Globular Clusters and Dark Satellite Galaxies through the Stream Velocity

The formation of purely baryonic globular clusters with no gravitationally bound dark matter is still a theoretical challenge. We show that these objects might form naturally whenever there is a relative stream velocity between baryons and dark matter. The stream velocity causes a phase shift between linear modes of baryonic and dark matter perturbations, which translates to a spatial offset between the two components when they collapse. For a 2sigma (3sigma) density fluctuation, baryonic clumps with masses in the range 1e5 - 2.5e6 Msun (1e5 - 4e6 Msun) collapse outside the virial radii of their counterpart dark matter halos. These objects could survive as long-lived dark matter-free objects and might conceivably become globular clusters. In addition, their dark matter counterparts, which were deprived of gas, might become dark satellite galaxies.Comment: 3 Figures, accepted for publication in ApJ Letter

Formation of Dark Matter Torii Around Supermassive Black Holes Via The Eccentric Kozai-Lidov Mechanism

We explore the effects of long term secular perturbations on the distribution of dark matter particles around Supermassive Black Hole (BH) binaries. We show that in the hierarchical (in separation) three-body problem, one of the BHs and a dark matter particle form an inner binary. Gravitational perturbations from the BH companion, on a much wider orbit, can cause the dark matter particle to reach extremely high eccentricities and even get accreted onto the BH, by what is known as the Eccentric Kozai-Lidov (EKL) mechanism. We show that this may produce a torus-like configuration for the dark matter distribution around the less massive member of the BH binary. We first consider an Intermediate BH (IMBH) in the vicinity of our Galactic Center, which may be a relic of a past minor merger. We show that if the IMBH is close enough (i.e., near the stellar disk) the EKL mechanism is very efficient in exciting the eccentricity of dark matter particles in near-polar configurations to extremely high values where they are accreted by the IMBH. We show that this mechanism is even more effective if the central BH grows in mass, where we have assumed adiabatic growth. Since near-polar configurations are disrupted, a torus-like shape is formed. We also show that this behavior is also likely to be relevant for Supermassive BH binaries. We suggest that if the BHs are spinning, the accreted dark matter particles may linger in the ergosphere and thereby may generate self-annihilations and produce an indirect signature of potential interest.Comment: Accepted to ApJ, 11 pages, 9 figure

Mergers and Obliquities in Stellar Triples

Many close stellar binaries are accompanied by a far-away star. The "eccentric Kozai-Lidov" (EKL) mechanism can cause dramatic inclination and eccentricity fluctuations, resulting in tidal tightening of inner binaries of triple stars. We run a large set of Monte-Carlo simulations including the secular evolution of the orbits, general relativistic precession and tides, and we determine the semimajor axis, eccentricity, inclination and spin-orbit angle distributions of the final configurations. We find that the efficiency of forming tight binaries (<~16 d) when taking the EKL mechanism into account is ~ 21%, and about 4% of all simulated systems ended up in a merger event. These merger events can lead to the formation of blue-stragglers. Furthermore, we find that the spin-orbit angle distribution of the inner binaries carries a signature of the initial setup of the system, thus observations can be used to disentangle close binaries' birth configuration. The resulting inner and outer final orbits' period distributions, and their estimated fraction, suggests secular dynamics may be a significant channel for the formation of close binaries in triples and even blue stragglers.Comment: Accepted to ApJ, 10 figure

The nonlinear evolution of baryonic overdensities in the early universe: Initial conditions of numerical simulations

We run very large cosmological N-body hydrodynamical simulations in order to study statistically the baryon fractions in early dark matter halos. We critically examine how differences in the initial conditions affect the gas fraction in the redshift range z = 11--21. We test three different linear power spectra for the initial conditions: (1) A complete heating model, which is our fiducial model; this model follows the evolution of overdensities correctly, according to Naoz & Barkana (2005), in particular including the spatial variation of the speed of sound of the gas due to Compton heating from the CMB. (2) An equal-{\delta} model, which assumes that the initial baryon fluctuations are equal to those of the dark matter, while conserving sigma8 of the total matter. (3) A mean cs model, which assumes a uniform speed of sound of the gas. The latter two models are often used in the literature. We calculate the baryon fractions for a large sample of halos in our simulations. Our fiducial model implies that before reionization and significant stellar heating took place, the minimum mass needed for a minihalo to keep most of its baryons throughout its formation was ~ 3 * 10^4 Msun. However, the alternative models yield a wrong (higher by about 50%) minimum mass, since the system retains a memory of the initial conditions. We also demonstrate this using the "filtering mass" from linear theory, which accurately describes the evolution of the baryon fraction throughout the simulated redshift range.Comment: 6 figures 1 table, accepted to MNRA