The impact of box size on the properties of dark matter haloes in cosmological simulations

ABSTRACTThe impact of 2-body scattering on the innermost density profiles of dark matter haloes is well established. We use a suite of cosmological simulations and idealized numerical experiments to show that 2-body scattering is exacerbated in situations where there are two species of unequal mass. This is a consequence of mass segregation and reflects a flow of kinetic energy from the more to less massive particles. This has important implications for the interpretation of galaxy sizes in cosmological hydrodynamic simulations, which nearly always model stars with less massive particles than are used for the dark matter. We compare idealized models as well as simulations from the eagle project that differ only in the mass resolution of the dark matter component, but keep subgrid physics, baryonic mass resolution, and gravitational force softening fixed. If the dark matter particle mass exceeds the mass of stellar particles, then galaxy sizes – quantified by their projected half-mass radii, R50 – increase systematically with time until R50 exceeds a small fraction of the redshift-dependent mean interparticle separation, l (${\rm R_{50}} \gtrsim 0.05\times l$). Our conclusions should also apply to simulations that adopt different hydrodynamic solvers, subgrid physics, or adaptive softening, but in that case may need quantitative revision. Any simulation employing a stellar-to-dark matter particle mass ratio greater than unity will escalate spurious energy transfer from dark matter to baryons on small scales.

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Numerical convergence of simulations of galaxy formation: the abundance and internal structure of cold dark matter haloes

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1821 ◽

2019 ◽

Vol 488 (3) ◽

pp. 3663-3684 ◽

Cited By ~ 15

Author(s):

Aaron D Ludlow ◽

Joop Schaye ◽

Richard Bower

Keyword(s):

Dark Matter ◽

Cold Dark Matter ◽

Particle Number ◽

Velocity Profiles ◽

Scaling Relations ◽

Cosmological Simulations ◽

Particle Spacing ◽

Circular Velocity ◽

Wide Range ◽

The Impact

ABSTRACT We study the impact of numerical parameters on the properties of cold dark matter haloes formed in collisionless cosmological simulations. We quantify convergence in the median spherically averaged circular velocity profiles for haloes of widely varying particle number, as well as in the statistics of their structural scaling relations and mass functions. In agreement with prior work focused on single haloes, our results suggest that cosmological simulations yield robust halo properties for a wide range of gravitational softening parameters, ϵ, provided: (1) ϵ is not larger than a ‘convergence radius’, rconv, which is dictated by two-body relaxation and determined by particle number, and (2) a sufficient number of time-steps are taken to accurately resolve particle orbits with short dynamical times. Provided these conditions are met, median circular velocity profiles converge to within ≈10 per cent for radii beyond which the local two-body relaxation time-scale exceeds the Hubble time by a factor $\kappa \equiv t_{\rm relax}/t_{\rm H}\rm{\,\, \buildrel\gt \over \sim \,\,}0.177$, with better convergence attained for higher κ. We provide analytic estimates of rconv that build on previous attempts in two ways: first, by highlighting its explicit (but weak) softening-dependence and, second, by providing a simpler criterion in which rconv is determined entirely by the mean inter-particle spacing, l, for example better than 10 per cent convergence in circular velocity for $r\rm{\,\, \buildrel\gt \over \sim \,\,}0.05\, l$. We show how these analytic criteria can be used to assess convergence in structural scaling relations for dark matter haloes as a function of their mass or maximum circular speed.

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A black box for dark sector physics: predicting dark matter annihilation feedback with conditional GANs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2759 ◽

2019 ◽

Vol 490 (3) ◽

pp. 3134-3143 ◽

Cited By ~ 2

Author(s):

Florian List ◽

Ishaan Bhat ◽

Geraint F Lewis

Keyword(s):

Dark Matter ◽

Generative Adversarial Networks ◽

Dark Sector ◽

Dark Matter Annihilation ◽

Average Deviation ◽

Simulation Code ◽

Cosmological Simulations ◽

Gas Density ◽

Adversarial Networks ◽

The Impact

Abstract Traditionally, incorporating additional physics into existing cosmological simulations requires re-running the cosmological simulation code, which can be computationally expensive. We show that conditional Generative Adversarial Networks (cGANs) can be harnessed to predict how changing the underlying physics alters the simulation results. To illustrate this, we train a cGAN to learn the impact of dark matter annihilation feedback (DMAF) on the gas density distribution. The predicted gas density slices are visually difficult to distinguish from their real brethren and the peak counts differ by less than 10 per cent for all test samples (the average deviation is <3 per cent). Finally, we invert the problem and show that cGANs are capable of endowing smooth density distributions with realistic substructure. The cGAN does however have difficulty generating new knots as well as creating/eliminating bubble-like structures. We conclude that trained cGANs can be an effective approach to provide mock samples of cosmological simulations incorporating DMAF physics from existing samples of standard cosmological simulations of the evolution of cosmic structure.

