Andrey V. Kravtsov

University of Chicago, Chicago, Illinois, United States

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Publications (130)601.82 Total impact

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    Benedikt Diemer, Andrey V. Kravtsov
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    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.
    07/2014;
  • Oscar Agertz, Andrey V. Kravtsov
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    ABSTRACT: Using high resolution cosmological zoom-in simulations of galaxy formation, we investigate the star formation-feedback cycle at high redshifts ($z>1$), focusing on progenitors of Milky Way-sized galaxies. Our star formation model is based on the local density of molecular hydrogen (H$_2$) forming on dust grains, as this may be an important ingredient for regulating star formation in the high redshift, metal-poor regime of galaxy formation. Our stellar feedback model accounts for energy and momentum from supernovae, stellar winds and radiation pressure. We use a suite of simulations with different parameters and assumptions about star formation and prescription recipes. We find that in order to reproduce global properties of the Milky Way progenitors, such as star formation history and stellar mass-halo mass relation, simulations should include 1) a combination of local early ($t\lesssim 4$ Myr) momentum feedback via radiation pressure and stellar winds and subsequent efficient supernovae feedback, and 2) the global star formation efficiency on kiloparsec scales should be feedback regulated. In particular, we find that in models with efficient feedback, the local efficiency of star formation per free fall time can be substantially larger than the global star formation efficiency inferred from the Kennicutt-Schmidt relation. We find that simulations that adopt inefficient star formation inferred from such relation fail to produce vigorous outflows and eject sufficient amounts of enriched gas in order to regulate the galactic baryon content. 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. (Abridged for arXiv)
    04/2014;
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    Benedikt Diemer, Andrey V. Kravtsov
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    ABSTRACT: We present a systematic study of the density profiles of halos forming in a LCDM cosmology, focusing on the outer regions of halos, 0.1 < r/Rvir < 9. We show that the median and mean density 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 NFW 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 steepening becomes more pronounced with increasing mass accretion rate. 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, a radius enclosing a fixed overdensity with respect to the critical density of the universe. We propose a new fitting formula that describes the profiles at all radii out to r ~ 9 Rvir, including the transition region around r ~ R200m where the steepening occurs. The formula fits the median and mean density profiles of halo samples selected by their peak height or mass accretion rate with accuracy < 10% at all redshifts and masses we studied, 0 < z < 6 and Mvir > 1.7E10 Msun/h. We discuss observational signatures of the density profile features described above, and show that the steepening of the outer profile should be detectable in future weak lensing analyses of massive clusters. (abridged)
    The Astrophysical Journal 01/2014; 789(1). · 6.73 Impact Factor
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    ABSTRACT: We use recent proper motion measurements of the tangential velocity of M31, along with its radial velocity and distance, to derive the likelihood of the sum of halo masses of the Milky Way and M31. This is done using a sample halo pairs in the Bolshoi cosmological simulation of $\Lambda$CDM cosmology selected to match properties and environment of the Local Group. The resulting likelihood gives estimate of the sum of masses of $M_{\rm MW,200}+M_{\rm M31,200}=$ $2.40_{-1.05}^{+1.95}\times10^{12}\,M_{\odot}$ ($90\%$ confidence interval). This estimate is consistent with individual mass estimates for the Milky Way and M31 and is consistent, albeit somewhat on the low side, with the mass estimated using the timing argument. We show that although the timing argument is unbiased on average for all pairs, for pairs constrained to have radial and tangential velocities similar to that of the Local Group the argument overestimates the sum of masses by a factor of $1.6$. Using similar technique we estimate the total dark matter mass enclosed within $1$ Mpc from the Local Group barycenter to be $M_{\rm LG}(r<1\, {\rm Mpc})=4.2_{-2.0}^{+3.4}\times10^{12}\,M_{\odot}$ ($90\%$ confidence interval).
