1014.7 h −1 M⊙. At the high-mass end, we find no strong evidence of assembly bias for masses above M vir ∼1014 h −1 M⊙. Secondary bias exists, however, for halo concentration and spin, up to cluster-size haloes, in agreement with previous findings. For halo spin, we report, for the first time, two different regimes: above M vir ∼ 1011.5 h −1 M⊙, haloes with larger values of spin have larger bias, at fixed mass, with the effect reaching almost a factor 2. This trend reverses below this characteristic mass. In addition to these results, we test, for the first time, the performance of a multitracer method for the determination of the relative bias between different subsets of haloes. We show that this method increases significantly the signal to noise of the secondary-bias measurement as compared to a traditional approach. This analysis serves as the basis for follow-up applications of our multitracer method to real data. [ABSTRACT FROM AUTHOR]

Cosmological simulations are fundamental tools to study structure formation and the astrophysics of evolving structures, in particular clusters of galaxies. While hydrodynamical simulations cannot sample efficiently large volumes and explore different cosmologies at the same time, N -body simulations lack the baryonic physics that is crucial to determine the observed properties of clusters. One solution is to use (semi-)analytical models to implement the needed baryonic physics. In this way, we can generate the many mock universes that will be required to fully exploit future large sky surveys, such as that from the upcoming eROSITA X-ray telescope. We developed a phenomenological model based on observations of clusters to implement gas density and temperature information on the dark-matter-only haloes of the MultiDark simulations. We generate several full-sky mock light-cones of clusters for the Wilkinson Microwave Anisotropy Probe and Planck cosmologies, adopting different parameters in our phenomenological model of the intracluster medium. For one of these simulations and models, we also generate 100 light-cones corresponding to 100 random observers and explore the variance among them in several quantities. In this first paper on MultiDark mock galaxy cluster light-cones, we focus on presenting our methodology and discuss predictions for eROSITA, in particular, exploring the potential of angular power spectrum analyses of its detected (and undetected) cluster population to study X-ray scaling relations, the intracluster medium, and the composition of the cosmic X-ray background. We make publicly available online more than 400 GB of light-cones, which include the expected eROSITA count rate, on Skies & Universes (http://www.skiesanduniverses.org). [ABSTRACT FROM AUTHOR]

We present the public release of the MultiDark-Galaxies: three distinct galaxy catalogues derived from one of the Planck cosmology MultiDark simulations (i.e. MDPL2, with a volume of (1 h−1 Gpc)3 and mass resolution of 1.5 × 109 h−1 M⊙) by applying the semi-analytic models galacticus, sag, and sage to it. We compare the three models and their conformity with observational data for a selection of fundamental properties of galaxies like stellar mass function, star formation rate, cold gas fractions, and metallicities – noting that they sometimes perform differently reflecting model designs and calibrations. We have further selected galaxy subsamples of the catalogues by number densities in stellar mass, cold gas mass, and star formation rate in order to study the clustering statistics of galaxies. We show that despite different treatment of orphan galaxies, i.e. galaxies that lost their dark-matter host halo due to the finite-mass resolution of the N-body simulation or tidal stripping, the clustering signal is comparable, and reproduces the observations in all three models – in particular when selecting samples based upon stellar mass. Our catalogues provide a powerful tool to study galaxy formation within a volume comparable to those probed by ongoing and future photometric and redshift surveys. All model data consisting of a range of galaxy properties – including broad-band SDSS magnitudes – are publicly available. [ABSTRACT FROM AUTHOR]

