Footballs, Conical Singularities and the Liouville Equation

We generalize the football shaped extra dimensions scenario to an arbitrary number of branes. The problem is related to the solution of the Liouville equation with singularities and explicit solutions are presented for the case of three branes. The tensions of the branes do not need to be tuned with each other but only satisfy mild global constraints.Comment: 15 pages, Refs. added, minor changes. Typo in eq. 4.3 corrected. Version to be published in PR

The Effects of a Photoionizing UV Background on the Formation of Disk Galaxies

We use high resolution N-body/gasdynamical simulations to investigate the effects of a photoionizing UV background on the assembly of disk galaxies in hierarchically clustering universes. We focus on the mass and rotational properties of gas that can cool to form centrifugally supported disks in dark matter halos of different mass. Photoheating can significantly reduce the amount of gas that can cool in galactic halos. Depending on the strength of the UV background field, the amount of cooled gas can be reduced by up to $50\%$ in systems with circular speeds in the range $80$-$200$ \kms. The magnitude of the effect, however, is not enough to solve the ``overcooling'' problem that plagues hierarchical models of galaxy formation if the UV background is chosen to be consistent with estimates based on recent observations of QSO absorption systems. Photoionization has little effect on the collapse of gas at high redshift and affects preferentially gas that is accreted at late times. Since disks form inside-out, accreting higher angular momentum gas at later times, disks formed in the presence of a UV background have spins that are even smaller than those formed in simulations that do not include the effects of photoionization. This exacerbates the angular momentum problem that afflicts hierarchical models of disk formation. We conclude that photoionization cannot provide the heating mechanism required to reconcile hierarchically clustering models with observations. Energy feedback and enrichment processes from the formation and evolution of stars must therefore be indispensable ingredients for any successful model of the formation of disk galaxies.Comment: 36 pages, w/ embedded figures, submitted to ApJ. Also available at http://penedes.as.arizona.edu/~jfn/preprints/dskform.ps.g

Dark-Halo Cusp: Asymptotic Convergence

We propose a model for how the buildup of dark halos by merging satellites produces a characteristic inner cusp, of a density profile \rho \prop r^-a with a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of a<1 exerts tidal compression which prevents local deposit of satellite material; the satellite sinks intact into the halo center thus causing a rapid steepening to a>1. Using merger N-body simulations, we learn that this cusp is stable under a sequence of mergers, and derive a practical tidal mass-transfer recipe in regions where the local slope of the halo profile is a>1. According to this recipe, the ratio of mean densities of halo and initial satellite within the tidal radius equals a given function psi(a), which is significantly smaller than unity (compared to being 1 according to crude resonance criteria) and is a decreasing function of a. This decrease makes the tidal mass transfer relatively more efficient at larger a, which means steepening when a is small and flattening when a is large, thus causing converges to a stable solution. Given this mass-transfer recipe, linear perturbation analysis, supported by toy simulations, shows that a sequence of cosmological mergers with homologous satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1. The slope depends only weakly on the fluctuation power spectrum, in agreement with cosmological simulations. During a long interim period the profile has an NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough compact satellite remnants make it intact into the inner halo. In order to maintain a flat core, satellites must be disrupted outside the core, possibly as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres

Simulations of galaxy formation in a Λ cold dark matter universe : I : dynamical and photometric properties of a simulated disk galaxy.

We present a detailed analysis of the dynamical and photometric properties of a disk galaxy simulated in the cold dark matter (CDM) cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z1 that lead to the formation of two distinct dynamical components: a spheroid of mostly old stars and a rotationally supported disk of younger stars. The surface brightness profile is very well approximated by the superposition of an R1/4 spheroid and an exponential disk. Each photometric component contributes a similar fraction of the total luminosity of the system, although less than a quarter of the stars form after the last merger episode at z1. In the optical bands the surface brightness profile is remarkably similar to that of Sab galaxy UGC 615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies 1 mag off the I-band Tully-Fisher relation of late-type spirals but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. In agreement with previous simulation work, the angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This again reflects the dominance of the slowly rotating, dense spheroidal component, to which most discrepancies with observation may be traced. On its own, the disk component has properties rather similar to those of late-type spirals: its luminosity, its exponential scale length, and its colors are all comparable to those of galaxy disks of similar rotation speed. This suggests that a different form of feedback than adopted here is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling, without fine-tuning, the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as CDM remains a challenging, yet so far elusive, proposition

