The structure of dark matter halos in hierarchical clustering theories

During hierarchical clustering, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small mass halo could be dense enough to resist disruption and survive undigested, when it is incorporated into a bigger object. We explore the possibility that a nested sequence of undigested cores in the center of the halo, which have survived the hierarchical, inhomogeneous collapse to form larger and larger objects, determines the halo structure in the inner regions. For a flat universe with $P(k) \propto k^n$, scaling arguments then suggest that the core density profile is, $\rho \propto r^{-\alpha}$ with $\alpha = (9+3n)/(5+n)$. But whether such behaviour obtains depends on detailed dynamics. We first examine the dynamics using a fluid approach to the self-similar collapse solutions for the dark matter phase space density, including the effect of velocity dispersions. We highlight the importance of tangential velocity dispersions to obtain density profiles shallower than $1/r^2$ in the core regions. If tangential velocity dispersions in the core are constrained to be less than the radial dispersion, a cuspy core density profile shallower than 1/r cannot obtain, in self-similar collapse. We then briefly look at the profiles of the outer halos in low density cosmological models where the total halo mass is convergent. Finally, we analyze a suite of dark halo density and velocity dispersion profiles obtained in cosmological N-body simulations of models with n= 0, -1 and -2. We find that the core-density profiles of dark halos, show considerable scatter in their properties, but nevertheless do appear to reflect a memory of the initial power spectrum, with steeper initial spectra producing flatter core profiles. (Abridged)Comment: 31 pages, 7 figures, submitted to Ap

Generic estimates for magnetic fields generated during inflation including Dirac-Born-Infeld theories

We estimate the strength of large-scale magnetic fields produced during inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis is sufficiently general in the sense that it covers most of conformal symmetry breaking theories in which the electromagnetic field is coupled to a scalar field. In DBI theories there is an additional factor associated with the speed of sound, which allows a possibility to lead to an extra amplification of the magnetic field in a ultra-relativistic region. We clarify the conditions under which seed magnetic fields to feed the galactic dynamo mechanism at a decoupling epoch as well as present magnetic fields on galactic scales are sufficiently generated to satisfy observational bounds.Comment: 7 pages, no figure, accepted in Phys. Rev.

Non-collinear Magnetic Order in the Double Perovskites: Double Exchange on a Geometrically Frustrated Lattice

Double perovskites of the form A_2BB'O_6 usually involve a transition metal ion, B, with a large magnetic moment, and a non magnetic ion B'. While many double perovskites are ferromagnetic, studies on the underlying model reveal the possibility of antiferromagnetic phases as well driven by electron delocalisation. In this paper we present a comprehensive study of the magnetic ground state and T_c scales of the minimal double perovskite model in three dimensions using a combination of spin-fermion Monte Carlo and variational calculations. In contrast to two dimensions, where the effective magnetic lattice is bipartite, three dimensions involves a geometrically frustrated face centered cubic (FCC) lattice. This promotes non-collinear spiral states and `flux' like phases in addition to collinear anti-ferromagnetic order. We map out the possible magnetic phases for varying electron density, `level separation' epsilon_B - epsilon_B', and the crucial B'-B' (next neighbour) hopping t'.Comment: 15 pages pdflatex + 19 figs, revision: removed redundant comment

Cosmological Magnetic Fields from Primordial Helical Seeds

Most early Universe scenarios predict negligible magnetic fields on cosmological scales if they are unprocessed during subsequent expansion of the Universe. We present a new numerical treatment of the evolution of primordial fields and apply it to weakly helical seeds as they occur in certain early Universe scenarios. We find that initial helicities not much larger than the baryon to photon number can lead to fields of about 10^{-13} Gauss with coherence scales slightly below a kilo-parsec today.Comment: 4 revtex pages, 2 postscript figures include

Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters

Using three-dimensional hydrodynamic simulations, we investigate heating and turbulence driving in an intracluster medium (ICM) by orbital motions of galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and isotropic orbits through an ICM typical of rich clusters. An introduction of the galaxies immediately produces gravitational wakes, providing perturbations that can potentially grow via resonant interaction with the background gas. When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11} Msun, the perturbations are in the linear regime and the resonant excitation of gravity waves is efficient to generate kinetic energy in the ICM, resulting in the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 > 100, on the other hand, nonlinear fluctuations of the background ICM destroy galaxy wakes and thus render resonant excitation weak or absent. In this case, the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s. The angle-averaged velocity power spectra of turbulence driven in our models have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of resonant excitation, none of the cooling models considered are able to halt cooling catastrophe, suggesting that the galaxy motions alone are unlikely to solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ

A Unified treatment of small and large- scale dynamos in helical turbulence

Helical turbulence is thought to provide the key to the generation of large-scale magnetic fields. Turbulence also generically leads to rapidly growing small-scale magnetic fields correlated on the turbulence scales. These two processes are usually studied separately. We give here a unified treatment of both processes, in the case of random fields, incorporating also a simple model non-linear drift. In the process we uncover an interesting plausible saturated state of the small-scale dynamo and a novel analogy between quantum mechanical (QM) tunneling and the generation of large scale fields. The steady state problem of the combined small/large scale dynamo, is mapped to a zero-energy, QM potential problem; but a potential which, for non-zero mean helicity, allows tunneling of bound states. A field generated by the small-scale dynamo, can 'tunnel' to produce large-scale correlations, which in steady state, correspond to a force-free 'mean' field.Comment: 4 pages, 1 figure, Physical Review Letters, in pres