We study the galaxy clustering dependence on the [O II] emission line luminosity in the SDSS DR7 Main galaxy sample at mean redshift z ∼ 0.1. We select volume-limited samples of galaxies with different [O II] luminosity thresholds and measure their projected, monopole and quadrupole two-point correlation functions. We model these observations using the 1 h-1 Gpc Multi Dark-Planck cosmological simulation and generate light cones with the SUrvey Gener A to R algorithm. To interpret our results, we adopt a modified (Sub)Halo Abundance Matching scheme, accounting for the stellar mass incompleteness of the emission line galaxies. The satellite fraction constitutes an extra parameter in this model and allows to optimize the clustering fit on both small and intermediate scales (i.e. rp ≲ 30 h-1 Mpc), with no need of any velocity bias correction. We find that, in the local Universe, the [O II] luminosity correlateswith all the clustering statistics explored and with the galaxy bias. This latter quantity correlates more strongly with the SDSS r-band magnitude than [O II] luminosity. In conclusion, we propose a straightforward method to produce reliable clustering models, entirely built on the simulation products, which provides robust predictions of the typical ELG host halo masses and satellite fraction values. The SDSS galaxy data, MultiDark mock catalogues and clustering results are made publicly available. [ABSTRACT FROM AUTHOR]

N-body cosmological simulations are an essential tool to understand the observed distribution of galaxies. We use the MultiDark simulation suite, run with the Planck cosmological parameters, to revisit the mass and velocity functions. At redshift z = 0, the simulations cover four orders of magnitude in halo mass from ~1011M⊙ with 8783 874 distinct haloes and 532 533 subhaloes. The total volume used is ~515 Gpc³, more than eight times larger than in previous studies. We measure and model the halo mass function, its covariance matrix w.r.t halo mass and the large-scale halo bias. With the formalism of the excursion-set mass function, we explicit the tight interconnection between the covariance matrix, bias and halo mass function. We obtain a very accurate (<2 per cent level) model of the distinct halo mass function. We also model the subhalo mass function and its relation to the distinct halo mass function. The set of models obtained provides a complete and precise framework for the description of haloes in the concordance Planck cosmology. Finally, we provide precise analytical fits of the Vmax maximum velocity function up to redshift z < 2.3 to push for the development of halo occupation distribution using Vmax. The data and the analysis code are made publicly available in the Skies and Universes data base. [ABSTRACT FROM AUTHOR]

In current and future surveys, quasars play a key role. The new data will extend our knowledge of the Universe as it will be used to better constrain the cosmological model at redshift z > 1 via baryon acoustic oscillation and redshift space distortion measurements. Here, we present the first clustering study of quasars observed by the extended Baryon Oscillation Spectroscopic Survey. We measure the clustering of ~70 000 quasars located in the redshift range 0.9 < z < 2.2 that cover 1168 deg2. We model the clustering and produce highfidelity quasar mock catalogues based on the BigMultiDark Planck simulation. Thus, we use a modified (sub)halo abundance matching model to account for the specificities of the halo population hosting quasars.We find that quasars are hosted by haloes with masses~1012.7M☉ and their bias evolves from 1.54 (z = 1.06) to 3.15 (z = 1.98). Using the current extended Baryon Oscillation Spectroscopic Survey data, we cannot distinguish between models with different fractions of satellites. The high-fidelity mock light-cones, including properties of haloes hosting quasars, are made publicly available. [ABSTRACT FROM AUTHOR]

In current and future surveys, quasars play a key role. The new data will extend our knowledge of the Universe as it will be used to better constrain the cosmological model at redshift z > 1 via baryon acoustic oscillation and redshift space distortion measurements. Here, we present the first clustering study of quasars observed by the extended Baryon Oscillation Spectroscopic Survey. We measure the clustering of ~70 000 quasars located in the redshift range 0.9 < z < 2.2 that cover 1168 deg2. We model the clustering and produce highfidelity quasar mock catalogues based on the BigMultiDark Planck simulation. Thus, we use a modified (sub)halo abundance matching model to account for the specificities of the halo population hosting quasars. We find that quasars are hosted by haloes with masses~1012.7M☉ and their bias evolves from 1.54 (z = 1.06) to 3.15 (z = 1.98). Using the current extended Baryon Oscillation Spectroscopic Survey data, we cannot distinguish between models with different fractions of satellites. The high-fidelity mock light-cones, including properties of haloes hosting quasars, are made publicly available. [ABSTRACT FROM AUTHOR]

