arXiv subjects

Volker Springel

Publications and source records attributed to Volker Springel.

At least 253 records · Page 14Linked to original sources

arXiv2017

Magnetic field formation in the Milky Way-like disk galaxies of the Auriga project

The magnetic fields observed in the Milky~Way and nearby galaxies appear to be in equipartition with the turbulent, thermal, and cosmic ray energy densities, and hence are expected to be dynamically important. However, the origin of these strong magnetic fields is still unclear, and most previous attempts to simulate galaxy formation from cosmological initial conditions have ignored them altogether. Here, we analyse the magnetic fields predicted by the simulations of the Auriga Project, a set of 30 high-resolution cosmological zoom simulations of Milky~Way-like galaxies, carried out with a moving-mesh magneto-hydrodynamics code and a detailed galaxy formation physics model. We find that the magnetic fields grow exponentially at early times owing to a small-scale dynamo with an e-folding time of roughly $100\,\rm{Myr}$ in the center of halos until saturation occurs around $z=2-3$, when the magnetic energy density reaches about $10\%$ of the turbulent energy density with a typical strength of $10-50\,\rm{μG}$. In the galactic centers the ratio between magnetic and turbulent energy remains nearly constant until $z=0$. At larger radii, differential rotation in the disks leads to linear amplification that typically saturates around $z=0.5$ to $z=0$. The final radial and vertical variations of the magnetic field strength can be well described by two joint exponential profiles, and are in good agreement with observational constraints. Overall, the magnetic fields have only little effect on the global evolution of the galaxies as it takes too long to reach equipartition. We also demonstrate that our results are well converged with numerical resolution.

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arXiv2017

Lessons from the Auriga discs: The hunt for the Milky Way's ex-situ disc is not yet over

We characterize the contribution from accreted material to the galactic discs of the Auriga Project, a set of high resolution magnetohydrodynamic cosmological simulations of late-type galaxies performed with the moving-mesh code AREPO. Our goal is to explore whether a significant accreted (or ex-situ) stellar component in the Milky Way disc could be hidden within the near-circular orbit population, which is strongly dominated by stars born in-situ. One third of our models shows a significant ex-situ disc but this fraction would be larger if constraints on orbital circularity were relaxed. Most of the ex-situ material ($\gtrsim 50\%$) comes from single massive satellites ($> 6 \times 10^{10}~M_{\odot}$). These satellites are accreted with a wide range of infall times and inclination angles (up to $85^{\circ}$). Ex-situ discs are thicker, older and more metal-poor than their in-situ counterparts. They show a flat median age profile, which differs from the negative gradient observed in the in-situ component. As a result, the likelihood of identifying an ex-situ disc in samples of old stars on near-circular orbits increases towards the outskirts of the disc. We show three examples that, in addition to ex-situ discs, have a strongly rotating dark matter component. Interestingly, two of these ex-situ stellar discs show an orbital circularity distribution that is consistent with that of the in-situ disc. Thus, they would not be detected in typical kinematic studies.

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arXiv2017

The inner structure of early-type galaxies in the Illustris simulation

Early-type galaxies provide unique tests for the predictions of the cold dark matter cosmology and the baryonic physics assumptions entering models for galaxy formation. In this work, we use the Illustris simulation to study correlations of three main properties of early-type galaxies, namely, the stellar orbital anisotropies, the central dark matter fractions and the central radial density slopes, as well as their redshift evolution since $z=1.0$. We find that lower-mass galaxies or galaxies at higher redshift tend to be bluer in rest-frame colour, have higher central gas fractions, and feature more tangentially anisotropic orbits and steeper central density slopes than their higher-mass or lower-redshift counterparts, respectively. The projected central dark matter fraction within the effective radius shows a very mild mass dependence but positively correlates with galaxy effective radii due to the aperture effect. The central density slopes obtained by combining strong lensing measurements with single aperture kinematics are found to differ from the true density slopes. We identify systematic biases in this measurement to be due to two common modelling assumptions, isotropic stellar orbital distributions and power-law density profiles. We also compare the properties of early-type galaxies in Illustris to those from existing galaxy and strong lensing surveys, we find in general broad agreement but also some tension, which poses a potential challenge to the stellar formation and feedback models adopted by the simulation.