    The Astrophysical Journal 12/2013; 793(2). · 6.73 Impact Factor
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    ABSTRACT: We present results from high-resolution hydrodynamic simulations of isolated SMC- and Milky Way-sized galaxies that include a model for feedback from galactic cosmic rays (CRs). We find that CRs are naturally able to drive winds with mass loading factors of up to ~10 in dwarf systems. The scaling of the mass loading factor with circular velocity between the two simulated systems is consistent with \propto v_c^{1-2} required to reproduce the faint end of the galaxy luminosity function. In addition, simulations with CR feedback reproduce both the normalization and the slope of the observed trend of wind velocity with galaxy circular velocity. We find that winds in simulations with CR feedback exhibit qualitatively different properties compared to SN driven winds, where most of the acceleration happens violently in situ near star forming sites. In contrast, the CR-driven winds are accelerated gently by the large-scale pressure gradient established by CRs diffusing from the star-forming galaxy disk out into the halo. The CR-driven winds also exhibit much cooler temperatures and, in the SMC-sized system, warm (T~10^4 K) gas dominates the outflow. The prevalence of warm gas in such outflows may provide a clue as to the origin of ubiquitous warm gas in the gaseous halos of galaxies detected via absorption lines in quasar spectra.
    The Astrophysical Journal 08/2013; 777(1). · 6.73 Impact Factor
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    ABSTRACT: We introduce the AGORA project, a comprehensive numerical study of well-resolved galaxies within the LCDM cosmology. Cosmological hydrodynamic simulations with force resolutions of ~100 proper pc or better will be run with a variety of code platforms to follow the hierarchical growth, star formation history, morphological transformation, and the cycle of baryons in and out of 8 galaxies with halo masses M_vir ~= 1e10, 1e11, 1e12, and 1e13 Msun at z=0 and two different ("violent" and "quiescent") assembly histories. The numerical techniques and implementations used in this project include the smoothed particle hydrodynamics codes GADGET and GASOLINE, and the adaptive mesh refinement codes ART, ENZO, and RAMSES. The codes will share common initial conditions and common astrophysics packages including UV background, metal-dependent radiative cooling, metal and energy yields of supernovae, and stellar initial mass function. These are described in detail in the present paper. Subgrid star formation and feedback prescriptions will be tuned to provide a realistic interstellar and circumgalactic medium using a non-cosmological disk galaxy simulation. Cosmological runs will be systematically compared with each other using a common analysis toolkit, and validated against observations to verify that the solutions are robust - i.e., that the astrophysical assumptions are responsible for any success, rather than artifacts of particular implementations. The goals of the AGORA project are, broadly speaking, to raise the realism and predictive power of galaxy simulations and the understanding of the feedback processes that regulate galaxy "metabolism." The proof-of-concept dark matter-only test of the formation of a galactic halo with a z=0 mass of M_vir ~= 1.7e11 Msun by 9 different versions of the participating codes is also presented to validate the infrastructure of the project.
    08/2013;
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    Benedikt Diemer, Andrey V. Kravtsov, Surhud More
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    ABSTRACT: We investigate whether the evolution of cluster scaling relations is affected by the spurious evolution of mass due to the evolving reference density with respect to which halo masses are defined (pseudo-evolution). We use the relation between mass, M, and velocity dispersion, sigma, as a test case, and find that the deviation from the M-sigma relation of cluster-sized halos due of pseudo-evolution is smaller than 10% for a wide range of mass definitions. The reason for this small impact is a tight relation between the velocity dispersion and mass profiles, sigma(<r) = const * (GM(<r) / r)^(1/2), which holds across a large radial range. We show that such a relation is generically expected for a wide range of density profiles, as long as halos are in approximate Jeans equilibrium. Thus, as the outer "virial" radius used to define the halo mass, R, increases due to pseudo-evolution, halos approximately preserve their M-sigma relation. This result explains why such relations are almost insensitive to the definition of the virial radius, and highlights the fact that tight scaling relations are the result of tight equilibrium relations between radial profiles of physical quantities, rather than a result of the virial theorem applied to the cluster as a whole within some unique boundary. We find exceptions at very small and very large radii, where the profiles deviate from the relations they exhibit at intermediate radii. We discuss the implications of these results for other cluster scaling relations, and argue that pseudo-evolution should have very small effects on most scaling relations, except for those which involve the stellar masses of galaxies. In particular, we show that the relation between stellar mass fraction and total mass is affected by pseudo-evolution, and is largely shaped by it for halo masses smaller than 1E14 Msun.