We report and provide fitting functions for the abundance of dark matter halos and subhalos as a function of mass, circular velocity, and redshift from the new Bolshoi-Planck and MultiDark-Planck ΛCDM cosmological simulations, based on the Planck cosmological parameters. We also report the halo mass accretion rates, which may be connected with galaxy star formation rates. We show that the higher cosmological matter density of the Planck parameters compared with the WMAP parameters leads to higher abundance of massive halos at high redshifts. We find that the median halo spin parameter λB=J(√2MvirRvirVvir)−1 is nearly independent of redshift, leading to predicted evolution of galaxy sizes that is consistent with observations, while the significant decrease with redshift in median λP=J|E|−1/2G−1M−5/2 predicts more decrease in galaxy sizes than is observed. Using the Tully-Fisher and Faber-Jackson relations between galaxy velocity and mass, we show that a simple model of how galaxy velocity is related to halo maximum circular velocity leads to increasing overprediction of cosmic stellar mass density as redshift increases beyond redshifts z∼1, implying that such velocity-mass relations must change at redshifts z ≳ 1. By making a realistic model of how observed galaxy velocities are related to halo circular velocity, we show that recent optical and radio observations of the abundance of galaxies are in good agreement with our ΛCDM simulations. Our halo demographics are based on updated versions of the \rockstar\ and \ctrees\ codes, and this paper includes appendices explaining all of their outputs. This paper is an introduction to a series of related papers presenting other analyses of the Bolshoi-Planck and MultiDark-Planck simulations. [ABSTRACT FROM AUTHOR]

Current and future large redshift surveys, as the Sloan Digital Sky Survey IV extended Baryon Oscillation Spectroscopic Survey (SDSS-IV/eBOSS) or the Dark Energy Spectroscopic Instrument (DESI), will use emission-line galaxies (ELGs) to probe cosmological models by mapping the large-scale structure of the Universe in the redshift range 0.6 < z < 1.7. With current data, we explore the halo-galaxy connection by measuring three clustering properties of g-selected ELGs as matter tracers in the redshift range 0.6 < z < 1: (i) the redshift-space two-point correlation function using spectroscopic redshifts from the BOSS ELG sample and VIPERS; (ii) the angular two-point correlation function on the footprint of the CFHT-LS; (iii) the galaxy-galaxy lensing signal around the ELGs using the CFHTLenS. We interpret these observations by mapping them on to the latest high-resolution MultiDark Planck N-body simulation, using a novel (Sub)Halo-Abundance Matching technique that accounts for the ELG incompleteness. ELGs at z ~ 0.8 live in haloes of (1 ± 0.5) × 1012 h-1M☉ and 22.5 ± 2.5 per cent of them are satellites belonging to a larger halo. The halo occupation distribution of ELGs indicates that we are sampling the galaxies in which stars form in the most efficient way, according to their stellar-to-halo mass ratio. [ABSTRACT FROM AUTHOR]

We present a study of the clustering and halo occupation distribution of Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies in the redshift range 0.43 < z < 0.7 drawn from the Final SDSS-III Data Release. We compare the BOSS results with the predictions of a halo abundance matching (HAM) clustering model that assigns galaxies to dark matter haloes selected from the large Big Multi Dark N-body simulation of a flat Λ cold dark matter Planck cosmology. We compare the observational data with the simulated ones on a light cone constructed from 20 subsequent outputs of the simulation. Observational effects such as incompleteness, geometry, veto masks and fibre collisions are included in the model, which reproduces within 1σ errors the observed monopole of the two-point correlation function at all relevant scales: from the smallest scales, 0.5 h-1 Mpc, up to scales beyond the baryon acoustic oscillation feature. This model also agrees remarkably well with the BOSS galaxy power spectrum (up to k ~ 1 h Mpc-1), and the three-point correlation function. The quadrupole of the correlation function presents some tensions with observations. We discuss possible causes that can explain this disagreement, including target selection effects. Overall, the standard HAM model describes remarkably well the clustering statistics of the CMASS sample. We compare the stellar-to-halo mass relation for the CMASS sample measured using weak lensing in the Canada--France--Hawaii Telescope Stripe 82 Survey with the prediction of our clustering model, and find a good agreement within 1σ. The BigMD-BOSS light cone including properties of BOSS galaxies and halo properties is made publicly available. [ABSTRACT FROM AUTHOR]

