On the effectiveness of mixing in violent relaxation

Relaxation processes in collisionless dynamics lead to peculiar behavior in systems with long-range interactions such as self-gravitating systems, non-neutral plasmas and wave-particle systems. These systems, adequately described by the Vlasov equation, present quasi-stationary states (QSS), i.e. long lasting intermediate stages of the dynamics that occur after a short significant evolution called "violent relaxation". The nature of the relaxation, in the absence of collisions, is not yet fully understood. We demonstrate in this article the occurrence of stretching and folding behavior in numerical simulations of the Vlasov equation, providing a plausible relaxation mechanism that brings the system from its initial condition into the QSS regime. Area-preserving discrete-time maps with a mean-field coupling term are found to display a similar behaviour in phase space as the Vlasov system.Comment: 10 pages, 11 figure

The Origin of the Brightest Cluster Galaxies

Most clusters and groups of galaxies contain a giant elliptical galaxy in their centres which far outshines and outweighs normal ellipticals. The origin of these brightest cluster galaxies is intimately related to the collapse and formation of the cluster. Using an N-body simulation of a cluster of galaxies in a hierarchical cosmological model, we show that galaxy merging naturally produces a massive, central galaxy with surface brightness and velocity dispersion profiles similar to observed BCG's. To enhance the resolution of the simulation, 100 dark halos at $z=2$ are replaced with self-consistent disk+bulge+halo galaxy models following a Tully-Fisher relation using 100000 particles for the 20 largest galaxies and 10000 particles for the remaining ones. This technique allows us to analyze the stellar and dark matter components independently. The central galaxy forms through the merger of several massive galaxies along a filament early in the cluster's history. Galactic cannibalism of smaller galaxies through dynamical friction over a Hubble time only accounts for a small fraction of the accreted mass. The galaxy is a flattened, triaxial object whose long axis aligns with the primordial filament and the long axis of the cluster galaxy distribution agreeing with observed trends for galaxy-cluster alignment.Comment: Revised and accepted in ApJ, 25 pages, 10 figures, online version available at http://www.cita.utoronto.ca/~dubinski/bcg

Quasi-equilibria in one-dimensional self-gravitating many body systems

The microscopic dynamics of one-dimensional self-gravitating many-body systems is studied. We examine two courses of the evolution which has the isothermal and stationary water-bag distribution as initial conditions. We investigate the evolution of the systems toward thermal equilibrium. It is found that when the number of degrees of freedom of the system is increased, the water-bag distribution becomes a quasi-equilibrium, and also the stochasticity of the system reduces. This results suggest that the phase space of the system is effectively not ergodic and the system with large degreees of freedom approaches to the near-integrable one.Comment: 21pages + 7 figures (available upon request), revtex, submitted to Physical Review

Forebrain Ptf1a Is Required for Sexual Differentiation of the Brain

The mammalian brain undergoes sexual differentiation by gonadal hormones during the perinatal critical period. However, the machinery at earlier stages has not been well studied. We found that Ptf1a is expressed in certain neuroepithelial cells and immature neurons around the third ventricle that give rise to various neurons in several hypothalamic nuclei. We show that conditional Ptf1a-deficient mice (Ptf1a cKO) exhibit abnormalities in sex-biased behaviors and reproductive organs in both sexes. Gonadal hormone administration to gonadectomized animals revealed that the abnormal behavior is caused by disorganized sexual development of the knockout brain. Accordingly, expression of sex-biased genes was severely altered in the cKO hypothalamus. In particular, Kiss1, important for sexual differentiation of the brain, was drastically reduced in the cKO hypothalamus, which may contribute to the observed phenotypes in the Ptf1a cKO. These findings suggest that forebrain Ptf1a is one of the earliest regulators for sexual differentiation of the brain

Topical Review: Development of overgrown semi-polar GaN for high efficiency green/yellow emission

The most successful example of large lattice-mismatched epitaxial growth of semiconductors is the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great success in developing InGaN-based blue emitters. However, the majority of achievements in the field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001) sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still a great challenge to develop longer wavelength devices such as green and yellow emitters. One clear way forward would be to grow III-nitride device structures along a semi-/non-polar direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium incorporation into GaN and reduced quantum confined Stark effects. This review presents recent progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates, the two kinds of major substrates which are cost-effective and thus industry-compatible, and also demonstrates the latest achievements on electrically injected InGaN emitters with long emission wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review presents a summary and outlook on further developments for semi-polar GaN based optoelectronics

Hypothalamic over-expression of VGF in the Siberian hamster increases energy expenditure and reduces body weight gain