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arXiv2017

Lens galaxies in the Illustris simulation: power-law models and the bias of the Hubble constant from time-delays

A power-law density model, i.e., $ρ(r) \propto r^{-γ'}$ has been commonly employed in strong gravitational lensing studies, including the so-called time-delay technique used to infer the Hubble constant $H_0$. However, since the radial scale at which strong lensing features are formed corresponds to the transition from the dominance of baryonic matter to dark matter, there is no known reason why galaxies should follow a power law in density. The assumption of a power law artificially breaks the mass-sheet degeneracy, a well-known invariance transformation in gravitational lensing which affects the product of Hubble constant and time delay and can therefore cause a bias in the determination of $H_0$ from the time-delay technique. In this paper, we use the Illustris hydrodynamical simulations to estimate the amplitude of this bias, and to understand how it is related to observational properties of galaxies. Investigating a large sample of Illustris galaxies that have velocity dispersion $σ_{SIE}$>160 km/s at redshifts below $z=1$, we find that the bias on $H_0$ introduced by the power-law assumption can reach 20%-50%, with a scatter of $10\%-30\%$ (rms). However, we find that by selecting galaxies with an inferred power-law model slope close to isothermal, it is possible to reduce the bias on $H_0$ to <5%, and the scatter to <10%. This could potentially be used to form less biased statistical samples for $H_0$ measurements in the upcoming large survey era.

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arXiv2017

Simulating the interaction of jets with the intra-cluster medium

Jets from supermassive black holes in the centres of galaxy clusters are a potential candidate for moderating gas cooling and subsequent star formation through depositing energy in the intra-cluster gas. In this work, we simulate the jet-intra-cluster medium interaction using the moving-mesh magnetohydrodynamics code Arepo. Our model injects supersonic, low density, collimated and magnetised outflows in cluster centres, which are then stopped by the surrounding gas, thermalise and inflate low-density cavities filled with cosmic-rays. We perform high-resolution, non-radiative simulations of the lobe creation, expansion and disruption, and find that its dynamical evolution is in qualitative agreement with simulations of idealised low-density cavities that are dominated by a large-scale Rayleigh-Taylor instability. The buoyant rising of the lobe does not create energetically significant small-scale chaotic motion in a volume-filling fashion, but rather a systematic upward motion in the wake of the lobe and a corresponding back-flow perpendicular to it. We find that, overall, 50 per cent of the injected energy ends up in material which is not part of the lobe, and about 25 per cent remains in the inner 100 kpc. We conclude that jet-inflated, buoyantly rising cavities drive systematic gas motions which play an important role in heating the central regions, while mixing of lobe material is sub-dominant. Encouragingly, the main mechanisms responsible for this energy deposition can be modelled already at resolutions within reach in future, high-resolution cosmological simulations of galaxy clusters.

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arXiv2017

Intrinsic Alignments of Galaxies in the Illustris Simulation

We study intrinsic alignments (IA) of galaxy image shapes within the Illustris cosmic structure formation simulations. We investigate how IA correlations depend on observable galaxy properties such as stellar mass, apparent magnitude, redshift, and photometric type, and on the employed shape measurement method. The correlations considered include the matter density-intrinsic ellipticity (mI), galaxy density-intrinsic ellipticity (dI), gravitational shear-intrinsic ellipticity (GI), and intrinsic ellipticity-intrinsic ellipticity (II) correlations. We find stronger correlations for more massive and more luminous galaxies, as well as for earlier photometric types, in agreement with observations. Moreover, the correlations significantly depend on the choice of shape estimator, even if calibrated to serve as unbiased shear estimators. In particular, shape estimators that down-weight the outer parts of galaxy images produce much weaker IA signals on intermediate and large scales than methods employing flat radial weights. The expected contribution of intrinsic alignments to the observed ellipticity correlation in tomographic cosmic shear surveys may be below one percent or several percent of the full signal depending on the details of the shape measurement method. A comparison of our results to a tidal alignment model indicates that such a model is able to reproduce the IA correlations well on intermediate and large scales, provided the effect of varying galaxy density is correctly taken into account. We also find that the GI contributions to the observed ellipticity correlations could be inferred directly from measurements of galaxy density-intrinsic ellipticity correlations, except on small scales, where systematic differences between mI and dI correlations are large.