We model the luminosity-dependent projected and redshift-space two-point correlation functions (2PCFs) of the Sloan Digital Sky Survey (SDSS) Data Release 7 Main galaxy sample, using the halo occupation distribution (HOD) model and the subhalo abundance matching (SHAM) model and its extension. All the models are built on the same high-resolution Nbody simulations. We find that the HOD model generally provides the best performance in reproducing the clustering measurements in both projected and redshift spaces. The SHAM model with the same halo-galaxy relation for central and satellite galaxies (or distinct haloes and subhaloes), when including scatters, has a best-fitting X2 around 2-3. We therefore extend the SHAM model to the subhalo clustering and abundance matching (SCAM) by allowing the central and satellite galaxies to have different galaxy-halo relations. We infer the corresponding halo/subhalo parameters by jointly fitting the galaxy 2PCFs and abundances and consider subhaloes selected based on three properties, the mass Macc at the time of accretion, the maximum circular velocity Vacc at the time of accretion, and the peak maximum circular velocity Vpeak over the history of the subhaloes. The three subhalo models work well for luminous galaxy samples (with luminosity above L*). For low-luminosity samples, the Vacc model stands out in reproducing the data, with the Vpeak model slightly worse, while the Macc model fails to fit the data. We discuss the implications of the modelling results. [ABSTRACT FROM AUTHOR]

Future dark energy experiments will require accurate theoretical predictions for the baryon acoustic oscillations (BAOs). Here, we use large N-body simulations to study any systematic shifts and damping in BAO due to non-linear effects. The impact of cosmic variance is largely reduced by dividing the tracer power spectrum by that from a 'BAO-free' simulation starting with the same random amplitudes and phases. The accuracy of our simulations allows us to resolve well dark matter (sub)haloes, which permits us to study with high accuracy (better than 0.02 per cent for dark matter and 0.07 per cent for low-bias haloes) small BAO shifts α towards larger k, and non-linear damping Σnl of BAO in the power spectrum. For dark matter, we provide an accurate parametrization of the evolution of α as a function of the linear growth factor D(z). For halo samples, with bias from 1.2 to 2.8, we measure a typical BAO shift of ≈0.25 per cent, with no appreciable evolution with redshift. Moreover, we report a constant shift as a function of halo bias. We find a different evolution of the BAO damping in all halo samples as compared to dark matter with haloes suffering less damping, and also find some weak dependence on bias. Larger BAO shift and damping are measured in redshift-space, which can be explained by linear theory due to redshift-space distortions. A clear modulation in phase with the acoustic scale is observed in the scale-dependent halo bias due to the presence of BAOs. We compare our results with previous works. [ABSTRACT FROM AUTHOR]

We reproduce the galaxy clustering catalogue from the SDSS-III Baryon Oscillation Spectroscopic Survey Final Data Release (BOSS DR11&DR12) with high fidelity on all relevant scales in order to allow a robust analysis of baryon acoustic oscillations and redshift space distortions. We have generated (6000) 12 288 MultiDark PATCHY BOSS (DR11) DR12 light cones corresponding to an effective volume of ∼192 000 [h-1 Gpc]³ (the largest ever simulated volume), including cosmic evolution in the redshift range from 0.15 to 0.75. The mocks have been calibrated using a reference galaxy catalogue based on the halo abundance matching modelling of the BOSS DR11&DR12 galaxy clustering data and on the data themselves. The production follows three steps. First, we apply the PATCHY code to generate a dark matter field and an object distribution including non-linear stochastic galaxy bias. Secondly, we run the halo/stellar distribution reconstruction HADRON code to assign masses to the various objects. This step uses the mass distribution as a function of local density and non-local indicators (i.e. tidal field tensor eigenvalues and relative halo exclusion separation for massive objects) from the reference simulation applied to the corresponding patchy dark matter and galaxy distribution. Finally, we apply the SUGAR code to build the light cones. The resulting MultiDarkPATCHY mock light cones reproduce the number density, selection function, survey geometry, and in general within 1s, for arbitrary stellar mass bins, the power spectrum up to k = 0.3 h Mpc-1, the two-point correlation functions down to a few Mpc scales, and the three-point statistics of the BOSS DR11&DR12 galaxy samples. [ABSTRACT FROM AUTHOR]

