29,197 research outputs found
The Phase-Space Density Profiles of Cold Dark Matter Halos
We examine the coarse-grained phase-space density profiles of a set of
recent, high-resolution simulations of galaxy-sized Cold Dark Matter (CDM)
halos. Over two and a half decades in radius the phase-space density closely
follows a power-law, , with . This behaviour matches the self-similar solution obtained by
Bertschinger for secondary infall in a uniformly expanding universe. On the
other hand, the density profile corresponding to Bertschinger's solution (a
power-law of slope ) differs significantly from the density
profiles of CDM halos. We show that isotropic mass distributions with power-law
phase-space density profiles form a one-parameter family of structures
controlled by , the ratio of the velocity dispersion to the peak
circular velocity. For one recovers the power-law
solution . For larger than some critical
value, , solutions become non-physical, leading to negative
densities near the center. The critical solution, , has
the narrowest phase-space density distribution compatible with the power-law
phase-space density stratification constraint. Over three decades in radius the
critical solution is indistinguishable from an NFW profile. Our results thus
suggest that the NFW profile is the result of a hierarchical assembly process
that preserves the phase-space stratification of Bertschinger's infall model
but which ``mixes'' the system maximally, perhaps as a result of repeated
merging.Comment: 16 pages, 4 figures; submitted to The Astrophysical Journa
Anomalous low temperature specific heat of He-3 inside nanotube bundles
Helium atoms and hydrogen molecules can be strongly bound inside interstitial
channels within bundles of carbon nanotubes. An exploration of the low energy
and low temperature properties of He-3 atoms is presented here. Recent study of
the analogous He-4 system has shown that the effect of heterogeneity is to
yield a density of states N(E) that is qualitatively different from the
one-dimensional (1D) form of N(E) that would occur for an ideal set of
identical channels. In particular, the functional form of N(E) is that of a 4D
gas near the very lowest energies and a 2D gas at somewhat higher energies.
Similar behavior is found here for He-3. The resulting thermodynamic behavior
of this fermi system is computed, yielding an anomalous form of the heat
capacity and its dependence on coverage.Comment: 11 pages, 6 figure
Wide Angle Redshift Distortions Revisited
We explore linear redshift distortions in wide angle surveys from the point
of view of symmetries. We show that the redshift space two-point correlation
function can be expanded into tripolar spherical harmonics of zero total
angular momentum . The
coefficients of the expansion are analogous to the 's of
the angular power spectrum, and express the anisotropy of the redshift space
correlation function. Moreover, only a handful of are
non-zero: the resulting formulae reveal a hidden simplicity comparable to
distant observer limit. The depend on spherical Bessel
moments of the power spectrum and . In the plane parallel
limit, the results of \cite{Kaiser1987} and \cite{Hamilton1993} are recovered.
The general formalism is used to derive useful new expressions. We present a
particularly simple trigonometric polynomial expansion, which is arguably the
most compact expression of wide angle redshift distortions. These formulae are
suitable to inversion due to the orthogonality of the basis functions. An
alternative Legendre polynomial expansion was obtained as well. This can be
shown to be equivalent to the results of \cite{SzalayEtal1998}. The simplicity
of the underlying theory will admit similar calculations for higher order
statistics as well.Comment: 6 pages, 1 figure, ApJL submitte
Correlation Between the Halo Concentration (c) and the Virial Mass (Mvir) Determined from X-ray Clusters
Numerical simulations of structure formation have suggested that there exists
a good correlation between the halo concentration c (or the characteristic
density delta_c) and the virial mass Mvir for any virialized dark halo
described by the Navarro, Frenk & White (1995) density profile. In this Letter,
we present an observational determination of the c-Mvir (or delta_c-Mvir)
relation in the mass range of 10^14< Mvir <10^16 (solar mass) using a sample of
63 X-ray luminous clusters. The best-fit power law relation, which is roughly
independent of the values of Omega_M and Lambda, is c propto Mvir^(-0.5) or
delta_c propto Mvir^(-1.2), indicating n=-0.7 for a scale-free power spectrum
of the primordial density fluctuations. We discuss the possible reasons for the
conflict with the predictions by typical CDM models such as SCDM, LCDM and
OCDM.Comment: 13 pages, 1 figure, two tables. Accepted for publication in ApJ
Majorana spinors and extended Lorentz symmetry in four-dimensional theory
An extended local Lorentz symmetry in four-dimensional (4D) theory is
considered. A source of this symmetry is a group of general linear
transformations of four-component Majorana spinors GL(4,M) which is isomorphic
to GL(4,R) and is the covering of an extended Lorentz group in a 6D Minkowski
space M(3,3) including superluminal and scaling transformations. Physical
space-time is assumed to be a 4D pseudo-Riemannian manifold. To connect the
extended Lorentz symmetry in the M(3,3) space with the physical space-time, a
fiber bundle over the 4D manifold is introduced with M(3,3) as a typical fiber.