    The Astrophysical Journal 06/2013; · 6.73 Impact Factor
  • Kyu-Hyun Chae, Mariangela Bernardi, Andrey V. Kravtsov
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    ABSTRACT: We assemble a statistical set of global mass models for ~2,000 nearly spherical SDSS galaxies at a mean redshift of 0.12 based on their aperture velocity dispersions and newly derived luminosity profiles in conjunction with published velocity dispersion profiles and empirical properties and relations of galaxy and halo parameters. We consider two-component mass models, in which stellar mass distribution is assumed to follow the observationally-derived luminosity profile and dark matter (DM) distribution is modelled using the generalized Navarro-Frenk-White (gNFW) or Einasto profile, as well as single component models of total mass distribution. When the two-component mass models are fitted to the SDSS aperture velocity dispersions, the predicted velocity dispersion profile (VP) slopes within the effective radius R_eff match well the distribution in observed elliptical galaxies. In contrast, the single-component models cannot reproduce the VP slope distribution. From a number of input variations the models exhibit for the radial range 0.1 R_eff < r < R_eff a tight correlation gamma_e=(1.865+/-0.008)+(-4.93+/-0.15) where gamma_e is the mean slope absolute value of the total mass density and eta is the mean slope of the velocity dispersion profile, which leads to a super-isothermal gamma_e = 2.15+/-0.04 for empirical eta=-0.058+/-0.008. Furthermore, the successful two-component models appear to imply a typical slope curvature pattern in the total mass profile. Finally, it is intriguing to note that a combination analogous to the pseudo phase-space density of DM haloes from N-body simulations Q'(r)=rho(r)/sigma^3(r)$, where rho(r) is the total density profile and sigma(r) is the radial stellar velocity dispersion profile, may well have a universal profile within R_eff with a mean slope of ~-1.87, strikingly similar to the corresponding slope of self-gravitating DM haloes. (abridged)
    Monthly Notices of the Royal Astronomical Society 05/2013; · 5.52 Impact Factor
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    ABSTRACT: Previous studies showed that an estimate of the likelihood distribution of the Milky Way halo mass can be derived using the properties of the satellites similar to the Large and Small Magellanic Clouds (LMC and SMC). However, it would be straightforward to interpret such an estimate only if the properties of the Magellanic Clouds (MCs) are fairly typical and are not biased by the environment. In this study we explore whether the environment of the Milky Way affects the properties of the SMC and LMC such as their velocities. To test for the effect of the environment, we compare velocity distributions for MC-sized subhalos around Milky Way hosts in a sample selected simply by mass and in the second sample of such halos selected with additional restrictions on the distance to the nearest cluster and the local galaxy density, designed to mimic the environment of the Local Group (LG). We find that satellites in halos in the LG-like environments do have somewhat larger velocities, as compared to the halos of similar mass in the sample without environmental constraints. We derive the host halo likelihood distribution for the samples in the LG-like envirionment and in the control sample and find that the environment does not significantly affect the derived likelihood. We use the updated properties of the SMC and LMC to derive the constraint on the MW halo mass $\log{({\rm M}_{200} /\msol)}=12.06^{+0.31}_{-0.19}$ (90% confidence interval). We also explore the incidence of close pairs with relative velocities and separations similar to those of the LMC and SMC and find that such pairs are quite rare among $\Lambda$CDM halos. Taking into account the close separation of the MCs in the Busha et al.\ 2011 method results in the shift of the MW halo mass estimate to smaller masses, with the peak shifting approximately by a factor of two.[Abridged]
    The Astrophysical Journal 01/2013; 770(2). · 6.73 Impact Factor
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    ABSTRACT: Upcoming wide-area sky surveys offer the power to test the source of cosmic acceleration by placing extremely precise constraints on existing cosmological model parameters. These observational surveys will employ multiple tests based on statistical signatures of galaxies and larger-scale structures such as clusters of galaxies. Simulations of large-scale structure provide the means to maximize the power of sky survey tests by characterizing key sources of systematic uncertainties. We describe an XSEDE program to produce multiple synthetic sky surveys of galaxies and large-scale cosmic structure in support of science analysis for the Dark Energy Survey. We explain our Airavata-enabled methods and report extensions to our workflow processing over the last year. We highlight science analysis focused on counts of clusters of galaxies.
    Proceedings of the Conference on Extreme Science and Engineering Discovery Environment: Gateway to Discovery, San Diego, California; 01/2013
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    Andrey V. Kravtsov
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    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<r_n. The results of this study imply that galaxy sizes and radial distribution of baryons are shaped primarily by properties of their parent halo and that sizes of both late type disks and early type spheroids are controlled by halo angular momentum.