We present the measurements and modelling of the small-to-intermediate scale (~0.1-25 h-1 Mpc) projected and three-dimensional redshift-space two-point correlation functions (2PCFs) of local galaxies in the Sloan Digital Sky Survey Data Release 7. We find a clear dependence of galaxy clustering on luminosity in both projected and redshift spaces, generally being stronger for more luminous samples. The measurements are successfully interpreted within the halo occupation distribution (HOD) framework with central and satellite velocity bias parameters to describe galaxy kinematics inside haloes and to model redshift-space distortion effects. In agreement with previous studies, we find that more luminous galaxies reside in more massive haloes. Including the redshift-space 2PCFs helps tighten the HOD constraints. Moreover, we find that luminous central galaxies are not at rest at the halo centres, with the velocity dispersion about 30 per cent that of the dark matter. Such a relative motion may reflect the consequence of galaxy and halo mergers, and we find that central galaxies in lower mass haloes tend to be more relaxed with respect to their host haloes. The motion of satellite galaxies in luminous samples is consistent with their following that of the dark matter. For faint samples, satellites tends to have slower motion, with velocity dispersion inside haloes about 85 per cent that of the dark matter. We discuss possible applications of the velocity bias constraints on studying galaxy evolution and cosmology. In the appendix, we characterize the distribution of galaxy redshift measurement errors, which is well described by a Gaussian-convolved double exponential distribution. [ABSTRACT FROM AUTHOR]

Despite recent success in forming realistic present-day galaxies, simulations still form the bulk of their stars earlier than observations indicate. We investigate the process of stellar mass assembly in low-mass field galaxies, a dwarf and a typical spiral, focusing on the effects of radiation from young stellar clusters on the star formation (SF) histories. We implement a novel model of SF with a deterministic low efficiency per free-fall time, as observed in molecular clouds. Stellar feedback is based on observations of star-forming regions, and includes radiation pressure from massive stars, photoheating in H II regions, supernovae and stellar winds. We find that stellar radiation has a strong effect on the formation of lowmass galaxies, especially at z > 1, where it efficiently suppresses SF by dispersing cold and dense gas, preventing runaway growth of the stellar component. This behaviour is evident in a variety of observations but had so far eluded analytical and numerical models without radiation feedback. Compared to supernovae alone, radiation feedback reduces the SF rate by a factor of ∼100 at z ≲ 2, yielding rising SF histories which reproduce recent observations of Local Group dwarfs. Stellar radiation also produces bulgeless spiral galaxies and may be responsible for excess thickening of the stellar disc. The galaxies also feature rotation curves and baryon fractions in excellent agreement with current data. Lastly, the dwarf galaxy shows a very slow reduction of the central dark matter density caused by radiation feedback over the last ∼7 Gyr of cosmic evolution. [ABSTRACT FROM AUTHOR]