Codimension Two Branes and Distributional Curvature

In general relativity, there is a well-developed formalism for working with the approximation that a gravitational source is concentrated on a shell, or codimension one surface. By contrast, there are obstacles to concentrating sources on surfaces that have a higher codimension, for example, a string in a spacetime with dimension greater than or equal to four. Here it is shown that, by giving up some of the generality of the codimension one case, curvature can be concentrated on submanifolds that have codimension two. A class of metrics is identified such that (1) the scalar curvature and Ricci densities exist as distributions with support on a co-dimension two submanifold, and (2) using the Einstein equation, the distributional curvature corresponds to a concentrated stress-energy with equation of state p equals minus the energy density, where p is the isotropic pressure tangent to the submanifold. This is the appropriate stress-energy to describe a self-gravitating brane that is governed by an area action, or a brane world deSitter cosmology. The possibility of having a different equation of state arise from a wider class of metrics is discussed.Comment: 18 pages; v2 references added; typos corrected, references added; additional references adde

Star Formation and Feedback in Dwarf Galaxies

We examine the star formation history and stellar feedback effects of dwarf galaxies under the influence of extragalactic ultraviolet radiation. We consider the dynamical evolution of gas in dwarf galaxies using a one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute the effects of radiative transfer and photoionization. We include a physically-motivated star formation recipe and consider the effects of feedback. Our results indicate that star formation in the severe environment of dwarf galaxies is a difficult and inefficient process. For intermediate mass systems, such as the dSphs around the Galaxy, star formation can proceed with in early cosmic epochs despite the intense background UV flux. Triggering processes such as merger events, collisions, and tidal disturbance can lead to density enhancements, reducing the recombination timescale, allowing gas to cool and star formation to proceed. However, the star formation and gas retention efficiency may vary widely in galaxies with similar dark matter potentials, because they depend on many factors, such as the baryonic fraction, external perturbation, IMF, and background UV intensity. We suggest that the presence of very old stars in these dwarf galaxies indicates that their initial baryonic to dark matter content was comparable to the cosmic value. This constraint suggests that the initial density fluctuation of baryonic matter may be correlated with that of the dark matter. For the more massive dwarf elliptical galaxies, the star formation efficiency and gas retention rate is much higher. Their mass to light ratio is regulated by star formation feedback, and is expected to be nearly independent of their absolute luminosity. The results of our theoretical models reproduce the observed $M/L-M_v$ correlation.Comment: 35 pages, 13 figure

Evidence of strong antiferromagnetic coupling between localized and itinerant electrons in ferromagnetic Sr2FeMoO6

Magnetic dc susceptibility ($\chi$) and electron spin resonance (ESR) measurements in the paramagnetic regime, are presented. We found a Curie-Weiss (CW) behavior for $\chi$(T) with a ferromagnetic $\Theta = 446(5)$ K and $\mu_{eff} = 4.72(9) \mu_{B}/f.u.$, this being lower than that expected for either $Fe^{3+}(5.9\mu_{B})$ or $Fe^{2+}(4.9\mu_{B})$ ions. The ESR g-factor $g = 2.01(2)$, is associated with $Fe^{3+}$. We obtained an excellent description of the experiments in terms of two interacting sublattices: the localized $Fe^{3+}$ ($3d^{5}$) cores and the delocalized electrons. The coupled equations were solved in a mean-field approximation, assuming for the itinerant electrons a bare susceptibility independent on $T$. We obtained $\chi_{e}^{0} = 3.7$ $10^{-4}$ emu/mol. We show that the reduction of $\mu_{eff}$ for $Fe^{3+}$ arises from the strong antiferromagnetic (AFM) interaction between the two sublattices. At variance with classical ferrimagnets, we found that $\Theta$ is ferromagnetic. Within the same model, we show that the ESR spectrum can be described by Bloch-Hasegawa type equations. Bottleneck is evidenced by the absence of a $g$-shift. Surprisingly, as observed in CMR manganites, no narrowing effects of the ESR linewidth is detected in spite of the presence of the strong magnetic coupling. These results provide evidence that the magnetic order in $Sr_{2}FeMoO_{6}$ does not originates in superexchange interactions, but from a novel mechanism recently proposed for double perovskites