    The Astrophysical Journal Letters 12/2012; 764(2). · 6.35 Impact Factor
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    ABSTRACT: Stellar feedback plays a key role in galaxy formation by regulating star formation, driving interstellar turbulence and generating galactic scale outflows. Although modern simulations of galaxy formation can resolve scales of 10-100 pc, star formation and feedback operate on smaller, "subgrid" scales. Great care should therefore be taken in order to properly account for the effect of feedback on global galaxy evolution. We investigate the momentum and energy budget of feedback during different stages of stellar evolution, and study its impact on the interstellar medium using simulations of local star forming regions and galactic disks at the resolution affordable in modern cosmological zoom-in simulations. In particular, we present a novel subgrid model for the momentum injection due to radiation pressure and stellar winds from massive stars during early, pre-supernova evolutionary stages of young star clusters. Early injection of momentum acts to clear out dense gas in star forming regions, hence limiting star formation. The reduced gas density mitigates radiative losses of thermal feedback energy from subsequent supernova explosions, leading to an increased overall efficiency of stellar feedback. The detailed impact of stellar feedback depends sensitively on the implementation and choice of parameters. Somewhat encouragingly, we find that implementations in which feedback is efficient lead to approximate self-regulation of global star formation efficiency. We compare simulation results using our feedback implementation to other phenomenological feedback methods, where thermal feedback energy is allowed to dissipate over time scales longer than the formal gas cooling time. We find that simulations with maximal momentum injection suppress star formation to a similar degree as is found in simulations adopting adiabatic thermal feedback.
    The Astrophysical Journal 10/2012; 770(1). · 6.73 Impact Factor
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    ABSTRACT: The next generation of wide-area sky surveys offer the power to place extremely precise constraints on cosmological parameters and to test the source of cosmic acceleration. These observational programs will employ multiple techniques based on a variety of statistical signatures of galaxies and large-scale structure. These techniques have sources of systematic error that need to be understood at the percent-level in order to fully leverage the power of next-generation catalogs. Simulations of large-scale structure provide the means to characterize these uncertainties. We are using XSEDE resources to produce multiple synthetic sky surveys of galaxies and large-scale structure in support of science analysis for the Dark Energy Survey. In order to scale up our production to the level of fifty 10^10-particle simulations, we are working to embed production control within the Apache Airavata workflow environment. We explain our methods and report how the workflow has reduced production time by 40% compared to manual management.
    10/2012;
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    Robert Feldmann, Nickolay Y. Gnedin, Andrey V. Kravtsov
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    ABSTRACT: There is ample observational evidence that the star formation rate (SFR) surface density, Sigma_SFR, is closely correlated with the surface density of molecular hydrogen, Sigma_H2. This empirical relation holds both for galaxy-wide averages and for individual >=kpc sized patches of the interstellar medium (ISM), but appears to degrade substantially at a sub-kpc scale. Identifying the physical mechanisms that determine the scale-dependent properties of the observed Sigma_H2-Sigma_SFR relation remains a challenge from a theoretical perspective. To address this question, we analyze the slope and scatter of the Sigma_H2-Sigma_SFR relation using a set of cosmological, galaxy formation simulations with a peak resolution of ~100 pc. These simulations include a chemical network for molecular hydrogen, a model for the CO emission, and a simple, stochastic prescription for star formation that operates on ~100 pc scales. Specifically, star formation is modeled as a Poisson process in which the average SFR is directly proportional to the present mass of H2. The predictions of our numerical model are in good agreement with the observed Kennicutt-Schmidt and Sigma_H2-Sigma_SFR relations. We show that observations based on CO emission are ill suited to reliably measure the slope of the latter relation at low (<20 M_sun pc^-2) H2 surface densities on sub-kpc scales. Our models also predict that the inferred Sigma_H2-Sigma_SFR relation steepens at high H2 surface densities as a result of the surface density dependence of the CO/H2 conversion factor. Finally, we show that on sub-kpc scales most of the scatter in the relation is a consequence of discreteness effects in the star formation process. In contrast, variations of the CO/H2 conversion factor are responsible for most of the scatter measured on super-kpc scales.
