If cold dark matter is present at the galactic center, as in current models of the dark halo, it is accreted by the central black hole into a dense spike. Particle dark matter then annihilates strongly inside the spike, making it a compact source of photons, electrons, positrons, protons, antiprotons, and neutrinos. The spike luminosity depends on the density profile of the inner halo: halos with finite cores have unnoticeable spikes, while halos with inner cusps may have spikes so bright that the absence of a detected neutrino signal from the galactic center already places interesting upper limits on the density slope of the inner halo. Future neutrino telescopes observing the galactic center could probe the inner structure of the dark halo, or indirectly find the nature of dark matter.The evidence is mounting for a massive black hole at the galactic center. Ghez et al. [1] have confirmed and sharpened the Keplerian behavior of the star velocity dispersion in the inner 0.1 pc of the galaxy found by Eckart and Genzel [2]. These groups estimate the mass of the black hole to be M = 2.6 ± 0.2 × 10 6 M . If cold dark matter is present at the galactic center, as in current models of the dark halo, it is redistributed by the black hole into a cusp. We call it the central 'spike,' to avoid confusion with the inner halo cusp favored by present N-body simulations of galaxy formation [3]. If cold dark matter contains neutral elementary particles that can annihilate with each other, like the supersymmetric neutralino, the annihilation rate in the spike is strongly increased as it depends on the square of the matter density. The steep spike profile, with index ≥ 3/2, then implies that most of the annihilations take place at the inner radius of the spike, determined either by self-annihilation or by capture into the black hole.Of the annihilation end-products, neutrinos escape the spike and propagate to us undisturbed. Current limits on the neutrino emission from the galactic center place upper limits on the slope of the inner halo. Future neutrino telescopes may improve on these limits or bring information on the nature of dark matter. I. ADIABATIC SPIKE AROUND THE CENTRAL BLACK HOLEWe find the dark matter density profile in the region where the black hole dominates the gravitational potential. From the data in [1,2], this is the region r < ∼ R M 0.2 pc. Other masses (the central star cluster, for example) also influence the dark matter distribution, but since they make the gravitational potential deeper, their effect is to increase the central dark matter density and the annihilation signals.We work under the assumption that the growth of the black hole is adiabatic. This assumption is supported by the collisionless behavior of particle dark matter. We can find the final density after black hole formation from the final phase-space distribution f (E , L ) aswithWe have neglected the contribution from unbound orbits (E > 0). The lower limit of integration L c , and the second factor in E m , are introduced to eliminate the ...

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2 < z < 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 × 10 10 M ⊙ . Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.