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Register a new userDark Matter Subhaloes in Numerical Simulations
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Authors
Darren Reed
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We use cosmological LCDM numerical simulations to model the evolution of the substructure population in sixteen dark matter haloes with resolutions of up to seven million particles within the virial radius. The combined substructure circular velocity distribution function (VDF) for hosts of 10^11 to 10^14 Msun at redshifts from zero to two or higher has a self-similar shape, is independent of host halo mass and redshift, and follows the relation: dn/dv=(1/8)(v_cmax/v_cmax,host)^-4. Halo to halo variance in the VDF is a factor of roughly two to four. At high redshifts, we find preliminary evidence for fewer large substructure haloes (subhaloes). Specific angular momenta are significantly lower for subhaloes nearer the host halo centre where tidal stripping is more effective. The radial distribution of subhaloes is marginally consistent with the mass profile for r >~ 0.3r_vir, where the possibility of artificial numerical disruption of subhaloes can be most reliably excluded by our convergence study, although a subhalo distribution that is shallower than the mass profile is favoured. Subhalo masses but not circular velocities decrease toward the host centre. Subhalo velocity dispersions hint at a positive velocity bias at small radii. There is a weak bias toward more circular orbits at lower redshift, especially at small radii. We additionally model a cluster in several power law cosmologies of P ~ k^n, and demonstrate that a steeper spectral index, n, results in significantly less substructure.
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The development of methods and algorithms to solve the $N$-body problem for classical, collisionless, non-relativistic particles has made it possible to follow the growth and evolution of cosmic dark matter structures over most of the Universes history. In the best studied case $-$ the cold dark matter or CDM model $-$ the dark matter is assumed to consist of elementary particles that had negligible thermal velocities at early times. Progress over the past three decades has led to a nearly complete description of the assembly, structure and spatial distribution of dark matter haloes, and their substructure in this model, over almost the entire mass range of astronomical objects. On scales of galaxies and above, predictions from this standard CDM model have been shown to provide a remarkably good match to a wide variety of astronomical data over a large range of epochs, from the temperature structure of the cosmic background radiation to the large-scale distribution of galaxies. The frontier in this field has shifted to the relatively unexplored subgalactic scales, the domain of the central regions of massive haloes, and that of low-mass haloes and subhaloes, where potentially fundamental questions remain. Answering them may require: (i) the effect of known but uncertain baryonic processes (involving gas and stars), and/or (ii) alternative models with new dark matter physics. Here we present a review of the field, focusing on our current understanding of dark matter structure from $N$-body simulations and on the challenges ahead.
We present N-body simulations of a new class of self-interacting dark matter models, which do not violate any astrophysical constraints due to a non-power-law velocity dependence of the transfer cross section which is motivated by a Yukawa-like new gauge boson interaction. Specifically, we focus on the formation of a Milky Way-like dark matter halo taken from the Aquarius project and re-simulate it for a couple of representative cases in the allowed parameter space of this new model. We find that for these cases, the main halo only develops a small core (~1 kpc) followed by a density profile identical to that of the standard cold dark matter scenario outside of that radius. Neither the subhalo mass function nor the radial number density of subhaloes are altered in these models but there is a significant change in the inner density structure of subhaloes resulting in the formation of a large density core. As a consequence, the inner circular velocity profiles of the most massive subhaloes differ significantly from the cold dark matter predictions and we demonstrate that they are compatible with the observational data of the brightest Milky Way dSphs in such a velocity-dependent self-interacting dark matter scenario. Specifically, and contrary to the cold dark matter case, there are no subhaloes that are more concentrated than what is inferred from the kinematics of the Milky Way dSphs. We conclude that these models offer an interesting alternative to the cold dark matter model that can reduce the recently reported tension between the brightest Milky Way satellites and the dense subhaloes found in cold dark matter simulations.
We study the shapes of subhalo distributions from four dark-matter-only simulations of Milky Way type haloes. Comparing the shapes derived from the subhalo distributions at high resolution to those of the underlying dark matter fields we find the former to be more triaxial if theanalysis is restricted to massive subhaloes. For three of the four analysed haloes the increased triaxiality of the distributions of massive subhaloes can be explained by a systematic effect caused by the low number of objects. Subhaloes of the fourth halo show indications for anisotropic accretion via their strong triaxial distribution and orbit alignment with respect to the dark matter field. These results are independent of the employed subhalo finder. Comparing the shape of the observed Milky Way satellite distribution to those of high-resolution subhalo samples from simulations, we find an agreement for samples of bright satellites, but significant deviations if faint satellites are included in the analysis. These deviations might result from observational incompleteness.
The presence of substructures in dark matter haloes is an unavoidable consequence of the cold dark matter paradigm. Indirect signals from these objects have been extensively searched for with cosmic rays and gamma-rays. At first sight, Cherenkov telescopes seem not very well suited for such searches, due to their small fields of view and the random nature of the possible dark matter substructure positions in the sky. However, with long enough exposure and an adequate observation strategy, the very good sensitivity of this experimental technique allows us to constrain particle dark matter models. We confront here the sensitivity map of the HESS experiment built out of their Galactic scan survey to the state-of-the-art cosmological N-body simulation Via Lactea II. We obtain competitive constraints on the annihilation cross section, at the level of 10^-24 -10^-23 cm^3s^-1. The results are extrapolated to the future Cherenkov Telescope Array, in the cases of a Galactic plane survey and of an even wider extragalactic survey. In the latter case, it is shown that the sensitivity of the Cherenkov Telescope Array will be sufficient to reach the most natural particle dark matter models.
It has been proposed that gravothermal collapse due to dark matter self-interactions (i.e. self-interacting dark matter, SIDM) can explain the observed diversity of the Milky Way (MW) satellites central dynamical masses. We investigate the process behind this hypothesis using an $N$-body simulation of a MW-analogue halo with velocity dependent self-interacting dark matter (vdSIDM) in which the low velocity self-scattering cross-section, $sigma_{T}/m_{x}$, reaches 100 cm$^{2}$g$^{-1}$; we dub this model the vd100 model. We compare the results of this simulation to simulations of the same halo that employ different dark models, including cold dark matter (CDM) and other, less extreme SIDM models. The masses of the vd100 haloes are very similar to their CDM counterparts, but the values of their maximum circular velocities, $V_{max}$, are significantly higher. We determine that these high $V_{max}$ subhaloes were objects in the mass range [$5times10^{6}$, $1times10^{8}$] $M_odot$ at $z=1$ that undergo gravothermal core collapse. These collapsed haloes have density profiles that are described by single power laws down to the resolution limit of the simulation, and the inner slope of this density profile is approximately $-3$. Resolving the ever decreasing collapsed region is challenging, and tailored simulations will be required to model the runaway instability accurately at scales $<1$ kpc.
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