arXiv subjects

Andrey V. Kravtsov

Publications and source records attributed to Andrey V. Kravtsov.

At least 55 records · Page 3Linked to original sources

arXiv2012

Constraining Cluster Physics with the Shape of X-ray Clusters: Comparison of Local X-ray Clusters versus LCDM Clusters

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.

astro-ph.CO↗

arXiv2012

The X-factor in Galaxies: II. The molecular hydrogen -- star formation relation

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.

astro-ph.CO↗

arXiv2011

The X-factor in Galaxies: I. Dependence on Environment and Scale

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^{-γ} with γ ~ 0.5 - 0.8.

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arXiv2011

Ultra-faint dwarf galaxies as a test of early enrichment and metallicity-dependent star formation

The tight relation of star formation with molecular gas indicated by observations and assumed in recent models implies that the efficiency with which galaxies convert their gas into stars depends on gas metallicity. This is because the abundance of molecular hydrogen is sensitive to the abundance of dust, which catalyzes the formation of H_2 and helps to shield it from dissociating radiation. In this study we point out that in the absence of significant pre-enrichment by Population III stars forming out of zero metallicity gas, such H_2-based star formation is expected to leave an imprint in the form of bi-modality in the metallicity distribution among dwarf galaxies and in the metallicity distribution of stars within individual galaxies. The bi-modality arises because when gas metallicity (and dust abundance) is low, formation of molecular gas is inefficient, the gas consumption time scale is long, and star formation and metal enrichment proceed slowly. When metallicity reaches a critical threshold value star formation and enrichment accelerate, which leads to rapid increase in both stellar mass and metallicity of galaxies. We demonstrate this process both using a simple analytical model and full cosmological simulations. In contrast, observed metallicity distributions of dwarf galaxies or stars within them are not bi-modal. We argue that this discrepancy points to substantial early stochastic pre-enrichment by population III stars to levels Z ~ 0.01 Z_sun in dense, star forming regions of early galaxies.

astro-ph.CO↗

arXiv2011

Halo Contraction Effect in Hydrodynamic Simulations of Galaxy Formation

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.

astro-ph.CO↗

arXiv2011

The impact of baryon physics on the structure of high-redshift galaxies

We study the detailed structure of galaxies at redshifts z > 2 using cosmological simulations with improved modeling of the interstellar medium and star formation. The simulations follow the formation and dissociation of molecular hydrogen, and include star formation only in cold molecular gas. The molecular gas is more concentrated towards the center of galaxies than the atomic gas, and as a consequence, the resulting stellar distribution is very compact. For halos with total mass above 10^{11} Mo, the median half-mass radius of the stellar disks is 0.8 kpc at z = 3. The vertical structure of the molecular disk is much thinner than that of the atomic neutral gas. Relative to the non-radiative run, the inner regions of the dark matter halo change shape from prolate to mildly oblate and align with the stellar disk. However, we do not find evidence for a significant dark disk of dark matter around the stellar disk. The outer halo regions retain the orientation acquired during accretion and mergers, and are significantly misaligned with the inner regions. The radial profile of the dark matter halo contracts in response to baryon dissipation, establishing an approximately isothermal profile throughout most of the halo. This effect can be accurately described by a modified model of halo contraction. The angular momentum of a fixed amount of inner dark matter is approximately conserved over time, while in the dissipationless case most of it is transferred outward during mergers. The conservation of the dark matter angular momentum provides supporting evidence for the validity of the halo contraction model in a hierarchical galaxy formation process.