VGF (non-acronymic) was first highlighted to have a role in energy homeostasis through experiments involving dietary manipulation in mice. Fasting increased VGF mRNA in the Arc and levels were subsequently reduced upon refeeding. This anabolic role for VGF was supported by observations in a VGF null (VGF-/-) mouse and in the diet-induced and gold-thioglucose obese mice. However, this anabolic role for VGF has not been supported by a number of subsequent studies investigating the physiological effects of VGF-derived peptides. Intracerebroventricular (ICV) infusion of TLQP-21 increased resting energy expenditure and rectal temperature in mice and protected against diet-induced obesity. Similarly, ICV infusion of TLQP-21 into Siberian hamsters significantly reduced body weight, but this was due to a decrease in food intake, with no effect on energy expenditure. Subsequently NERP-2 was shown to increase food intake in rats via the orexin system, suggesting opposing roles for these VGF-derived peptides. Thus to further elucidate the role of hypothalamic VGF in the regulation of energy homeostasis we utilised a recombinant adeno-associated viral vector to over-express VGF in adult male Siberian hamsters, thus avoiding any developmental effects or associated functional compensation. Initially, hypothalamic over-expression of VGF in adult Siberian hamsters produced no effect on metabolic parameters, but by 12 weeks post-infusion hamsters had increased oxygen consumption and a tendency to increased carbon dioxide production; this attenuated body weight gain, reduced interscapular white adipose tissue and resulted in a compensatory increase in food intake. These observed changes in energy expenditure and food intake were associated with an increase in the hypothalamic contents of the VGF-derived peptides AQEE, TLQP and NERP-2. The complex phenotype of the VGF-/- mice is a likely consequence of global ablation of the gene and its derived peptides during development, as well as in the adult

A dynamical origin for early mass segregation in young star clusters

Some young star clusters show a degree of mass segregation that is inconsistent with the effects of standard two-body relaxation from an initially unsegregated system without substructure, in virial equilibrium, and it is unclear whether current cluster formation models can account for this degree of initial segregation in clusters of significant mass. In this Letter we demonstrate that mergers of small clumps that are either initially mass segregated, or in which mass segregation can be produced by two-body relaxation before they merge, generically lead to larger systems which inherit the progenitor clumps' segregation. We conclude that clusters formed in this way are naturally mass segregated, accounting for the anomalous observations and suggesting that this process of prompt mass segregation due to initial clumping should be taken fully into account in constructing cluster dynamical models.Comment: 13 pages, 4 figures, submitted to ApJ

Hubble Space Telescope Observations of cD Galaxies and their Globular Cluster Systems

We have used WFPC2 on the Hubble Space Telescope to obtain F450W and F814W images of four cD galaxies (NGC 541 in Abell 194, NGC 2832 in Abell 779, NGC 4839 in Abell 1656 and NGC 7768 in Abell 2666) in the range 5400 < cz < 8100 km s^{-1}. For NGC 541, the HST data are supplemented by ground-based B and I images obtained with the FORS1 on the VLT. We present surface brightness and color profiles for each of the four galaxies, confirming their classification as cD galaxies. Isophotal analyses reveal the presence of subarcsecond-scale dust disks in the nuclei of NGC 541 and NGC 7768. Despite the extreme nature of these galaxies in terms of spatial extent and luminosity, our analysis of their globular cluster systems reveals no anomalies in terms of specific frequencies, metallicity gradients, average metallicities, or the metallicity offset between the globulars and the host galaxy. We show that the latter offset appears roughly constant at \Delta [Fe/H] ~ 0.8 dex for early-type galaxies spanning a luminosity range of roughly four orders of magnitude. We combine the globular cluster metallicity distributions with an empirical technique described in a series of earlier papers to investigate the form of the protogalactic mass spectrum in these cD galaxies. We find that the observed GC metallicity distributions are consistent with those expected if cD galaxies form through the cannibalism of numerous galaxies and protogalactic fragments which formed their stars and globular clusters before capture and disruption. However, the properties of their GC systems suggest that dynamical friction is not the primary mechanism by which these galaxies are assembled. We argue that cDs instead form rapidly, via hierarchical merging, prior to cluster virialization.Comment: 36 pages, 20 postscript figures, uses emulateapj. Accepted for publication in the Astronomical Journa

On the Stability of Quasi-Equilibrium Self-Gravitating Configurations in a Tidal Field

The possibility that quasi-equilibrium self-gravitating galaxy-like configurations exist in a tidal field is analyzed in this paper. More specifically, we address the question of how to predict initial configurations modeling galaxies that are able to survive environmental effects in a dense environment for a Hubble time or so, provided thay dynamical friction is neglected. The initial quasi-equilibrium configurations have been built up taking into account the external tidal field produced by the halo. It modifies the escape velocity field of the configuration, compared with isolated configurations. The survival of the configurations as they orbit inside the halos has been studied through N-body simulations. As a general result, it has been found out that the bulk of the models is conserved along 12.5 Gyears of evolution, and that the low rates of mass losses they experience are consistent with those expected when the adiabatic protection hypothesis is at work. So, solutions for galaxy configurations in tidal quasi-equilibrium have been found, showing that tidal stripping in quiescent phases does not seem to be very important, unless that the density of the galaxy environment at its formation had been much lower than that of the galaxy environment at the point of its orbit where the tidal perturbation is maximum. (Abridged)Comment: 36 pages, 7 postscript figures, to be published in Ap