Instability of brane cosmological solutions with flux compactifications

We discuss the stability of the higher-dimensional de Sitter (dS) brane solutions with two-dimensional internal space in the Einstein-Maxwel theory. We show that an instability appears in the scalar-type perturbations with respect to the dS spacetime. We derive a differential relation which has the very similar structure to the ordinary laws of thermodynamics as an extension of the work for the six-dimensional model [20]. In this relation, the area of dS horizon (integrated over the two internal dimensions) exactly behaves as the thermodynamical entropy. The dynamically unstable solutions are in the thermodynamically unstable branch. An unstable dS compactification either evolves toward a stable configuration or two-dimensional internal space is decompactified. These dS brane solutions are equivalent to the accelerating cosmological solutions in the six-dimensional Einstein-Maxwell-dilaton theory via dimensional reduction. Thus, if the seed higher-dimensional solution is unstable, the corresponding six-dimensional solution is also unstable. From the effective four-dimensional point of view, a cosmological evolution from an unstable cosmological solution in higher dimensions may be seen as a process of the transition from the initial cosmological inflation to the current dark energy dominated Universe.Comment: 11 pages, 3 figures, references added, to appear in CQ

Label-free electrical detection of DNA hybridization using carbon nanotubes and graphene

The interface between biosystems and nanomaterials is emerging for detection of various biomolecules and subtle cellular activities. In particular, the development of cost-effective and sequence-selective DNA detection is urgent for the diagnosis of genetic or pathogenic diseases. Graphene-based nanocarbon materials, such as carbon nanotubes and thin graphene layers, have been employed as biosensors because they are biocompatible, extraordinarily sensitive, and promising for large-area detection. Electrical and label-free detection of DNA can be achieved by monitoring the conductance change of devices fabricated from these carbon materials. Here, the recent advances in this research area are briefly reviewed. The key issues and perspectives of future development are also discussed

Ca II 8542 \AA\ brightenings induced by a solar microflare

We study small-scale brightenings in Ca II 8542 \AA\ line-core images to determine their nature and effect on localized heating and mass transfer in active regions. High-resolution 2D spectroscopic observations of an active region in the Ca II 8542 \AA\ line were acquired with the GFPI attached to the 1.5-meter GREGOR telescope. Inversions of the spectra were carried out using NICOLE. We identified three brightenings of sizes up to 2"x2". We found evidence that the brightenings belonged to the footpoints of a microflare (MF). The properties of the observed brightenings disqualified the scenarios of Ellerman bombs or IRIS bombs. However, this MF shared some common properties with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs and MFs are both apparent in chromospheric and coronal layers according to the AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5 min and lengths of about 20". The inversions revealed heating by 600 K at the footpoint location in the ambient chromosphere during the impulsive phase. Connecting the footpoints, a dark filamentary structure appeared in the Ca II line-core images. Before the start of the MF, the spectra of this structure already indicated average blueshifts, meaning upward motions of the plasma along the LOS. During the impulsive phase, these velocities increased up to -2.2 km/s. Downflows dominated at the footpoints. However, in the upper photosphere, slight upflows occurred during the impulsive phase. Hence, bidirectional flows are present in the footpoints of the MF. Conclusions: We detected Ca II brightenings that coincided with the footpoint location of an MF. The MF event led to a rise of plasma in the upper photosphere, both before and during the impulsive phase. Excess mass, previously raised to at most chromospheric layers, slowly drained downward along arches toward the footpoints of the MF.Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 6 figures, 1 online movi