Formation and Evolution of Self-Interacting Dark Matter Halos

We study the formation and evolution of self-interacting dark matter (SIDM) halos. We find analytical, fully cosmological similarity solutions taking account of the collisional interaction of SIDM particles. This interaction results in a thermal conductivity that heats the halo core and flattens its density profile. These similarity solutions are relevant to galactic and cluster halo formation in the CDM model. We assume an initial mass profile dM/M M^{-eps}, as in the familiar secondary infall model. If eps=1/6, SIDM halos will evolve self-similarly, with a cold, supersonic infall terminated by a strong accretion shock. Different solutions arise for different values of the collisionality parameter, Q= sigma rho_b r_s, where sigma is the scattering cross section, rho_b is the cosmic mean density, and r_s is the shock radius. For all these solutions, a flat-density, isothermal core is present which grows in size as a fixed fraction of r_s. We find two different regimes for these solutions: 1) for Q \leq Q_{th}, the core density decreases and core size increases as Q increases; 2) for Q \geq Q_{th}, the core density increases and core size decreases as Q increases. Our similarity solutions are in agreement with previous N-body simulations of SIDM halos, which correspond to the low-Q regime, if Q=[8.4e-4 - 4.9e-2]Q_{th} (low-Q), or sigma=[0.56-5.6]cm^2/g. As Q=\infty, our similarity solution aquires a central density cusp, in agreement with some simulation results which used an ordinary collisional fluid to approximate the effects of SIDM collisionality. When Q=[18.6-231]Q_{th} or sigma=[1.2e4 - 2.71e4]cm^2/g, for which we find flat-density cores comparable to those of the observationally acceptable low-Q solutions, has not previously been identified. Further study of this regime is warranted.Comment: 7 pages, 5 figures, talk presented at the Second Korean Astrophysics Workshop (APCTP Workshop) on Formation and Interaction of Galaxies, published in a special issue of Journal of Korean Astronomical Society, ed. H. M Le

Meta-Stable Brane Configurations by Adding an Orientifold-Plane to Giveon-Kutasov

In hep-th/0703135, they have found the type IIA intersecting brane configuration where there exist three NS5-branes, D4-branes and anti-D4-branes. By analyzing the gravitational interaction for the D4-branes in the background of the NS5-branes, the phase structures in different regions of the parameter space were studied in the context of classical string theory. In this paper, by adding the orientifold 4-plane and 6-plane to the above brane configuration, we describe the intersecting brane configurations of type IIA string theory corresponding to the meta-stable nonsupersymmetric vacua of these gauge theories.Comment: 21 pp, 6 figures; reduced bytes of figures, DBI action analysis added and to appear in JHE

Spacelike boundaries from the c=1 Matrix Model

We find classical solutions of two dimensional noncritical string theory which give rise to geometries with spacelike boundaries, similar to spacetimes with cosmological event horizons. In the c=1 matrix model, these solutions have a representation as simple time dependent configurations. We obtain the causal structure of the resulting spacetimes. Using the macroscopic loop transform, we probe the form of the tachyon condensate in the asymptotic regions.Comment: 22 pages, 5 figures, v2: reference added, v3: minor correction

Nanocrystalline and Thin Film Germanium Electrodes with High Lithium Capacity and High Rate Capabilities

Germanium nanocrystals (12 nm mean diam) and amorphous thin films (60-250 nm thick) were prepared as anodes for lithium secondary cells. Amorphous thin film electrodes prepared on planar nickel substrates showed stable capacities of 1700 mAh/g over 60 cycles. Germanium nanocrystals showed reversible gravimetric capacities of up to 1400 mAh/g with 60% capacity retention after 50 cycles. Both electrodes were found to be crystalline in the fully lithiated state. The enhanced capacity, rate capability (1000C), and cycle life of nanophase germanium over bulk crystalline germanium is attributed to the high surface area and short diffusion lengths of the active material and the absence of defects in nanophase materials