astro-ph.CO↗

arXiv2017

Moving mesh simulations of star forming cores in magneto-gravo-turbulence

Star formation in our Galaxy occurs in molecular clouds that are self-gravitating, highly turbulent, and magnetized. We study the conditions under which cloud cores inherit large-scale magnetic field morphologies and how the field is governed by cloud turbulence. We present four moving-mesh simulations of supersonic, turbulent, isothermal, self-gravitating gas with a range of magnetic mean-field strengths characterized by the Alfvénic Mach number $\mathcal{M}_{{\rm A}, 0}$, resolving pre-stellar core formation from parsec to a few AU scales. In our simulations with the turbulent kinetic energy density dominating over magnetic pressure ($\mathcal{M}_{{\rm A}, 0}>1$), we find that the collapse is approximately isotropic with $B\proptoρ^{2/3}$, core properties are similar regardless of initial mean-field strength, and the field direction on $100$ AU scales is uncorrelated with the mean field. However, in the case of a dominant large-scale magnetic field ($\mathcal{M}_{{\rm A}, 0}=0.35$), the collapse is anisotropic with $B\proptoρ^{1/2}$. This transition at $\mathcal{M}_{{\rm A}, 0}\sim1$ is not expected to be sharp, but clearly signifies two different paths for magnetic field evolution in star formation. Based on observations of different star forming regions, we conclude that star formation in the interstellar medium may occur in both regimes. Magnetic field correlation with the mean-field extends to smaller scales as $\mathcal{M}_{{\rm A}, 0}$ decreases, making future ALMA observations useful for constraining $\mathcal{M}_{{\rm A}, 0}$ of the interstellar medium.

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arXiv2017

Simulating galaxy formation with black hole driven thermal and kinetic feedback

The inefficiency of star formation in massive elliptical galaxies is widely believed to be caused by the interactions of an active galactic nucleus (AGN) with the surrounding gas. Achieving a sufficiently rapid reddening of moderately massive galaxies without expelling too many baryons has however proven difficult for hydrodynamical simulations of galaxy formation, prompting us to explore a new model for the accretion and feedback effects of supermassive black holes. For high accretion rates relative to the Eddington limit, we assume that a fraction of the accreted rest mass energy heats the surrounding gas thermally, similar to the `quasar mode' in previous work. For low accretion rates, we invoke a new, pure kinetic feedback model which imparts momentum into the surrounding gas in a stochastic manner. These two modes of feedback are motivated both by theoretical conjectures for the existence of different types of accretion flows as well as recent observational evidence for the importance of kinetic AGN winds in quenching galaxies. We find that a large fraction of the injected kinetic energy in this mode thermalises via shocks in the surrounding gas, thereby providing a distributed heating channel. In cosmological simulations, the resulting model produces red, non star-forming massive elliptical galaxies, and achieves realistic gas fractions, black hole growth histories and thermodynamic profiles in large haloes.

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arXiv2017

The role of mergers and halo spin in shaping galaxy morphology

Mergers and the spin of the dark matter halo are factors traditionally believed to determine the morphology of galaxies within a $Λ$CDM cosmology. We study this hypothesis by considering approximately 18,000 central galaxies at $z=0$ with stellar masses $M_{\ast} = 10^{9}-10^{12} \, {\rm M}_{\odot}$ selected from the Illustris cosmological hydrodynamic simulation. The fraction of accreted stars -- which measures the importance of massive, recent and dry mergers -- increases steeply with galaxy stellar mass, from less than 5 per cent in dwarfs to 80 per cent in the most massive objects, and the impact of mergers on galaxy morphology increases accordingly. For galaxies with $M_{\ast} \gtrsim 10^{11} \, {\rm M}_{\odot}$, mergers have the expected effect: if gas-poor they promote the formation of spheroidal galaxies, whereas gas-rich mergers favour the formation and survivability of massive discs. This trend, however, breaks at lower masses. For objects with $M_{\ast} \lesssim 10^{11} \, {\rm M}_{\odot}$, mergers do not seem to play any significant role in determining the morphology, with accreted stellar fractions and mean merger gas fractions that are indistinguishable between spheroidal and disc-dominated galaxies. On the other hand, halo spin correlates with morphology primarily in the least massive objects in the sample ($M_{\ast} \lesssim 10^{10} \, {\rm M}_{\odot}$), but only weakly for galaxies above that mass. Our results support a scenario where (1) mergers play a dominant role in shaping the morphology of massive galaxies, (2) halo spin is important for the morphology of dwarfs, and (3) the morphology of medium-sized galaxies -- including the Milky Way -- shows little dependence on galaxy assembly history or halo spin, at least when these two factors are considered individually.

