The Role of the Radial Orbit Instability in Dark Matter Halo Formation and Structure

For a decade, N-body simulations have revealed a nearly universal dark matter density profile, which appears to be robust to changes in the overall density of the universe and the underlying power spectrum. Despite its universality, the physical origin of this profile has not yet been well understood. Semi--analytic models by Barnes et al. (2005) have suggested that the density structure of dark matter halos is determined by the onset of the radial orbit instability (ROI). We have tested this hypothesis using N-body simulations of collapsing dark matter halos with a variety of initial conditions. For dynamically cold initial conditions, the resulting halo structures are triaxial in shape, due to the mild aspect of the instability. We examine how variations in initial velocity dispersion affect the onset of the instability, and find that an isotropic velocity dispersion can suppress the ROI entirely, while a purely radial dispersion does not. The quantity sigma^2/vc^2 is a criterion for instability, where regions with sigma^2/vc^2 <~1 become triaxial due to the ROI or other perturbations. We also find that the radial orbit instability sets a scale length at which the velocity dispersion changes rapidly from isotropic to radially anisotropic. This scale length is proportional to the radius at which the density profile changes shape, as is the case in the semi--analytic models; however, the coefficient of proportionality is different by a factor of ~2.5. We conclude that the radial orbit instability is likely to be a key physical mechanism responsible for the nearly universal profiles of simulated dark matter halos.Comment: 13 pages, 12 figures, accepted to Ap

Nearly horizon skimming orbits of Kerr black holes

An unusual set of orbits about extreme Kerr black holes resides at the Boyer-Lindquist radius $r = M$, the coordinate of the hole's event horizon. These ``horizon skimming'' orbits have the property that their angular momentum $L_z$ {\it increases} with inclination angle, opposite to the familiar behavior one encounters at larger radius. In this paper, I show that this behavior is characteristic of a larger family of orbits, the ``nearly horizon skimming'' (NHS) orbits. NHS orbits exist in the very strong field of any black hole with spin a\agt 0.952412M. Their unusual behavior is due to the locking of particle motion near the event horizon to the hole's spin, and is therefore a signature of the Kerr metric's extreme strong field. An observational hallmark of NHS orbits is that a small body spiraling into a Kerr black hole due to gravitational-wave emission will be driven into orbits of progressively smaller inclination angle, toward the equator. This is in contrast to the ``normal'' behavior. For circular orbits, the change in inclination is very small, and unlikely to be of observational importance. I argue that the change in inclination may be considerably larger when one considers the evolution of inclined eccentric orbits. If this proves correct, then the gravitational waves produced by evolution through the NHS regime may constitute a very interesting and important probe of the strong-field nature of rotating black holes.Comment: 9 pages, 5 figures, accepted for publication in PR

Gravitational Waves from a Compact Star in a Circular, Inspiral Orbit, in the Equatorial Plane of a Massive, Spinning Black Hole, as Observed by LISA

Results are presented from high-precision computations of the orbital evolution and emitted gravitational waves for a stellar-mass object spiraling into a massive black hole in a slowly shrinking, circular, equatorial orbit. The focus of these computations is inspiral near the innermost stable circular orbit (isco)---more particularly, on orbits for which the angular velocity Omega is 0.03 < Omega/Omega_{isco} < 1. The computations are based on the Teukolsky-Sasaki-Nakamura formalism, and the results are tabulated in a set of functions that are of order unity and represent relativistic corrections to low-orbital-velocity formulas. These tables can form a foundation for future design studies for the LISA space-based gravitational-wave mission. A first survey of applications to LISA is presented: Signal to noise ratios S/N are computed and graphed as functions of the time-evolving gravitational-wave frequency for representative values of the hole's mass M and spin a and the inspiraling object's mass \mu, with the distance to Earth chosen to be r_o = 1 Gpc. These S/N's show a very strong dependence on the black-hole spin, as well as on M and \mu. A comparison with predicted event rates shows strong promise for detecting these waves, but not beyond about 1Gpc if the inspiraling object is a white dwarf or neutron star. This argues for a modest lowering of LISA's noise floor. A brief discussion is given of the prospects for extracting information from the observed wavesComment: Physical Review D, in press; 21 pages, 9 figures, 10 tables it is present in the RevTeX fil

Evolutionary life history theory as an organising framework for cohort studies : insights from the Cebu Longitudinal Health and Nutrition Survey