    The Astrophysical Journal 04/2012; 758(2). · 6.73 Impact Factor
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    ABSTRACT: With SDSS galaxy data and halo data from up-to-date N-body simulations we construct a semi-empirical catalog (SEC) of early-type systems by making a self-consistent bivariate statistical match of stellar mass (M_star) and velocity dispersion (sigma) with halo virial mass (M_vir). We then assign stellar mass profile and velocity dispersion profile parameters to each system in the SEC using their observed correlations with M_star and sigma. Simultaneously, we solve for dark matter density profile of each halo using the spherical Jeans equation. The resulting dark matter density profiles deviate in general from the dissipationless profile of NFW or Einasto and their mean inner density slope and concentration vary systematically with M_vir. Statistical tests of the distribution of profiles at fixed M_vir rule out the null hypothesis that it follows the distribution predicted by N-body simulations for M_vir ~< 10^{13.5-14.5} M_solar. These dark matter profiles imply that dark matter density is, on average, enhanced significantly in the inner region of halos with M_vir ~< 10^{13.5-14.5} M_solar supporting halo contraction. The main characteristics of halo contraction are: (1) the mean dark matter density within the effective radius has increased by a factor varying systematically up to ~ 3-4 at M_vir = 10^{12} M_solar, and (2) the inner density slope has a mean of ~ 1.3 with rho(r) ~ r^{-alpha} and a halo-to-halo rms scatter of rms(alpha) ~ 0.4-0.5 for 10^{12} M_solar ~< M_vir ~< 10^{13-14} M_solar steeper than the NFW profile (alpha=1). Based on our results we predict that halos of nearby elliptical and lenticular galaxies can, in principle, be promising targets for gamma-ray emission from dark matter annihilation.
    Journal of Cosmology and Astroparticle Physics 02/2012; 2012(11). · 6.04 Impact Factor
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    Denis Erkal, Nickolay Y. Gnedin, Andrey V. Kravtsov
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    ABSTRACT: We study the high column density regime of the HI column density distribution function and argue that there are two distinct features: a turnover at NHI ~ 10^21 cm^-2 which is present at both z=0 and z ~ 3, and a lack of systems above NHI ~ 10^22 cm^-2 at z=0. Using observations of the column density distribution, we argue that the HI-H2 transition does not cause the turnover at NHI ~ 10^21 cm^-2, but can plausibly explain the turnover at NHI > 10^22 cm^-2. We compute the HI column density distribution of individual galaxies in the THINGS sample and show that the turnover column density depends only weakly on metallicity. Furthermore, we show that the column density distribution of galaxies, corrected for inclination, is insensitive to the resolution of the HI map or to averaging in radial shells. Our results indicate that the similarity of HI column density distributions at z=3 and z=0 is due to the similarity of the maximum HI surface densities of high-z and low-z disks, set presumably by universal processes that shape properties of the gaseous disks of galaxies. Using fully cosmological simulations, we explore other candidate physical mechanisms that could produce a turnover in the column density distribution. We show that while turbulence within GMCs cannot affect the DLA column density distribution, stellar feedback can affect it significantly if the feedback is sufficiently effective in removing gas from the central 2-3 kpc of high-redshift galaxies. Finally, we argue that it is meaningful to compare column densities averaged over ~ kpc scales with those estimated from quasar spectra which probe sub-pc scales due to the steep power spectrum of HI column density fluctuations observed in nearby galaxies.
    The Astrophysical Journal 01/2012; 761(1). · 6.73 Impact Factor
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    ABSTRACT: Simulations of cluster formation have demonstrated that condensation of baryons into central galaxies during cluster formation can drive the shape of the gas distribution in galaxy clusters significantly rounder, even at radii as large as half of the virial radius. However, such simulations generally predict stellar fractions within cluster virial radii that are ~2 to 3 times larger than the stellar masses deduced from observations. In this work we compare ellipticity profiles of clusters simulated with and without baryonic cooling to the cluster ellipticity profiles derived from Chandra and ROSAT observations in an effort to constrain the fraction of gas that cools and condenses into the central galaxies within clusters. We find that the observed ellipticity profiles are fairly constant with radius, with an average ellipticity of 0.18 +/- 0.05. The observed ellipticity profiles are in good agreement with the predictions of non-radiative simulations. On the other hand, the ellipticity profiles of the clusters in simulations that include radiative cooling, star formation, and supernova feedback (but no AGN feedback) deviate significantly from the observed ellipticity profiles at all radii. The simulations with cooling overpredict (underpredict) ellipticity in the inner (outer) regions of galaxy clusters. By comparing the simulations with and without cooling, we show that the cooling of gas via cooling flows in the central regions of simulated clusters causes the gas distribution to be more oblate in the central regions, but makes the outer gas distribution more spherical. We find that late-time gas cooling and star formation are responsible for the significantly oblate gas distributions in cluster cores, but the gas shapes outside of cluster cores are set primarily by baryon dissipation at high redshift z > 2.