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arXiv2017

Baryonic impact on the dark matter orbital properties of Milky Way-sized haloes

We study the orbital properties of dark matter haloes by combining a spectral method and cosmological simulations of Milky Way-sized galaxies. We compare the dynamics and orbits of individual dark matter particles from both hydrodynamic and $N$-body simulations, and find that the fraction of box, tube and resonant orbits of the dark matter halo decreases significantly due to the effects of baryons. In particular, the central region of the dark matter halo in the hydrodynamic simulation is dominated by regular, short-axis tube orbits, in contrast to the chaotic, box and thin orbits dominant in the $N$-body run. This leads to a more spherical dark matter halo in the hydrodynamic run compared to a prolate one as commonly seen in the $N$-body simulations. Furthermore, by using a kernel based density estimator, we compare the coarse-grained phase-space densities of dark matter haloes in both simulations and find that it is lower by $\sim0.5$ dex in the hydrodynamic run due to changes in the angular momentum distribution, which indicates that the baryonic process that affects the dark matter is irreversible. Our results imply that baryons play an important role in determining the shape, kinematics and phase-space density of dark matter haloes in galaxies.

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arXiv2017

Properties of HI discs in the Auriga cosmological simulations

We analyse the properties of the HI gas distribution in the Auriga project, a set of magnetohydrodynamic cosmological simulations performed with the moving-mesh code AREPO and a physics model for galaxy formation that succeeds in forming realistic late-type galaxies in the 30 Milky Way-sized haloes simulated in this project. We use a simple approach to estimate the neutral hydrogen fraction in our simulation set, which treats low-density and star-forming gas separately, and we explore two different prescriptions to subtract the contribution of molecular hydrogen from the total HI content. The HI gas in the vast majority of the systems forms extended discs although more disturbed morphologies are present. Notwithstanding the general good agreement with observed HI properties -- such as radial profiles and the mass-diameter relation -- the Auriga galaxies are systematically larger and more gas-rich than typical nearby galaxies. Interestingly, the amount of HI gas outside the disc plane correlates with the star formation rate, consistent with a picture where most of this extra-planar HI gas originates from a fountain-like flow. Our findings are robust with respect to the different assumptions adopted for computing the molecular hydrogen fraction and do not vary significantly over a wide range of numerical resolution. The HI modelling introduced in this paper can be used in future work to build artificial interferometric HI data cubes, allowing an even closer comparison of the gas dynamics in simulated galaxies with observations.

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arXiv2017

Baryonic impact on the dark matter distribution in Milky Way-size galaxies and their satellites

We study the impact of baryons on the distribution of dark matter in a Milky Way-size halo by comparing a high-resolution, moving-mesh cosmological simulation with its dark matter-only counterpart. We identify three main processes related to baryons -- adiabatic contraction, tidal disruption and reionization -- which jointly shape the dark matter distribution in both the main halo and its subhalos. The relative effect of each baryonic process depends strongly on the subhalo mass. For massive subhalos with maximum circular velocity $v_{\rm max} > 35 km/s$, adiabatic contraction increases the dark matter concentration, making these halos less susceptible to tidal disruption. For low-mass subhalos with $v_{\rm max} < 20 km/s$, reionization effectively reduces their mass on average by $\approx$ 30% and $v_{\rm max}$ by $\approx$ 20%. For intermediate subhalos with $20 km/s < v_{\rm max} < 35 km/s$, which share a similar mass range as the classical dwarf spheroidals, strong tidal truncation induced by the main galaxy reduces their $v_{\rm max}$. Moreover, the stellar disk of the main galaxy effectively depletes subhalos near the central region. As a combined result of reionization and increased tidal disruption, the total number of low-mass subhalos in the hydrodynamic simulation is nearly halved compared to that of the $\textit{N-}$body simulation. We do not find dark matter cores in dwarf galaxies, unlike previous studies that employed bursty feedback-driven outflows. The substantial impact of baryons on the abundance and internal structure of subhalos suggests that galaxy formation and evolution models based on $\textit{N}$-body simulations should include these physical processes as major components.