By tracking a group of individuals through time, cohort studies provide fundamental insights into the developmental time course and causes of health and disease. Evolutionary life history theory seeks to explain patterns of growth, development, reproduction and senescence, and inspires a range of hypotheses that are testable using the longitudinal data from cohort studies. Here we review two decades of life history theory-motivated work conducted in collaboration with the Cebu Longitudinal Health and Nutrition Survey (CLHNS), a birth cohort study that enrolled more than 3000 pregnant women in the Philippines in 1983 and has since followed these women, their offspring and grandoffspring. This work has provided evidence that reproduction carries “costs” to cellular maintenance functions, potentially speeding senescence, and revealed an unusual form of genetic plasticity in which the length of telomeres inherited across generations is influenced by reproductive timing in paternal ancestors. Men in Cebu experience hormonal and behavioural changes in conjunction with changes in relationship and fatherhood status that are consistent with predictions based upon other species that practice bi-parental care. The theoretical expectation that early life cues of mortality or environmental unpredictability will motivate a “fast” life history strategy are confirmed for behavioural components of reproductive decision making, but not for maturational tempo, while our work points to a broader capacity for early life developmental calibration of systems like immunity, reproductive biology and metabolism. Our CLHNS findings illustrate the power of life history theory as an integrative, lifecourse framework to guide longitudinal studies of human populations

Corporate governance and financial constraints on strategic turnarounds

The paper extends the Robbins and Pearce (1992) two-stage turnaround response model to include governance factors. In addition to the retrenchment and recovery, the paper proposes the addition of a realignment stage, referring specifically to the re-alignment of expectations of principal and agent groups. The realignment stage imposes a threshold that must be crossed before the retrenchment and hence recovery stage can be entered. Crossing this threshold is problematic to the extent that the interests of governance-stakeholder groups diverge in a crisis situation. The severity of the crisis impacts on the bases of strategy contingent asset valuation leading to the fragmentation of stakeholder interests. In some cases the consequence may be that management are prevented from carrying out turnarounds by governance constraints. The paper uses a case study to illustrate these dynamics, and like the Robbins and Pearce study, it focuses on the textile industry. A longitudinal approach is used to show the impact of the removal of governance constraints. The empirical evidence suggests that such financial constraints become less serious to the extent that there is a functioning market for corporate control. Building on governance research and turnaround literature, the paper also outlines the general case necessary and sufficient conditions for successful turnarounds

Two Stellar Components in the Halo of the Milky Way

The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, which can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components -- an inner and an outer halo -- that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.Comment: Two stand-alone files in manuscript, concatenated together. The first is for the main paper, the second for supplementary information. The version is consistent with the version published in Natur

Orbit size and estimated eye size in dinosaurs and other archosaurs and their implications for the evolution of visual capabilities

Vision is one of the most important senses for animals, allowing them to interact with their environment and with further implications for evolutionary histories. However, relevant soft tissues, such as the eye and associated structures, are not preserved in fossil vertebrates, limiting our knowledge of their visual capabilities. Here, we quantified absolute and relative orbit size for 400 species of dinosaurs and other extinct archosaurs using linear measurements of the preserved skeletal elements as a proxy for visual capabilities. Our results demonstrate that the orbit makes up on average 20% of skull size with a strong and consistent correlation across all sampled groups. This trend is largely independent of temporal distribution, species richness, and phylogeny. In fact, relative orbit size is narrowly constrained and did not surpass 45% of skull size, suggesting physiological and functional controls. Estimated eye size was found to be absolutely larger in herbivores, whereas carnivores tended to have smaller eyes absolutely and compared with skull size. Relatively large eyes only occurred in small-bodied species and vice versa. However, eye size alone was not sufficient to discriminate between different activity patterns or to characterize visual capabilities in detail.</p

The Fifth Data Release of the Sloan Digital Sky Survey

This paper describes the Fifth Data Release (DR5) of the Sloan Digital Sky Survey (SDSS). DR5 includes all survey quality data taken through June 2005 and represents the completion of the SDSS-I project (whose successor, SDSS-II will continue through mid-2008). It includes five-band photometric data for 217 million objects selected over 8000 square degrees, and 1,048,960 spectra of galaxies, quasars, and stars selected from 5713 square degrees of that imaging data. These numbers represent a roughly 20% increment over those of the Fourth Data Release; all the data from previous data releases are included in the present release. In addition to "standard" SDSS observations, DR5 includes repeat scans of the southern equatorial stripe, imaging scans across M31 and the core of the Perseus cluster of galaxies, and the first spectroscopic data from SEGUE, a survey to explore the kinematics and chemical evolution of the Galaxy. The catalog database incorporates several new features, including photometric redshifts of galaxies, tables of matched objects in overlap regions of the imaging survey, and tools that allow precise computations of survey geometry for statistical investigations.Comment: ApJ Supp, in press, October 2007. This paper describes DR5. The SDSS Sixth Data Release (DR6) is now public, available from http://www.sdss.or