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Dark Matter Substructure and Dwarf Galactic Satellites
Andrey Kravtsov
Advances in Astronomy , 2010, DOI: 10.1155/2010/281913
Abstract: A decade ago cosmological simulations of increasingly higher resolution were used to demonstrate that virialized regions of Cold Dark Matter (CDM) halos are filled with a multitude of dense, gravitationally bound clumps. These dark matter subhalos are central regions of halos that survived strong gravitational tidal forces and dynamical friction during the hierarchical sequence of merging and accretion via which the CDM halos form. Comparisons with observations revealed that there is a glaring discrepancy between abundance of subhalos and luminous satellites of the Milky Way and Andromeda as a function of their circular velocity or bound mass within a fixed aperture. This large discrepancy, which became known as the “substructure” or the “missing satellites” problem, begs for an explanation. In this paper, the author reviews the progress made during the last several years both in quantifying the problem and in exploring possible scenarios in which it could be accommodated and explained in the context of galaxy formation in the framework of the CDM paradigm of structure formation. In particular, he shows that the observed luminosity function, radial distribution, and the remarkable similarity of the inner density profiles of luminous satellites can be understood within hierarchical CDM framework using a simple model in which efficiency of star formation monotonically decreases with decreasing virial mass satellites had before their accretion without any actual sharp galaxy formation threshold. 1. Introduction In the hierarchical scenario of galaxy formation [1], theoretically rooted in the Cold Dark Matter (CDM) structure formation model [2], galaxies form via cooling and condensation of gas in dark matter halos, which grow via an hierarchical sequence of mergers and accretion. The density perturbations in these models have amplitude that increases with decreasing scale down to 1 comoving parsec or below [3], with the smallest fluctuation scale defined by the specific properties of the particles assumed to constitute the majority of the CDM. Smaller perturbations thus collapse first and then grow and merge to form larger and larger objects, with details of the evolution determined by expansion history of the universe (i.e., by parameters describing the background cosmological model) and by the shape of the density fluctuation power spectrum [4]. An example of such evolution in the flat CDM model is illustrated in Figure 1, which shows collapse of a object. The figure shows that during the early stages of evolution the matter that is incorporated into the
Large-scale distribution of quasars in deep pencil-beam surveys
Andrey V. Kravtsov
Physics , 1996,
Abstract: We have used integral two-point spatial correlation function and its second derivative to analyze the distribution of quasars in three very deep surveys published in the literature. Statistically significant ($\sim 2-3\sigma $) correlations were found at scales of $\sim 50-100h^{-1}$ Mpc in all of the analyzed surveys. We have used the friends-of-friends cluster analysis to show that these correlations can be explained by the presence of relatively small quasar clusters (3-6 objects) which may possibly belong to larger structures such as Large Quasar Groups found in the bigger surveys. The sizes of these clusters along the redshift direction and distances between them are similar to those for structures found recently in studies of CIV absorption systems. These results present further evidence for the existence of large-scale structures at redshifts $z\sim 1-2$.
Cosmological simulations of galaxy clusters
Stefano Borgani,Andrey Kravtsov
Physics , 2009, DOI: 10.1166/asl.2011.1209
Abstract: We review recent progress in the description of the formation and evolution of galaxy clusters in a cosmological context by using numerical simulations. We focus our presentation on the comparison between simulated and observed X-ray properties, while we will also discuss numerical predictions on properties of the galaxy population in clusters. Many of the salient observed properties of clusters, such as X-ray scaling relations, radial profiles of entropy and density of the intracluster gas, and radial distribution of galaxies are reproduced quite well. In particular, the outer regions of cluster at radii beyond about 10 per cent of the virial radius are quite regular and exhibit scaling with mass remarkably close to that expected in the simplest case in which only the action of gravity determines the evolution of the intra-cluster gas. However, simulations generally fail at reproducing the observed cool-core structure of clusters: simulated clusters generally exhibit a significant excess of gas cooling in their central regions, which causes an overestimate of the star formation and incorrect temperature and entropy profiles. The total baryon fraction in clusters is below the mean universal value, by an amount which depends on the cluster-centric distance and the physics included in the simulations, with interesting tensions between observed stellar and gas fractions in clusters and predictions of simulations. Besides their important implications for the cosmological application of clusters, these puzzles also point towards the important role played by additional physical processes, beyond those already included in the simulations. We review the role played by these processes, along with the difficulty for their implementation, and discuss the outlook for the future progress in numerical modeling of clusters.