astro-ph.GA↗

arXiv2011

On the Accuracy of Weak Lensing Cluster Mass Reconstructions

We study the bias and scatter in mass measurements of galaxy clusters resulting from fitting a spherically-symmetric Navarro, Frenk & White model to the reduced tangential shear profile measured in weak lensing observations. The reduced shear profiles are generated for ~10^4 cluster-sized halos formed in a LCDM cosmological N-body simulation of a 1 Gpc/h box. In agreement with previous studies, we find that the scatter in the weak lensing masses derived using this fitting method has irreducible contributions from the triaxial shapes of cluster-sized halos and uncorrelated large-scale matter projections along the line-of-sight. Additionally, we find that correlated large-scale structure within several virial radii of clusters contributes a smaller, but nevertheless significant, amount to the scatter. The intrinsic scatter due to these physical sources is ~20% for massive clusters, and can be as high as ~30% for group-sized systems. For current, ground-based observations, however, the total scatter should be dominated by shape noise from the background galaxies used to measure the shear. Importantly, we find that weak lensing mass measurements can have a small, ~5%-10%, but non-negligible amount of bias. Given that weak lensing measurements of cluster masses are a powerful way to calibrate cluster mass-observable relations for precision cosmological constraints, we strongly emphasize that a robust calibration of the bias requires detailed simulations which include more observational effects than we consider here. Such a calibration exercise needs to be carried out for each specific weak lensing mass estimation method, as the details of the method determine in part the expected scatter and bias. We present an iterative method for estimating mass M500c that can eliminate the bias for analyses of ground-based data.

astro-ph.CO↗

arXiv2011

On determining the shape of matter distributions

A basic property of objects, like galaxies and halos that form in cosmological structure formation simulations, is their shape. Here, we critically investigate shape determination methods that are commonly used in the literature. It is found that using an enclosed integration volume and weight factors r^{-2} and r_{ell}^{-2} (elliptical radius) for the contribution of each particle or volume element in the shape tensor leads to biased axis ratios and smoothing of details when calculating the local shape as a function of distance from the center. To determine the local shape of matter distributions as a function of distance for well resolved objects (typically more than O(10^4) particles), we advocate a method that (1) uses an ellipsoidal shell (homoeoid) as an integration volume without any weight factors in the shape tensor and (2) removes subhalos.

astro-ph.IM↗

arXiv2011

Shapes of Gas, Gravitational Potential and Dark Matter in Lambda-CDM Clusters

We present analysis of the three-dimensional shape of intracluster gas in clusters formed in cosmological simulations of the Lambda-CDM cosmology and compare it to the shape of dark matter distribution and the shape of the overall isopotential surfaces. We find that in simulations with radiative cooling, star formation and stellar feedback (CSF), intracluster gas outside the cluster core is more spherical compared to non-radiative (NR) simulations, while in the core the gas in the CSF runs is more triaxial and has a distinctly oblate shape. The latter reflects the ongoing cooling of gas, which settles into a thick oblate ellipsoid as it loses thermal energy. The shape of the gas in the inner regions of clusters can therefore be a useful diagnostic of gas cooling. We find that gas traces the shape of the underlying potential rather well outside the core, as expected in hydrostatic equilibrium. At smaller radii, however, the gas and potential shapes differ significantly. In the CSF runs, the difference reflects the fact that gas is partly rotationally supported. Interestingly, we find that in NR simulations the difference between gas and potential shape at small radii is due to random gas motions, which make the gas distribution more spherical than the equipotential surfaces. Finally, we use mock Chandra X-ray maps to show that the differences in shapes observed in three-dimensional distribution of gas are discernible in the ellipticity of X-ray isophotes. Contrasting the ellipticities measured in simulated clusters against observations can therefore constrain the amount of cooling of the intracluster medium and the presence of random gas motions in cluster cores.

astro-ph.CO↗

arXiv2011

Implementing the DC Mode in Cosmological Simulations with Supercomoving Variables

As emphasized by previous studies, proper treatment of the density fluctuation on the fundamental scale of a cosmological simulation volume - the "DC mode" - is critical for accurate modeling of spatial correlations on scales > 10% of simulation box size. We provide further illustration of the effects of the DC mode on the abundance of halos in small boxes and show that it is straightforward to incorporate this mode in cosmological codes that use the "supercomoving" variables. The equations governing evolution of dark matter and baryons recast with these variables are particularly simple and include the expansion factor, and hence the effect of the DC mode, explicitly only in the Poisson equation.