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Baryons Shaping Dark Matter Haloes

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3988 ◽

2020 ◽

Author(s):

P Cataldi ◽

S E Pedrosa ◽

P B Tissera ◽

M C Artale

Keyword(s):

Dark Matter ◽

Mass Range ◽

Orbital Structure ◽

Intrinsic Properties ◽

Cosmological Simulations ◽

Velocity Anisotropy ◽

Relative Decrease ◽

Central Regions ◽

The Impact ◽

Disc Galaxies

Abstract In this work we aim at investigating the effects of baryons on the dark matter (DM) haloes structure, focusing on the correlation between the presence and importance of stellar discs and the halo shapes. We study the properties of a subsample of DM haloes from Fenix and eagle cosmological simulations. We inspect the central regions of haloes in the mass range $[10.9-992.3] \times 10^{10} \ \rm M_{\odot }$ at z = 0, comparing the hydrodynamic runs and their dark matter only (DMo) counterparts. Our results indicate that baryons have a significant impact on the shape of the inner halo, mainly within ∼ 20 percent of the virial radius. We find haloes to be more spherical when hosting baryons. While the impact of baryons depends on the mass of the haloes, we also find a trend with morphology which suggests that the way baryons are assembled is also relevant in agreement with previous works. Our findings also indicate that disc galaxies preferentially form in haloes whose DMo counterparts were originally more spherical and with stronger velocity anisotropy. The presence of baryons alter the orbital structure of the DM particles of the haloes, which show a decrease in their velocity anisotropy, towards more tangentially biased orbits. This relative decrease is weaker in the case of disc-dominated galaxies. Our results point out to a cosmological connection between the final morphology of galaxies and the intrinsic properties of their DM haloes, which gets reinforce by the growth of the discs.

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The impact of baryonic physics on the structure of dark matter haloes: the view from the FIRE cosmological simulations

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stv2165 ◽

2015 ◽

Vol 454 (3) ◽

pp. 2981-3001 ◽

Cited By ~ 173

Author(s):

T. K. Chan ◽

D. Kereš ◽

J. Oñorbe ◽

P. F. Hopkins ◽

A. L. Muratov ◽

...

Keyword(s):

Dark Matter ◽

Cosmological Simulations ◽

The Impact

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How biased are halo properties in cosmological simulations?

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3388 ◽

2020 ◽

Vol 500 (3) ◽

pp. 3309-3328

Author(s):

Philip Mansfield ◽

Camille Avestruz

Keyword(s):

Dark Matter ◽

Empirical Model ◽

Empirical Models ◽

Rotation Curves ◽

Cosmological Simulations ◽

Wide Range ◽

Practical Convergence ◽

Halo Population ◽

The Impact

ABSTRACT Cosmological N-body simulations have been a major tool of theorists for decades, yet many of the numerical issues that these simulations face are still unexplored. This paper measures numerical biases in these large, dark matter-only simulations that affect the properties of their dark matter haloes. We compare many simulation suites in order to provide several tools for simulators and analysts which help mitigate these biases. We summarize our comparisons with practical ‘convergence limits’ that can be applied to a wide range of halo properties, including halo properties which are traditionally overlooked by the testing literature. We also find that the halo properties predicted by different simulations can diverge from one another at unexpectedly high resolutions. We demonstrate that many halo properties depend strongly on force softening scale and that this dependence leads to much of the measured divergence between simulations. We offer an empirical model to estimate the impact of such effects on the rotation curves of a halo population. This model can serve as a template for future empirical models of the biases in other halo properties.

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Black Hole Feeding and Feedback in the Context of Galaxy Formation

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310006939 ◽

2009 ◽

Vol 5 (S267) ◽

pp. 411-420

Author(s):

Rachel S. Somerville

Keyword(s):

Black Holes ◽

Black Hole ◽

Dark Matter ◽

Galaxy Formation ◽

Cold Dark Matter ◽

State Of The Art ◽

Supermassive Black Holes ◽

Cosmological Simulations ◽

Evolution Of Galaxies ◽

The Impact

AbstractI describe ways in which state-of-the-art cosmological simulations are modeling the growth and evolution of supermassive black holes (feeding), and the impact of the energy that they release on galaxies and their surroundings (feedback). I then discuss how this new picture of interconnected co-evolution of galaxies and black holes provides plausible explanations for several of the mysteries that have long vexed theorists studying galaxy formation within the hierarchical cold dark matter paradigm.

