Galactic Dynamics and Local Dark Matter (original) (raw)
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Mon Notic Roy Astron Soc, 2011
We make detailed theoretical predictions for the assembly properties of the Local Group (LG) in the standard Λ cold dark matter cosmological model. We use three cosmological N-body dark matter simulations from the Constrained Local Universe Simulations project, which are designed to reproduce the main dynamical features of the matter distribution down to the scale of a few Mpc around the LG. Additionally, we use the results of an unconstrained simulation with a 60 times larger volume to calibrate the influence of cosmic variance. We characterize the mass aggregation history (MAH) for each halo by three characteristic times: the formation, assembly and last major merger times. A major merger is defined by a minimal mass ratio of 10: 1. We find that the three LGs share a similar MAH with formation and last major merger epochs placed on average ≈10-12 Gyr ago. Between 12 and 17 per cent of the haloes in the mass range 5 × 1011 < Mh < 5 × 1012 h-1 M⊙ have a similar MAH. In a set of pairs of haloes within the same mass range, a fraction of 1-3 per cent share similar formation properties as both haloes in the simulated LG. An unsolved question posed by our results is the dynamical origin of the MAH of the LGs. The isolation criteria commonly used to define LG-like haloes in unconstrained simulations do not narrow down the halo population into a set with quiet MAHs, nor does a further constraint to reside in a low-density environment. The quiet MAH of the LGs provides a favourable environment for the formation of disc galaxies like the Milky Way and M31. The timing for the beginning of the last major merger in the Milky Way dark matter halo matches with the gas-rich merger origin for the thick component in the galactic disc. Our results support the view that the specific large- and mid-scale environments around the LG play a critical role in shaping its MAH and hence its baryonic structure at present.
Dark Matters on the Scale of Galaxies
The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some of the solutions proposed within the cold dark-matter model, and solutions when including warm dark matter, self-interacting dark matter, axion-like particles, or fuzzy dark matter. We also illustrate some modifications of the theory of gravity: Modified Newtonian Dynamics (MOND), MOdified Gravity (MOG), and f (R) gravity.
A survey on galactic dark matter
This paper contributes an extensive analysis of Dark Matter (DM) along with associated materials regarding the interpretation of its evidence from 'cosmic microwave background radiation’, ‘gravitational lensing’, ‘galactic mergers and collisions’, ‘evolution and formation of galaxies’, ‘galactic clusters’, ‘Lambda-CDM models of the observable universe. Massive Astrophysical Compact Halo Objects (MACHOs), Weakly Interacting Massive Particles (WIMP’s), Strongly Interacting Massive Particles (SIMPs), along with hot, warm, and cold DM, shedding an extensive overview of the γ_elastic of the cold DM after the chemical equilibrium is reached and kinetic decoupling happens earlier for γ_(in-elastic) accounting for 85% of the dominated matter in the universe. Scaler-Vector-Tensor (SVT) Gravity, MOdified Newtonian Dynamics (MONDs), and Gravity of Entropy (GoE) have emerged as a supplement of the modified General Relativity (GR) as a source of non-baryonic DM scenarios in the universe. Obs...
The core density of dark matter halos: a critical challenge to the Lambda-CDM paradigm?
1999
We compare the central mass concentration of Cold Dark Matter halos found in cosmological N-body simulations with constraints derived from the Milky Way disk dynamics and from the Tully-Fisher relation. For currently favored values of the cosmological parameters ($\Omega_0 \sim 0.3$; Lambda0=1−Omega0sim0.7\Lambda_0=1-\Omega_0 \sim 0.7Lambda0=1−Omega0sim0.7; hsim0.7h \sim 0.7hsim0.7; COBE- and cluster abundance-normalized sigma8\sigma_8sigma8; Big-Bang nucleosynthesis Omegab\Omega_bOmegab), we find that halos with circular velocities comparable to the rotation speed of the Galaxy have typically {\it three times} more dark matter inside the solar circle than inferred from observations of Galactic dynamics. Such high central concentrations of dark matter on the scale of galaxy disks also imply that stellar mass-to-light ratios much lower than expected from population synthesis models must be assumed in order to reproduce the zero-point of the Tully-Fisher relation. Indeed, even under the extreme assumption that {\it all} baryons in a dark halo are turned into stars, disks with conventional III-band stellar mass-to-light ratios ($M/L_I \sim 2 \pm 1 (M/L_I)_{\odot}$) are about two magnitudes fainter than observed at a given rotation speed. We examine several modifications to the Lambda\LambdaLambdaCDM model that may account for these discrepancies and conclude that agreement can only be accomplished at the expense of renouncing other major successes of the model. Reproducing the observed properties of disk galaxies thus appears to demand substantial revision to the currently most successful model of structure formation.