Formation of Galaxy Clusters
Andrey Kravtsov,Stefano Borgani
Physics , 2012, DOI: 10.1146/annurev-astro-081811-125502
Abstract: In this review, we describe our current understanding of cluster formation: from the general picture of collapse from initial density fluctuations in an expanding Universe to detailed simulations of cluster formation including the effects of galaxy formation. We outline both the areas in which highly accurate predictions of theoretical models can be obtained and areas where predictions are uncertain due to uncertain physics of galaxy formation and feedback. The former includes the description of the structural properties of the dark matter halos hosting cluster, their mass function and clustering properties. Their study provides a foundation for cosmological applications of clusters and for testing the fundamental assumptions of the standard model of structure formation. The latter includes the description of the total gas and stellar fractions, the thermodynamical and non-thermal processes in the intracluster plasma. Their study serves as a testing ground for galaxy formation models and plasma physics. In this context, we identify a suitable radial range where the observed thermal properties of the intra-cluster plasma exhibit the most regular behavior and thus can be used to define robust observational proxies for the total cluster mass. We put particular emphasis on examining assumptions and limitations of the widely used self-similar model of clusters. Finally, we discuss the formation of clusters in non-standard cosmological models, such as non-Gaussian models for the initial density field and models with modified gravity, along with prospects for testing these alternative scenarios with large cluster surveys in the near future.
On the Origin of the Global Schmidt Law of Star Formation
Andrey V. Kravtsov
Physics , 2003, DOI: 10.1086/376674
Abstract: One of the most puzzling properties of observed galaxies is the universality of the empirical correlation between the star formation rate and average gas surface density on kiloparsec scales (the Schmidt law). In this study I present results of self-consistent cosmological simulations of high-redshift galaxy formation that reproduce the Schmidt law naturally, without assuming it, and provide some clues to this puzzle. The simulations incorporate the main physical processes critical to various aspects of galaxy formation and have a dynamic range high enough to identify individual star forming regions. The results indicate that the global Schmidt law is a manifestation of the overall density distribution of the interstellar medium (ISM). In particular, the density probability distribution function (PDF) in the simulated disks is similar to that observed in recent state-of-the-art modeling of the turbulent ISM and has a well-defined generic shape. The shape of the PDF in a given region of the disk depends on the local average surface density Sigma_g. The dependence is such that the fraction of gas mass in the high-density tail of the distribution scales as Sigma_g^{n-1} with n~1.4, which gives rise to the Schmidt-like correlation. The high-density tail of the PDF is remarkably insensitive to the inclusion of feedback and details of the cooling and heating processes. This indicates that the global star formation rate is determined by the supersonic turbulence driven by gravitational instabilities on large scales, rather than stellar feedback or thermal instabilities on small scales.
Origin and evolution of halo bias in linear and non-linear regimes
Andrey Kravtsov,Anatoly Klypin
Physics , 1998, DOI: 10.1086/307495
Abstract: We present results from a study of bias and its evolution for galaxy-size halos in a large, high-resolution simulation of a LCDM model. We consider the evolution of bias estimated using two-point correlation function (b_xi), power spectrum (b_P), and a direct correlation of smoothed halo and matter overdensity fields (b_d). We present accurate estimates of the evolution of the matter power spectrum probed deep into the stable clustering regime (k~[0.1-200]h/Mpc at z=0). The halo power spectrum evolves much slower than the power spectrum of matter and has a different shape which indicates that the bias is time- and scale-dependent. At z=0, the halo power spectrum is anti-biased with respect to the matter power spectrum at wavenumbers k~[0.15-30]h/Mpc, and provides an excellent match to the power spectrum of the APM galaxies at all probed k. In particular, it nicely matches the inflection observed in the APM power spectrum at k~0.15h/Mpc. We complement the power spectrum analysis with a direct estimate of bias using smoothed halo and matter overdensity fields and show that the evolution observed in the simulation in linear and mildly non-linear regimes can be well described by the analytical model of Mo & White (1996), if the distinction between formation redshift of halos and observation epoch is introduced into the model. We present arguments and evidence that at higher overdensities, the evolution of bias is significantly affected by dynamical friction and tidal stripping operating on the satellite halos in high-density regions of clusters and groups; we attribute the strong anti-bias observed in the halo correlation function and power spectrum to these effects. (Abridged)
The size - virial radius relation of galaxies
Andrey V. Kravtsov
Physics , 2012, DOI: 10.1088/2041-8205/764/2/L31