astro-ph.CO↗

arXiv2011

Fuel Efficient Galaxies: Sustaining Star Formation with Stellar Mass Loss

We examine the importance of secular stellar mass loss for fueling ongoing star formation in disk galaxies during the late stages of their evolution. For a galaxy of a given stellar mass, we calculate the total mass loss rate of its entire stellar population using star formation histories derived from the observed evolution of the M*-star formation rate relation, along with the predictions of standard stellar evolution models for stellar mass loss for a variety of initial stellar mass functions. Our model shows that recycled gas from stellar mass loss can provide most or all of the fuel required to sustain the current level of star formation in late type galaxies. Stellar mass loss can therefore remove the tension between the low gas infall rates that are derived from observations and the relatively rapid star formation occurring in disk galaxies. For galaxies where cold gas infall rates have been estimated, we demonstrate explicitly that stellar mass loss can account for most of the deficit between their star formation and infall rates.

astro-ph.GA↗

arXiv2010

The distribution of annihilation luminosities in dark matter substructure

We calculate the probability distribution function (PDF) of the expected annihilation luminosities of dark matter subhalos as a function of subhalo mass and distance from the Galactic center using a semi-analytical model of halo evolution. We find that the PDF of luminosities is relatively broad, exhibiting a spread of as much as an order of magnitude at fixed subhalo mass and halo-centric distance. The luminosity PDF allows for simple construction of mock samples of gamma-ray luminous subhalos and assessment of the variance in among predicted gamma-ray signals from dark matter annihilation. Other applications include quantifying the variance among the expected luminosities of dwarf spheroidal galaxies, assessing the level at which dark matter annihilation can be a contaminant in the expected gamma-ray signal from other astrophysical sources, as well as estimating the level at which nearby subhalos can contribute to the antimatter flux.

astro-ph.HE↗

arXiv2010

The Large Scale Bias of Dark Matter Halos: Numerical Calibration and Model Tests

We measure the clustering of dark matter halos in a large set of collisionless cosmological simulations of the flat LCDM cosmology. Halos are identified using the spherical overdensity algorithm, which finds the mass around isolated peaks in the density field such that the mean density is Delta times the background. We calibrate fitting functions for the large scale bias that are adaptable to any value of Delta we examine. We find a ~6% scatter about our best fit bias relation. Our fitting functions couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare halos is higher than that predicted in the modified ellipsoidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest halos. Halo bias results based on friends-of-friends halos identified with linking length 0.2 are systematically lower than for halos with the canonical Delta=200 overdensity by ~10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak-background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but ~20% residuals remain, both at high and low masses.

astro-ph.CO↗

arXiv2010

The Metal-Enriched Outer Disk of NGC 2915

We present optical emission-line spectra for outlying HII regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be enriched to a metallicity of 0.4 Z_solar. An analysis of the metal yields shows that the outer disk of NGC 2915 is overabundant for its gas fraction, while the central star-foming core is similarly under-abundant for its gas fraction. Star formation rates derived from very deep ~14 ks GALEX FUV exposures indicate that the low-level of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider 3 plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC 2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.

astro-ph.CO↗

arXiv2010

Environmental Dependence of the Kennicutt-Schmidt Relation in Galaxies

We present a detailed description of a phenomenological H2 formation model and local star formation prescription based on the density of molecular (rather than total) gas. Such approach allows us to avoid the arbitrary density and temperature thresholds typically used in star formation recipes. We present results of the model based on realistic cosmological simulations of high-z galaxy formation for a grid of numerical models with varied dust-to-gas ratios and interstellar far UV (FUV) fluxes. Our results show that both the atomic-to-molecular transition on small, ~10 pc scales and the Kennicutt-Schmidt (KS) relation on ~kpc scales are sensititive to the dust-to-gas ratio and the FUV flux. The atomic-to-molecular transition as a function of gas density or column density has a large scatter but is rather sharp and shifts to higher densities with decreasing dust-to-gas ratio and/or increasing FUV flux. Consequently, star formation is concentrated to higher gas surface density regions, resulting in steeper slope and lower amplitude of the KS relation at a given gas surface density, in less dusty and/or higher FUV flux environments. These trends should have a particularly strong effect on the evolution of low-mass, low surface brightness galaxies which typically have low dust content and anemic star formation, but are also likely to be important for evolution of the Milky Way-sized systems. We parameterize the dependencies observed in our simulations in convenient fitting formulae, which can be used to model the dependence of the KS relation on the dust-to-gas ratio and FUV flux in semi-analytic models and in cosmological simulations that do not include radiative transfer and H2 formation.