We present results on the clustering of 282 068 galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) sample of massive galaxies with redshifts 0.4 < z < 0.7 which is part of the Sloan Digital Sky Survey III project. Our results cover a large range of scales from ∼500 to ∼90 h−1 Mpc. We compare these estimates with the expectations of the flat Λ cold dark matter (ΛCDM) standard cosmological model with parameters compatible with Wilkinson Microwave Anisotropy Probe 7 data. We use the MultiDark cosmological simulation, one of the largest N-body runs presently available, together with a simple halo abundance matching technique, to estimate galaxy correlation functions, power spectra, abundance of subhaloes and galaxy biases. We find that the ΛCDM model gives a reasonable description to the observed correlation functions at z ≈ 0.5, which is remarkably good agreement considering that the model, once matched to the observed abundance of BOSS galaxies, does not have any free parameters. However, we find a ≳10 per cent deviation in the correlation functions for scales ≲ 1 and ∼10–40 h−1 Mpc. A more realistic abundance matching model and better statistics from upcoming observations are needed to clarify the situation. We also estimate that about 12 per cent of the ‘galaxies’ in the abundance-matched sample are satellites inhabiting central haloes with mass M ≳ 1014 h−1 M⊙. Using the MultiDark simulation, we also study the real-space halo bias b of the matched catalogue finding that b = 2.00 ± 0.07 at large scales, consistent with the one obtained using the measured BOSS-projected correlation function. Furthermore, the linear large-scale bias, defined using the extrapolated linear matter power spectrum, depends on the number density n of the abundance-matched sample as b = −0.048 − (0.594 ± 0.02)log10(n/ h3 Mpc−3). Extrapolating these results to baryon acoustic oscillation scales, we measure a scale-dependent damping of the acoustic signal produced by non-linear evolution that leads to ∼2–4 per cent dips at ≳ 3σ level for wavenumbers k ≳ 0.1 h Mpc−1 in the linear large-scale bias. [ABSTRACT FROM AUTHOR]

ABSTRACT We make detailed theoretical predictions for the assembly properties of the Local Group (LG) in the standard Λ cold dark matter cosmological model. We use three cosmological N-body dark matter simulations from the Constrained Local Universe Simulations 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 60 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 ≈10-12 Gyr ago. Between 12 and 17 per cent of the haloes in the mass range 5 × 1011 < Mh < 5 × 1012 h−1 M⊙ have a similar MAH. In a set of pairs of haloes within the same mass range, a fraction of 1-3 per cent share similar formation properties as both haloes 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 haloes 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 favourable environment for the formation of disc 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 disc. Our results support the view that the specific large- and mid-scale environments around the LG play a critical role in shaping its MAH and hence its baryonic structure at present. [ABSTRACT FROM AUTHOR]

ABSTRACT We present a detailed comparison of fundamental dark matter halo properties retrieved by a substantial number of different halo finders. These codes span a wide range of techniques including friends-of-friends, spherical-overdensity and phase-space-based algorithms. We further introduce a robust (and publicly available) suite of test scenarios that allow halo finder developers to compare the performance of their codes against those presented here. This set includes mock haloes containing various levels and distributions of substructure at a range of resolutions as well as a cosmological simulation of the large-scale structure of the universe. All the halo-finding codes tested could successfully recover the spatial location of our mock haloes. They further returned lists of particles (potentially) belonging to the object that led to coinciding values for the maximum of the circular velocity profile and the radius where it is reached. All the finders based in configuration space struggled to recover substructure that was located close to the centre of the host halo, and the radial dependence of the mass recovered varies from finder to finder. Those finders based in phase space could resolve central substructure although they found difficulties in accurately recovering its properties. Through a resolution study we found that most of the finders could not reliably recover substructure containing fewer than 30-40 particles. However, also here the phase-space finders excelled by resolving substructure down to 10-20 particles. By comparing the halo finders using a high-resolution cosmological volume, we found that they agree remarkably well on fundamental properties of astrophysical significance (e.g. mass, position, velocity and peak of the rotation curve). We further suggest to utilize the peak of the rotation curve, vmax, as a proxy for mass, given the arbitrariness in defining a proper halo edge. [ABSTRACT FROM AUTHOR]