astro-ph.CO↗

arXiv2017

The Auriga Project: the properties and formation mechanisms of disc galaxies across cosmic time

We introduce a suite of thirty cosmological magneto-hydrodynamical zoom simulations of the formation of galaxies in isolated Milky Way mass dark haloes. These were carried out with the moving mesh code \textlcsc{AREPO}, together with a comprehensive model for galaxy formation physics, including AGN feedback and magnetic fields, which produces realistic galaxy populations in large cosmological simulations. We demonstrate that our simulations reproduce a wide range of present-day observables, in particular, two component disc dominated galaxies with appropriate stellar masses, sizes, rotation curves, star formation rates and metallicities. We investigate the driving mechanisms that set present-day disc sizes/scale lengths, and find that they are related to the angular momentum of halo material. We show that the largest discs are produced by quiescent mergers that inspiral into the galaxy and deposit high angular momentum material into the pre-existing disc, simultaneously increasing the spin of dark matter and gas in the halo. More violent mergers and strong AGN feedback play roles in limiting disc size by destroying pre-existing discs and by suppressing gas accretion onto the outer disc, respectively. The most important factor that leads to compact discs, however, is simply a low angular momentum for the halo. In these cases, AGN feedback plays an important role in limiting central star formation and the formation of a massive bulge.

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arXiv2017

The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars

We use the Illustris simulation to study the relative contributions of in situ star formation and stellar accretion to the build-up of galaxies over an unprecedentedly wide range of masses ($M_{\ast} = 10^9-10^{12} \, {\rm M_{\odot}}$), galaxy types, environments, and assembly histories. We find that the `two-phase' picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in our simulation -- namely, the stellar mass growth of galaxies below a few times $10^{11} \, {\rm M_{\odot}}$ is dominated by in situ star formation at all redshifts. The fraction of the total stellar mass of galaxies at $z=0$ contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10 per cent for Milky Way-sized galaxies to over 80 per cent for $M_{\ast} \approx 10^{12} \, {\rm M_{\odot}}$ objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centres of younger haloes exhibit larger fractions of ex situ stars than disc-like galaxies and those formed in older haloes. On average, $\sim$50 per cent of the ex situ stellar mass comes from major mergers (stellar mass ratio $μ> 1/4$), $\sim$20 per cent from minor mergers ($1/10 < μ< 1/4$), $\sim$20 per cent from very minor mergers ($μ< 1/10$), and $\sim$10 per cent from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with in situ stars dominating the innermost regions of galaxies, and ex situ stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio.

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arXiv2016

Rotation curve fitting and its fatal attraction to cores in realistically simulated galaxy observations

We study the role of systematic effects in observational studies of the cusp-core problem under the minimum disc approximation using a suite of high-resolution (25-pc softening length) hydrodynamical simulations of dwarf galaxies. We mimic realistic kinematic observations and fit the mock rotation curves with two analytic models commonly used to differentiate cores from cusps in the dark matter distribution. We find that the cored pseudo-isothermal sphere (ISO) model is strongly favoured by the reduced $χ^2_ν$ of the fits in spite of the fact that our simulations contain cuspy Navarro-Frenk-White profiles (NFW). We show that even idealized measurements of the gas circular motions can lead to the incorrect answer if velocity underestimates induced by pressure support, with a typical size of order $\sim$5 km s$^{-1}$ in the central kiloparsec, are neglected. Increasing the spatial resolution of the mock observations leads to more misleading results because the inner region, where the effect of pressure support is most significant, is better sampled. Fits to observations with a spatial resolution of 100 pc (2 arcsec at 10 Mpc) favour the ISO model in 78-90 per cent of the cases, while at 800-pc resolution, 40-78 per cent of the galaxies indicate the fictitious presence of a dark matter core. The coefficients of our best-fit models agree well with those reported in observational studies; therefore, we conclude that NFW haloes can not be ruled out reliably from this type of analysis.