The action is constructed which is invariant with respect to both general 4D
coordinate and local GL(4,M) spinor transformations. The components of the
metric on the 6D fiber are expressed in terms of the 4D pseudo-Riemannian
metric and two extra complex fields: 4D vector and scalar ones. These extra
fields describe in the general case massive particles interacting with an extra
U(1) gauge field and weakly interacting with ordinary particles, i.e.
possessing properties of invisible (dark) matter.Comment: 24 page
Supporting and enabling scholarship: developing and sharing expertise in online learning and teaching
In a highly competitive, rapidly changing higher education market, universities need to be able to generate pedagogical expertise quickly and ensure that it is applied to practice. Since teaching approaches are constantly evolving, partly responding to emerging learning technologies, there is a need to foster ways to keep abreast on an ongoing basis. This paper explores how a small-scale project, the Teaching Online Panel (TOP), used scholarship investigations and a bottom-up approach to enhance one particular aspect of academic practice ? online learning and teaching. The experiences of TOP are useful for identifying: - how a scholarship approach can help develop academic expertise - its contribution to enhancing understanding of staff?s different roles in the University - ways of developing the necessary supportive network for those undertaking such scholarship - the effectiveness of staff development which is peer-led rather than imposed from above - how practical examples can stimulate practice development - the relevance of literature on communities of practice and landscapes of practice for scholarship - the important role of ?brokers? to facilitate the dissemination of scholarship findings - the benefits to the brokers? own professional roles - the challenges of sustaining such an approach and lessons learnt. This study has relevance for those involved in supporting scholarship or delivering staff development in Higher Education
The Evolution of the Galaxy Sizes in the NTT Deep Field: a Comparison with CDM Models
The sizes of the field galaxies with I<25 have been measured in the NTT Deep
Field. Intrinsic sizes have been obtained after deconvolution of the PSF with a
multigaussian method. The reliability of the method has been tested using both
simulated data and HST observations of the same field. The distribution of the
half light radii is peaked at r_{hl} 0.3 arcsec, in good agreement with that
derived from HST images at the same magnitude. An approximate morphological
classification has been obtained using the asymmetry and concentration
parameters. The intrinsic sizes of the galaxies are shown as a function of
their redshifts and absolute magnitudes using photometric redshifts derived
from the multicolor catalog. While the brighter galaxies with morphological
parameters typical of the normal spirals show a flat distribution in the range
r_{d}=1-6 kpc, the fainter population at 0.4<z<0.8 dominates at small sizes. To
explore the significance of this behaviour, an analytical rendition of the
standard CDM model for the disc size evolution has been computed. The model
showing the best fit to the local luminosity function and the Tully-Fisher
relation is able to reproduce at intermediate redshifts a size distribution in
general agreement with the observations, although it tends to underestimate the
number of galaxies fainter than M_B~ -19 with disk sizes r_d~ 1-2 kpc.Comment: 16 pages, 11 figures, ApJ in press, Dec 199
Comparing the Evolution of the Galaxy Disk Sizes with CDM Models: The Hubble Deep Field
The intrinsic sizes of the field galaxies with I<26 in the Hubble and ESO-NTT
Deep Fields are shown as a function of their redshifts and absolute magnitudes
using photometric redshifts derived from the multicolor catalogs and are
compared with the CDM predictions. Extending to lower luminosities and to
higher z our previous analysis performed on the NTT field alone, we find that
the distribution of the galaxy disk sizes at different cosmic epochs is within
the range predicted by typical CDM models. However, the observed size
distribution of faint (M_B>-19) galaxies is skewed with respect to the CDM
predictions and an excess of small-size disks (R_d<2 kpc) is already present at
z~ 0.5. The excess persists up to z~3 and involves brighter galaxies . Such an
excess may be reduced if luminosity-dependent effects, like starburst activity
in interacting galaxies, are included in the physical mechanisms governing the
star formation history in CDM models.Comment: 9 pages, 3 figures, ApJ Letters in pres
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