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The impact of inelastic self-interacting dark matter on the dark matter structure of a Milky Way halo

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3315 ◽

2020 ◽

Author(s):

Kun Ting Eddie Chua ◽

Karia Dibert ◽

Mark Vogelsberger ◽

Jesús Zavala

Keyword(s):

Dark Matter ◽

High Speed ◽

Cold Dark Matter ◽

Milky Way ◽

Direct Detection ◽

Major Axis ◽

Inelastic Collisions ◽

Axis Ratio ◽

Lower Velocity ◽

The Impact

Abstract We study the effects of inelastic dark matter self-interactions on the internal structure of a simulated Milky Way (MW)-size halo. Self-interacting dark matter (SIDM) is an alternative to collisionless cold dark matter (CDM) which offers a unique solution to the problems encountered with CDM on sub-galactic scales. Although previous SIDM simulations have mainly considered elastic collisions, theoretical considerations motivate the existence of multi-state dark matter where transitions from the excited to the ground state are exothermic. In this work, we consider a self-interacting, two-state dark matter model with inelastic collisions, implemented in the Arepo code. We find that energy injection from inelastic self-interactions reduces the central density of the MW halo in a shorter timescale relative to the elastic scale, resulting in a larger core size. Inelastic collisions also isotropize the orbits, resulting in an overall lower velocity anisotropy for the inelastic MW halo. In the inner halo, the inelastic SIDM case (minor-to-major axis ratio s ≡ c/a ≈ 0.65) is more spherical than the CDM (s ≈ 0.4), but less spherical than the elastic SIDM case (s ≈ 0.75). The speed distribution f(v) of dark matter particles at the location of the Sun in the inelastic SIDM model shows a significant departure from the CDM model, with f(v) falling more steeply at high speeds. In addition, the velocity kicks imparted during inelastic collisions produce unbound high-speed particles with velocities up to 500 km s−1 throughout the halo. This implies that inelastic SIDM can potentially leave distinct signatures in direct detection experiments, relative to elastic SIDM and CDM.

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The anisotropy of the power spectrum in periodic cosmological simulations

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab874 ◽

2021 ◽

Vol 503 (4) ◽

pp. 5638-5645

Author(s):

Gábor Rácz ◽

István Szapudi ◽

István Csabai ◽

László Dobos

Keyword(s):

Dark Matter ◽

Power Spectrum ◽

Large Scale ◽

Cold Dark Matter ◽

Initial Conditions ◽

Power Spectra ◽

Cosmological Simulations ◽

Spherical Collapse ◽

Lower Amplitude ◽

Hydrodynamical Simulations

ABSTRACT The classical gravitational force on a torus is anisotropic and always lower than Newton’s 1/r2 law. We demonstrate the effects of periodicity in dark matter only N-body simulations of spherical collapse and standard Lambda cold dark matter (ΛCDM) initial conditions. Periodic boundary conditions cause an overall negative and anisotropic bias in cosmological simulations of cosmic structure formation. The lower amplitude of power spectra of small periodic simulations is a consequence of the missing large-scale modes and the equally important smaller periodic forces. The effect is most significant when the largest mildly non-linear scales are comparable to the linear size of the simulation box, as often is the case for high-resolution hydrodynamical simulations. Spherical collapse morphs into a shape similar to an octahedron. The anisotropic growth distorts the large-scale ΛCDM dark matter structures. We introduce the direction-dependent power spectrum invariant under the octahedral group of the simulation volume and show that the results break spherical symmetry.

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Gravitational polarization of the quantum vacuum caused by two point-like bodies

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab768 ◽

2021 ◽

Vol 503 (4) ◽

pp. 5091-5099

Author(s):

Dragan Slavkov Hajdukovic ◽

Sergej Walter

Keyword(s):

Dark Matter ◽

Solar System ◽

Charge Density ◽

Numerical Method ◽

Quantum Vacuum ◽

Body System ◽

Complex Fluid ◽

The Impact ◽

The Universe ◽

Gravitational Charge

ABSTRACT In a recent paper, quantum vacuum was considered as a source of gravity, and the simplest, phenomenon, the gravitational polarization of the quantum vacuum by an immersed point-like body, was studied. In this paper, we have derived the effective gravitational charge density of the quantum vacuum, caused by two immersed point-like bodies. Among others, the obtained result proves that quantum vacuum can have regions with a negative effective gravitational charge density. Hence, quantum vacuum, the ‘ocean’ in which all matter of the Universe is immersed, acts as a complex fluid with a very variable gravitational charge density that might include both positive and negative densities; a crucial prediction that can be tested within the Solar system. In the general case of ${N \ge {\rm{3}}}$ point-like bodies, immersed in the quantum vacuum, the analytical solutions are not possible, and the use of numerical methods is inevitable. The key point is that an appropriate numerical method, for the calculation of the effective gravitational charge density of the quantum vacuum induced by N immersed bodies, might be crucial in description of galaxies, without the involvement of dark matter or a modification of gravity. The development of such a valuable numerical method, is not possible, without a previous (and in this study achieved) understanding of the impact of a two-body system.

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