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arXiv2016

Increasing blackhole feedback induced quenching with anisotropic thermal conduction

Feedback from central supermassive blackholes is often invoked to explain the low star formation rates in massive galaxies at the centers of galaxy clusters. However, the detailed physics of the coupling of the injected feedback energy with the intracluster medium is still unclear. Using high-resolution magnetohydrodynamic cosmological simulations of galaxy cluster formation, we investigate the role of anisotropic thermal conduction in shaping the thermodynamic structure of clusters, and, in particular, in modifying the impact of black hole feedback. Stratified anisotropically conducting plasmas are formally always unstable, and thus more prone to mixing, an expectation borne out by our results. The increased mixing efficiently isotropizes the injected feedback energy which in turn significantly improves the coupling between the feedback energy and the intracluster medium. This facilitates an earlier disruption of the cool core, reduces the star formation rate by more than an order of magnitude, and results in earlier quenching despite an overall lower amount of feedback energy injected into the cluster core. With conduction, the metallicity gradients and dispersions are lowered, aligning them better with observational constraints. These results highlight the important role of thermal conduction in establishing and maintaining quiescence of massive galaxies.

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arXiv2016

Constructing stable 3D hydrodynamical models of giant stars

Hydrodynamical simulations of stellar interactions require stable models of stars as initial conditions. Such initial models, however, are difficult to construct for giant stars because of the wide range in spatial scales of the hydrostatic equilibrium and in dynamical timescales between the core and the envelope of the giant. They are needed for, e.g., modeling the common envelope phase where a giant envelope encompasses both the giant core and a companion star. Here, we present a new method of approximating and reconstructing giant profiles from a stellar evolution code to produce stable models for multi-dimensional hydrodynamical simulations. We determine typical stellar stratification profiles with the 1D stellar evolution code MESA. After an appropriate mapping, hydrodynamical simulations are conducted using the moving-mesh code AREPO. The giant profiles are approximated by replacing the core of the giant with a point mass and by constructing a suitable continuation of the profile to the center. Different reconstruction methods are tested that can specifically control the convective behaviour of the model. After mapping to a grid, a relaxation procedure that includes damping of spurious velocities yields stable models in three-dimensional hydrodynamical simulations. Initially convectively stable configurations lead to stable hydrodynamical models while for stratifications that are convectively unstable in the stellar evolution code, simulations recover the convective behaviour of the initial model and show large convective plumes with Mach numbers up to 0.8. Examples are shown for a $2M_\odot$ red giant and a $0.67M_\odot$ asymptotic giant branch star. A detailed analysis shows that the improved method reliably provides stable models of giant envelopes that can be used as initial conditions for subsequent hydrodynamical simulations of stellar interactions involving giant stars.

astro-ph.SR↗

arXiv2016

Angular momentum properties of haloes and their baryon content in the Illustris simulation

The angular momentum properties of virialised dark matter haloes have been measured with good statistics in collisionless N-body simulations, but an equally accurate analysis of the baryonic spin is still missing. We employ the Illustris simulation suite, one of the first simulations of galaxy formation with full hydrodynamics that produces a realistic galaxy population in a sizeable volume, to quantify the baryonic spin properties for more than $\sim$ 320,000 haloes. We first compare the systematic differences between different spin parameter and halo definitions, and the impact of sample selection criteria on the derived properties. We confirm that dark matter only haloes exhibit a close to self-similar spin distribution in mass and redshift of lognormal form. However, the physics of galaxy formation radically changes the baryonic spin distribution. While the dark matter component remains largely unaffected, strong trends with mass and redshift appear for the spin of diffuse gas and the formed stellar component. With time the baryons staying bound to the halo develop a misalignment of their spin vector with respect to dark matter, and increase their specific angular momentum by a factor of $\sim$ 1.3 in the non-radiative case and $\sim$ 1.8 in the full physics setup at z = 0. We show that this enhancement in baryonic spin can be explained by the combined effect of specific angular momentum transfer from dark matter onto gas during mergers and from feedback expelling low specific angular momentum gas from the halo. Our results challenge certain models for spin evolution and underline the significant changes induced by baryonic physics in the structure of haloes.

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