astro-ph.CO↗

arXiv2009

On the Kennicutt-Schmidt relation of low-metallicity high-redshift galaxies

We present results of self-consistent, high-resolution cosmological simulations of galaxy formation at z~3. The simulations employ recently developed recipe for star formation based on the local abundance of molecular hydrogen, which is tracked self-consistently during the course of simulation. The phenomenological H2 formation model accounts for the effects of dissociating UV radiation of stars in each galaxy, as well as self-shielding and shielding of H2 by dust, and therefore allows us to explore effects of lower metallicities and higher UV fluxes prevalent in high redshift galaxies on their star formation. We compare stellar masses, metallicities, and star formation rates of the simulated galaxies to available observations of the Lyman Break Galaxies (LBGs) and find a reasonable agreement. We find that the Kennicutt-Schmidt (KS) relation exhibited by our simulated galaxies at z~3 is substantially steeper and has a lower amplitude than the z=0 relation at Sigma_gas < 100 Msun/pc^2. The predicted relation, however, is consistent with existing observational constraints for the z~3 Damped Lyman $α$ (DLA) and LBGs. Our tests show that the main reason for the difference from the local KS relation is lower metallicity of the ISM in high redshift galaxies. We discuss several implications of the metallicity-dependence of the KS relation for galaxy evolution and interpretation of observations.

astro-ph.CO↗

arXiv2009

Effects of Baryon Dissipation on the Dark Matter Virial Scaling Relation

We investigate effects of baryon dissipation on the dark matter virial scaling relation between total mass and velocity dispersion and the velocity bias of galaxies in groups and clusters using self-consistent cosmological simulations. We show that the baryon dissipation increases the velocity dispersion of dark matter within the virial radius by 5% - 10%. The effect is mainly driven by the change in density and gravitational potential in inner regions of cluster, and it is larger in lower mass systems where gas cooling and star formation are more efficient. We also show that the galaxy velocity bias depends on how galaxies are selected. Galaxies selected based on their stellar mass exhibit no velocity bias, while galaxies selected based on their total mass show positive bias of ~10%, consistent with previous results based on collisionless dark matter- only simulations. We further find that observational estimates of galaxy velocity dispersion are unbiased with respect to the velocity dispersion of dark matter, provided galaxies are selected using their stellar masses and and their velocity dispersions are computed with more than twenty most massive galaxies. Velocity dispersions estimated with fewer galaxies, on the other hand, can lead to significant underestimate of dynamical masses. Results presented in this paper should be useful in interpretating high-redshift groups and clusters as well as cosmological constraints derived from upcoming optical cluster surveys.

astro-ph.CO↗

arXiv2009

Computational Eulerian Hydrodynamics and Galilean Invariance

Eulerian hydrodynamical simulations are a powerful and popular tool for modeling fluids in astrophysical systems. In this work, we critically examine recent claims that these methods violate Galilean invariance of the Euler equations. We demonstrate that Eulerian hydrodynamics methods do converge to a Galilean-invariant solution, provided a well-defined convergent solution exists. Specifically, we show that numerical diffusion, resulting from diffusion-like terms in the discretized hydrodynamical equations solved by Eulerian methods, accounts for the effects previously identified as evidence for the Galilean non-invariance of these methods. These velocity-dependent diffusive terms lead to different results for different bulk velocities when the spatial resolution of the simulation is kept fixed, but their effect becomes negligible as the resolution of the simulation is increased to obtain a converged solution. In particular, we find that Kelvin-Helmholtz instabilities develop properly in realistic Eulerian calculations regardless of the bulk velocity provided the problem is simulated with sufficient resolution (a factor of 2-4 increase compared to the case without bulk flows for realistic velocities). Our results reiterate that high-resolution Eulerian methods can perform well and obtain a convergent solution, even in the presence of highly supersonic bulk flows.

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