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Search Results: 1 - 10 of 132357 matches for " Andrey V. Kravtsov "
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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 $z\sim 1-2$.
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.
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)
Dependence of the outer density profiles of halos on their mass accretion rate
Benedikt Diemer,Andrey V. Kravtsov
Physics , 2014, DOI: 10.1088/0004-637X/789/1/1
Abstract: We present a systematic study of the density profiles of LCDM halos, focusing on the outer regions, 0.1 < r/Rvir < 9. We show that the median and mean profiles of halo samples of a given peak height exhibit significant deviations from the universal analytic profiles discussed previously in the literature, such as the Navarro-Frenk-White and Einasto profiles, at radii r > 0.5 R200m. In particular, at these radii the logarithmic slope of the median density profiles of massive or rapidly accreting halos steepens more sharply than predicted. The steepest slope of the profiles occurs at r ~ R200m, and its absolute value increases with increasing peak height or mass accretion rate, reaching slopes of -4 and steeper. Importantly, we find that the outermost density profiles at r > R200m are remarkably self-similar when radii are rescaled by R200m. This self-similarity indicates that radii defined with respect to the mean density are preferred for describing the structure and evolution of the outer profiles. However, the inner density profiles are most self-similar when radii are rescaled by R200c. We propose a new fitting formula that describes the median and mean profiles of halo samples selected by their peak height or mass accretion rate with accuracy < 10% at all radii, redshifts and masses we studied, r < 9 Rvir, 0 < z < 6 and Mvir > 1.7E10 Msun/h. We discuss observational signatures of the profile features described above, and show that the steepening of the outer profile should be detectable in future weak-lensing analyses of massive clusters. Such observations could be used to estimate the mass accretion rate of cluster halos.
A universal model for halo concentrations
Benedikt Diemer,Andrey V. Kravtsov
Physics , 2014, DOI: 10.1088/0004-637X/799/1/108
Abstract: We present a numerical study of dark matter halo concentrations in $\Lambda$CDM and self-similar cosmologies. We show that the relation between concentration, $c$, and peak height, $\nu$, exhibits the smallest deviations from universality if halo masses are defined with respect to the critical density of the universe. These deviations can be explained by the residual dependence of concentration on the local slope of the matter power spectrum, $n$, which affects both the normalization and shape of the $c$-$\nu$ relation. In particular, there is no well-defined floor in the concentration values. Instead, the minimum concentration depends on redshift: at fixed $\nu$, halos at higher $z$ experience steeper slopes $n$, and thus have lower minimum concentrations. We show that the concentrations in our simulations can be accurately described by a universal seven-parameter function of only $\nu$ and $n$. This model matches our $\Lambda$CDM results to $\lesssim 5\%$ accuracy up to $z = 6$, and matches scale-free $\Omega_{\rm m} = 1$ models to $\lesssim 15\%$. The model also reproduces the low concentration values of Earth--mass halos at $z \approx 30$, and thus correctly extrapolates over $16$ orders of magnitude in halo mass. The predictions of our model differ significantly from all models previously proposed in the literature at high masses and redshifts. Our model is in excellent agreement with recent lensing measurements of cluster concentrations.
The impact of stellar feedback on the structure, size and morphology of galaxies in Milky Way size dark matter haloes
Oscar Agertz,Andrey V. Kravtsov
Physics , 2015,
Abstract: We use cosmological zoom-in simulations of galaxy formation in a Milky Way (MW)-sized halo started from identical initial conditions to investigate the evolution of galaxy sizes, baryon fractions, morphologies and angular momenta in runs with different parameters of the star formation--feedback cycle. Our fiducial model with a high local star formation efficiency, which results in efficient feedback, produces a realistic late-type galaxy that matches the evolution of basic properties of late-type galaxies: stellar mass, disk size, morphology dominated by a kinematically cold disk, stellar and gas surface density profiles, and specific angular momentum. We argue that feedback's role in this success is twofold: (1) removal of low-angular momentum gas and (2) maintaining a low disk-to-halo mass fraction which suppresses disk instabilities that lead to angular momentum redistribution and a central concentration of baryons. However, our model with a low local star formation efficiency, but large energy input per supernova, chosen to produce a galaxy with a similar star formation history as our fiducial model, leads to a highly irregular galaxy with no kinematically cold component, overly extended stellar distribution and low angular momentum. This indicates that only when feedback is allowed to become vigorous via locally efficient star formation in dense cold gas, resulting galaxy sizes, gas/stellar surface density profiles and stellar disk angular momenta agree with observed $z=0$ galaxies.
On the interplay between star formation and feedback in galaxy formation simulations
Oscar Agertz,Andrey V. Kravtsov
Physics , 2014, DOI: 10.1088/0004-637X/804/1/18
Abstract: We investigate the star formation-feedback cycle in cosmological galaxy formation simulations, focusing on progenitors of Milky Way (MW)-sized galaxies. We find that in order to reproduce key properties of the MW progenitors, such as semi-empirically derived star formation histories and the shape of rotation curves, our implementation of star formation and stellar feedback requires 1) a combination of local early momentum feedback via radiation pressure and stellar winds and subsequent efficient supernovae feedback, and 2) efficacy of feedback that results in self-regulation of the global star formation rate on kiloparsec scales. We show that such feedback-driven self-regulation is achieved globally for a local star formation efficiency per free fall time of $\epsilon_{\rm ff}\approx 10\%$. Although this value is larger that the $\epsilon_{\rm ff}\sim 1\%$ value usually inferred from the Kennicutt-Schmidt (KS) relation, we show that it is consistent with direct observational estimates of $\epsilon_{\rm ff}$ in molecular clouds. Moreover, we show that simulations with local efficiency of $\epsilon_{\rm ff}\approx 10\%$ reproduce the global observed KS relation. Such simulations also reproduce the cosmic star formation history of the Milky Way sized galaxies and satisfy a number of other observational constraints. Conversely, we find that simulations that a priori assume an inefficient mode of star formation, instead of achieving it via stellar feedback regulation, fail to produce sufficiently vigorous outflows and do not reproduce observations. This illustrates the importance of understanding the complex interplay between star formation and feedback and the detailed processes that contribute to the feedback-regulated formation of galaxies.
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