Abstract: Sizes of galaxies are an important diagnostic for galaxy formation models. In this study I use the abundance matching ansatz, which has proven to be successful in reproducing galaxy clustering and other statistics, to derive estimates of the virial radius, R200, for galaxies of different morphological types and wide range of stellar mass. I show that over eight of orders of magnitude in stellar mass galaxies of all morphological types follow an approximately linear relation between 3D half-mass radius of their stellar distribution, rhalf and virial radius, rhalf~0.015R200 with a scatter of ~0.2 dex. Such scaling is in remarkable agreement with expectation of models which assume that galaxy sizes are controlled by halo angular momentum, which implies rhalf\propto lambda R200, where lambda is the spin of galaxy parent halo. The scatter about the relation is comparable with the scatter expected from the distribution of $\lambda$ and normalization of the relation agrees with that predicted by the model of Mo, Mao & White (1998), if galaxy sizes were set on average at z~1-2. Moreover, I show that when stellar and gas surface density profiles of galaxies of different morphological types are rescaled using radius r_n= 0.015 R200, the rescaled surface density profiles follow approximately universal exponential (for late types) and de Vaucouleurs (for early types) profiles with scatter of only 30-50% at R~1-3r_n. Remarkably, both late and early type galaxies have similar mean stellar surface density profiles at R>r_n. The main difference between their stellar distributions is thus at R
Sample Variance Considerations for Cluster Surveys
Wayne Hu,Andrey V. Kravtsov
Physics , 2002, DOI: 10.1086/345846
Abstract: We present a general statistical framework for describing the effect of sample variance in the number counts of virialized objects and examine its effect on cosmological parameter estimation. Specifically, we consider effects of sample variance on the power spectrum normalization and properties of dark energy extracted from current and future local and high-redshift samples of clusters. We show that for future surveys that probe ever lower cluster masses and temperatures, sample variance is generally comparable to or greater than shot noise and thus cannot be neglected in deriving precision cosmological constraints. For example, sample variance is usually more important than shot variance in constraints on the equation of state of the dark energy from z < 1 clusters. Although we found that effects of sample variance on the sigma_8-Omega_m constraints from the current flux and temperature limited X-ray surveys are not significant, they may be important for future studies utilizing the shape of the temperature function to break the sigma_8-Omega_m degeneracy. We also present numerical tests clarifying the definition of cluster mass employed in cosmological modelling and accurate fitting formula for the conversion between different definitions of halo mass (e.g., virial vs. fixed overdensity).
Cold Fronts in CDM clusters
Daisuke Nagai,Andrey V. Kravtsov
Physics , 2002, DOI: 10.1086/368303
Abstract: Recently, high-resolution Chandra observations revealed the existence of very sharp features in the X-ray surface brightness and temperature maps of several clusters (Vikhlinin et. al., 2001). These features, called ``cold fronts'', are characterized by an increase in surface brightness by a factor >2 over 10-50 kpc, accompanied by a drop in temperature of a similar magnitude. The existence of such sharp gradients can be used to put interesting constraints on the physics of the intracluster medium (ICM), if their mechanism and longevity are well understood. Here, we present results of a search for cold fronts in high-resolution simulations of galaxy clusters in cold dark matter (CDM) models. We show that sharp gradients with properties similar to those of observed cold fronts naturally arise in cluster mergers when the shocks heat gas surrounding the merging sub-cluster, while its dense core remains relatively cold. The compression induced by supersonic motions and shock heating during the merger enhance the amplitude of gas density and temperature gradients across the front. Our results indicate that cold fronts are non-equilibrium transient phenomena and can be observed for a period of less than a billion years. We show that the velocity and density fields of gas surrounding the cold front can be very irregular which would complicate analyses aiming to put constraints on the physical conditions of the intracluster medium in the vicinity of the front.
On the supernovae heating of intergalactic medium
Andrey V. Kravtsov,Gustavo Yepes
Physics , 2000, DOI: 10.1046/j.1365-8711.2000.03771.x
Abstract: We present estimates of the energy input from supernovae (SNe) into the intergalactic medium using (i) recent measurements of Si and Fe abundances in the intracluster medium (ICM) and (ii) self-consistent gasdynamical galaxy formation simulations that include processes of cooling, star formation, SNe feedback, and a multi-phase model of the interstellar medium. We estimate the energy input from observed abundances using two different assumptions: (i) spatial uniformity of metal abundances in the ICM and (ii) radial abundance gradients. We show that these two cases lead to energy input estimates which are different by an order of magnitude, highlighting a need for observational data on large-scale abundance gradients in clusters. Analysis of galaxy formation results and estimates from observed Fe and Si abundances indicates that the SNe energy input can be important for heating of the entire ICM (providing energy of ~1 keV per particle) only if the ICM abundances are uniform and the efficiency of gas heating by SN explosions is close to 100% (implying that all of the initial kinetic energy of the explosion goes into heating of the ICM). We conclude that unless these most favorable conditions are met, SNe alone are unlikely to provide sufficient energy input to heat all of the cluster ICM and may need to be supplemented or even substituted by some other heating process(es). (Abridged)
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