Nonlinear Bias of Cosmological Halo Formation in the Early Universe

We present estimates of the nonlinear bias of cosmological halo formation, spanning a wide range in the halo mass from $\sim 10^{5} M_\odot$ to $\sim 10^{12} M_\odot$, based upon both a suite of high-resolution cosmological N-body simulations and theoretical predictions. The halo bias is expressed in terms of the mean bias and stochasticity as a function of local overdensity ($\delta$), under different filtering scales, which is realized as the density of individual cells in uniform grids. The sampled overdensities span a range wide enough to provide the fully nonlinear bias effect on the formation of haloes. A strong correlation between $\delta$ and halo population overdensity $\delta_h$ is found, along with sizable stochasticity. We find that the empirical mean halo bias matches, with good accuracy, the prediction by the peak-background split method based on the excursion set formalism, as long as the empirical, globally-averaged halo mass function is used. Consequently, this bias formalism is insensitive to uncertainties caused by varying halo identification schemes, and can be applied generically. We also find that the probability distribution function of biased halo numbers has wider distribution than the pure Poisson shot noise, which is attributed to the sub-cell scale halo correlation. We explicitly calculate this correlation function and show that both overdense and underdense regions have positive correlation, leading to stochasticity larger than the Poisson shot noise in the range of haloes and halo-collapse epochs we study.Comment: 18 pages, 8 figures, in press for publication in MNRAS; supplementary material (additional 16 figures) separately supplied (supplement.pdf) as a part of source file

Supersymmetry Breaking Vacua from M Theory Fivebranes

We consider intersecting brane configurations realizing N=2 supersymmetric gauge theories broken to N=1 by multitrace superpotentials, and softly to N=0. We analyze, in the framework of M5-brane wrapping a curve, the supersymmetric vacua and the analogs of spontaneous supersymmetry breaking and soft supersymmetry breaking in gauge theories. We show that the M5-brane does not exhibit the analog of metastable spontaneous supersymmetry breaking, and does not have non-holomorphic minimal volume curves with holomorphic boundary conditions. However, we find that any point in the N=2 moduli space can be rotated to a non-holomorphic minimal volume curve, whose boundary conditions break supersymmetry. We interpret these as the analogs of soft supersymmetry breaking vacua in the gauge theory.Comment: 32 pages, 8 figures, harvmac; v2: corrections in eq. 3.6 and in section 6, reference adde

Highly Reversible Lithium Storage in Nanostructured Silicon

Anode materials of nanostructured silicon have been prepared by physical vapor deposition and characterized using electrochemical methods. The electrodes were prepared in thin-film form as nanocrystalline particles (12 nm mean diameter) and as continuous amorphous thin films (100 nm thick). The nanocrystalline silicon exhibited specific capacities of around 1100 mAh/g with a 50% capacity retention after 50 cycles. The amorphous thin-film electrodes exhibited initial capacities of 3500 mAh/g with a stable capacity of 2000 mAh/g over 50 cycles. We suggest that the nanoscale dimensions of the silicon circumvents conventional mechanisms of mechanical deterioration, permitting good cycle life

Environment of The Gamma-Ray Burst GRB971214 : A Giant H II Region surrounded by A Galactic Supershell

Among a number of gamma ray bursts whose host galaxies are known, GRB971214 stands out for its high redshift $z\ge 3$ and the Ly$\alpha$ emission line having a P-Cygni type profile, which is interpreted to be a direct consequence of the expanding supershell. From a profile fitting analysis we estimate the expansion velocity of the supershell v_{exp} = 1500\kms and the neutral column density N_{HI}=10^{20}\cm^{-2}. The redshift $z=3.418$ of the host galaxy proposed by Kulkarni et al. (1998) has been revised to be $z=3.425$ from our profile analysis. The observed Ly$\alpha$ profile is fitted well by a Gaussian curve, which yields the Ly$\alpha$ luminosity L_{Ly\alpha}=(1.8\pm0.8)\times10^{42}\ergs \s^{-1}. Assuming that the photon source is a giant H II region, we deduce the electron number density in the H II region n_e=(40\pm10) ({R \over {100 \pc}})^{-1.5}\cm^{-3}, which corresponds to the illumination by about $10^4$ O5 stars. We estimate the star-formation rate to be R_{SF} = (7\pm3){\rm M}_\odot\yr^{-1} with the internal and the Galactic extinction corrected. The theory on the evolution of supernova remnants is used to propose that the supershell is at the adiabatic phase, with its radius R = 18 E_{53}^{1/2} \pc, its age $t = 4.7\times10^3\ E_{53}^{1/2} \yrs$, and the density of the ambient medium$n_1 = 5.4\ E_{53}^{-1/2}\cm^{-3}$, where$E_{53}= E/10^{53}\ergs$.And we estimate the kinetic energy of the supershell to be$E_k=7.3\times10^{52} E_{53} \ergs$. These values are consistent with the hypothesis that the supershell is the remnant of a gamma ray burst.Comment: 5 papges, 3 figures, Accepted for publication in Astrophysical Journal Letter