Dynamics of Dark-Matter Cusps

Abstract

Formation and disruption of dark-matter cusps are reviewed. Accumulation of baryons at the center of a halo can displace the dark matter, converting singular density cusps into low-density cores. The displaced mass can be of order ∼ 10M• with M• the mass of the infalling population. If M• is identified with the masses of the black holes currently observed at the centers of bright galaxies, predicted core radii are ∼ a few × 10 2 pc. Other mechanisms, such as early mass outflow, may explain the large dark-matter cores in dwarf and low-surface-brightness galaxies. Predictions of dark matter annihilation radiation from the center of the Milky Way galaxy are shown to be strongly dependent on the galaxy's merger history. CDM CuspsThe influential paper of Navarro, Frenk & White [1] proposed a universal, broken power-law profile for dark matter halos in cold dark matter (CDM) cosmologies. The dependence of ρ on distance from the halo center was found to be well described by a two-parameter family of distributions,The central density of the NFW profile diverges as ρ ∼ r −1 , similar to what was found in earlier N -body studies (e.g. [2]). However the small-radius dependence was little more than an ansatz since the relevant scales were barely resolved in the simulations. A debate ensued as to whether the profiles are indeed universal, and if so, what power of radius describes the dark matter density in the limit r → 0. A number of subsequent studies [3,4,5,6,7] found steeper central profiles, e.g. ρ ∼ r −1.5 , the "Moore" profile. At about the same time, the luminous (stellar) densities in galaxies were found to be centrally divergent [8,9]; in contrast with CDM halos, the power law index of stellar nuclei varies widely but correlates well with galaxy properties, in the sense that fainter galaxies have steeper cusps [10]. While the power-law nature of stellar cusps suggests a gravitational origin, gas dynamics probably also played a role in their formation, including processes like radiative dissipation, star formation, and energy infusion from supernova explosions.The power-law nature of density stratification in virialized objects arises naturally in the standard cosmological paradigm where the growth of objects proceeds from smallest to largest scales [11] starting from a featureless spectrum of primordial fluctuations P (k) ∝ k n . Building blocks of structure can

Cold Dark Matter and Strong Gravitational Lensing: Concord or Conflict?

“…A high-density CDM cosmology can therefore be ruled out at () M \ 1) better than 95% conÐdence, while a low-density, Ñat cosmology is at best marginally consis-() M \ 0.2, ) " \ 0.8) tent with the lens data. By implying that CDM model galaxies have too much mass on kiloparsec scales, lensing independently supports the evidence against CDM from spiral galaxy dynamics (e.g., Moore 1994 ;de Blok et al 2001 ;Salucci 2001 ;Weiner et al 2001). The evidence from lensing is important because its only uncertainty is the decomposition into stellar and dark matter components, and lensing extends the argument to elliptical galaxies.…”

“…straints from odd images can only get strongerÈand perhaps substantially, if more lenses turn out to have upper limits as strong as in B0218]357. f odd \ 0.0005 The odd image problem stands in stark contrast to most observational tests of CDM. Spiral galaxy dynamics are said to imply that CDM halos are too concentrated to agree with observed galaxies (e.g., Moore 1994 ;de Blok et al 2001 ;Salucci 2001 ;Weiner et al 2001), although there is still substantial debate about whether beam smearing a †ects this conclusion for low surface brightness galaxies (van den Bosch et al 2000 ;van den Bosch & Swaters 2001). Independent of the dynamical arguments, the number and sizes of observed lenses lead to a similar conclusion, as shown in°4.…”

“…Debattista & Sellwood (1998) and Weiner, Sellwood, & Williams (2001) argue that fast-rotating bars require dark matter densities lower than predicted by CDM. The slowly rising rotation curves of dwarf galaxies and low surface brightness galaxies also seem to imply constant-density cores (e.g., Flores & Primack 1994 ;Moore 1994 ;McGaugh & de Blok 1998 ;Blais-Ouellette, Amram, & Carignan 2001 ;. However, several authors have argued that the H I rotation curves may have appeared artiÐcially shallow because of beam smearing and that better data are actually consistent with cuspy CDM mass distributions (van den Bosch et al 2000 ; Moore (2001) argues that only two of the 19 galaxies analyzed by van den Bosch & Swaters (2001) are actually consistent with CDM.…”

“…The slowly rising rotation curves of dwarf galaxies and low surface brightness galaxies also seem to imply constant-density cores (e.g., Flores & Primack 1994 ;Moore 1994 ;McGaugh & de Blok 1998 ;Blais-Ouellette, Amram, & Carignan 2001 ;. However, several authors have argued that the H I rotation curves may have appeared artiÐcially shallow because of beam smearing and that better data are actually consistent with cuspy CDM mass distributions (van den Bosch et al 2000 ; Moore (2001) argues that only two of the 19 galaxies analyzed by van den Bosch & Swaters (2001) are actually consistent with CDM. To summarize, many argue that CDM mass distributions are too concentrated to agree with the dynamics of spiral galaxies, but the conclusions are still subject to some vigorous debate.…”

Apoastron shift constraints on dark matter distribution at the Galactic Center

“…In this case the Galactic Center could be a strong source of γ-rays and neutrinos [4,7,15,16,17,18,19,20,21,22] due to DM annihilation. Since it is also expected that DM forms spikes at galaxy centers [23,24,25] the γ-ray flux from the Galactic Center should increase significantly in that case.…”