    The Astrophysical Journal 01/2012; 755(2). · 6.73 Impact Factor
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    ABSTRACT: Cosmology simulations are highly communication-intensive, thus it is critical to exploit topology-aware task mapping techniques for performance optimization. To exploit the architectural properties of multiprocessor clusters (the performance gap between inter-node and intra-node communication as well as the gap between inter-socket and intra-socket communication), we design and develop a hierarchical task mapping scheme for cell-based AMR (Adaptive Mesh Refinement) cosmology simulations, in particular, the ART application. Our scheme consists of two parts: (1) an inter-node mapping to map application processes onto nodes with the objective of minimizing network traffic among nodes and (2) an intra-node mapping within each node to minimize the maximum size of messages transmitted between CPU sockets. Experiments on production supercomputers with 3D torus and fat-tree topologies show that our scheme can significantly reduce application communication cost by up to 50%. More importantly, our scheme is generic and can be extended to many other applications.
    High Performance Computing, Networking, Storage and Analysis (SC), 2012 International Conference for; 01/2012
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    Robert Feldmann, Nickolay Y. Gnedin, Andrey V. Kravtsov
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    ABSTRACT: Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X_CO, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds (GMCs) in the Milky Way, except possibly for mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. The XCO factor may also depend on the scale on which CO emission is averaged due to effects of limited resolution. We study the dependence of X_CO on gas properties and averaging scale using a model that is based on a combination of results of sub-pc scale magneto-hydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. Our model predicts a value of X_CO that is consistent with the Galactic value for interstellar medium conditions typical for the Milky Way. For such conditions the predicted X_CO varies by only a factor of two for gas surfaced densities in the range \sim 50 - 500 M_sun / pc^2. However, the model also predicts that more generally on the scale of GMCs, X_CO is a strong function of metallicity, and depends on the column density and the interstellar UV flux. We show explicitly that neglecting these dependencies in observational estimates can strongly bias the inferred distribution of H2 column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We find that when averaged on \sim kpc scales the X-factor depends only weakly on radiation field and column density, but is still a strong function of metallicity. The predicted metallicity dependence can be approximated as X_CO \sim Z^{-{\gamma}} with {\gamma} ~ 0.5 - 0.8.
    The Astrophysical Journal 12/2011; 747(2). · 6.73 Impact Factor
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    ABSTRACT: The condensation of gas and stars in the inner regions of dark matter halos leads to a more concentrated dark matter distribution. While this effect is based on simple gravitational physics, the question of its validity in hierarchical galaxy formation has led to an active debate in the literature. We use a collection of several state-of-the-art cosmological hydrodynamic simulations to study the halo contraction effect in systems ranging from dwarf galaxies to clusters of galaxies, at high and low redshift. The simulations are run by different groups with different codes and include hierarchical merging, gas cooling, star formation, and stellar feedback. We show that in all our cases the inner dark matter density increases relative to the matching simulation without baryon dissipation, at least by a factor of several. The strength of the contraction effect varies from system to system and cannot be reduced to a simple prescription. We present a revised analytical model that describes the contracted mass profile to an rms accuracy of about 10%. The model can be used to effectively bracket the response of the dark matter halo to baryon dissipation. The halo contraction effect is real and must be included in modeling of the mass distribution of galaxies and galaxy clusters.
    08/2011;

Publication Stats

9k Citations
601.82 Total Impact Points

Institutions

  • 2002–2014
    • University of Chicago
      • • Kavli Institute for Cosmological Physics
      • • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States
  • 2013
    • California College San Diego
      San Diego, California, United States
  • 2012
    • Yale University
      • Department of Physics
      New Haven, Connecticut, United States
  • 2007–2012
    • Fermi National Accelerator Laboratory (Fermilab)
      • Center for Particle Astrophysics
      Batavia, Illinois, United States
    • University of Pittsburgh
      • Physics and Astronomy
      Pittsburgh, Pennsylvania, United States
  • 1999–2009
    • New Mexico State University
      • Department of Astronomy
      Las Cruces, NM, United States
  • 2008
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2006–2008
    • Space Research Institute
      Moskva, Moscow, Russia
  • 1999–2008
    • The Ohio State University
      • • Department of Physics
      • • Department of Astronomy
      Columbus, Ohio, United States