We study the distribution function (DF) of dark matter particles in haloes of mass range . In the numerical part of this work we measure the DF for a sample of relaxed haloes formed in the simulation of a standard Λ cold dark matter (ΛCDM) model. The DF is expressed as a function of energy E and the absolute value of the angular momentum L, a form suitable for comparison with theoretical models. By proper scaling we obtain the results that do not depend on the virial mass of the haloes. We demonstrate that the DF can be separated into energy and angular momentum components and propose a phenomenological model of the DF in the form . This formulation involves three parameters describing the anisotropy profile in terms of its asymptotic values (β0 and ) and the scale of transition between them ( L0). The energy part is obtained via inversion of the integral for spatial density. We provide a straightforward numerical scheme for this procedure as well as a simple analytical approximation for a typical halo formed in the simulation. The DF model is extensively compared with the simulations: using the model parameters obtained from fitting the anisotropy profile, we recover the DF from the simulation as well as the profiles of the dispersion and kurtosis of radial and tangential velocities. Finally, we show that our DF model reproduces the power-law behaviour of phase-space density . [ABSTRACT FROM AUTHOR]

We investigate the origin and evolution of fossil groups in a concordance ΛCDM cosmological simulation. We consider haloes with masses between and , and study the physical mechanisms that lead to the formation of the large gap in magnitude between the brightest and the second most bright group member, which is typical for these fossil systems. Fossil groups are found to have high dark matter concentrations, which we can relate to their early formation time. The large magnitude gaps arise after the groups have built up half of their final mass, due to merging of massive group members. We show that the existence of fossil systems is primarily driven by the relatively early infall of massive satellites, and that we do not find a strong environmental dependence for these systems. In addition, we find tentative evidence for fossil group satellites falling in on orbits with typically lower angular momentum, which might lead to a more efficient merger on to the host. We find a population of groups at higher redshifts that go through a ‘fossil phase’: a stage where they show a large magnitude gap, which is terminated by renewed infall from their environment. [ABSTRACT FROM AUTHOR]

We present a comparison of the properties of galaxies in the most underdense regions of the Universe, where the galaxy number density is less than 10 per cent of the mean density, with galaxies from more typical regions. We have compiled a sample of galaxies in 46 large nearby voids that were identified using the Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky. We study the u− r colour distribution, morphology, specific star formation rate (SFR) and radial number density profiles for a total of 495 galaxies fainter than located inside the voids and compare these properties with a control sample of field galaxies. We show that there is an excess of blue galaxies inside the voids. However, inspecting the properties of blue and red galaxies separately, we find that galaxy properties such as colour distribution, bulge-to-total ratios and concentrations are remarkably similar between the void and overall sample. The void galaxies also show the same specific SFR at fixed colour as the control galaxies. We compare our results with the predictions of cosmological simulations of galaxy formation using the Millennium Run semi-analytic galaxy catalogue. We show that the properties of the simulated galaxies in large voids are in reasonably good agreement with those found in similar environments in the real Universe. To summarize, in spite of the fact that galaxies in voids live in the least dense large-scale environment, this environment makes very little impact on the properties of galaxies. [ABSTRACT FROM AUTHOR]

We present a statistical analysis of voids in the two-degree Field Galaxy Redshift Survey (2dFGRS). In order to detect the voids, we have developed two robust algorithms. We define voids as non-overlapping maximal spheres empty of haloes or galaxies with mass or luminosity above a given value. We search for voids in cosmological N-Body simulations to test the performance of our void finders. We obtain and analyse the void statistics for several volume-limited samples for the North Galactic Pole (NGP) and the South Galactic Pole (SGP) constructed from the 2dFGRS full data release. We find that the results obtained from the NGP and the SGP are statistically compatible. From the results of several statistical tests we conclude that voids are essentially uncorrelated, with at most a mild anticorrelation and that at the 99.5 per cent confidence level there is a dependence of the void number density on redshift. We develop a technique to correct the distortion caused by the fact that we use the redshift as the radial coordinate. We calibrate this technique with mock catalogues and find that the correction might be of some relevance to carry out accurate inferences from void statistics. We study the statistics of the galaxies inside nine nearby voids. We find that galaxies in voids are not randomly distributed: they form structures like filaments. We also obtain the galaxy number density profile in voids. This profile follow a similar but steeper trend to that followed by haloes in voids. [ABSTRACT FROM AUTHOR]