869 research outputs found
Radio and optical orientations of galaxies
We investigate the correlations between optical and radio isophotal position
angles for 14302 SDSS galaxies with magnitudes brighter than 18 and which
have been associated with extended FIRST radio sources. We identify two
separate populations of galaxies using the colour, concentration and their
principal components. Surprisingly strong statistical alignments are found:
late-type galaxies are overwhelmingly biased towards a position angle
differences of and early-type galaxies to . The
late-type alignment can be easily understood in terms of the standard picture
in which the radio emission is intimately related to areas of recent
star-formation. In early-type galaxies the radio emission is expected to be
driven by accretion on to a nuclear black hole. We argue that the observed
correlation of the radio axis with the minor axis of the large-scale stellar
distribution gives a fundamental insight into the structure of elliptical
galaxies, for example, whether or not the nuclear kinematics are decoupled form
the rest of the galaxy. Our results imply that the galaxies are oblate
spheroids with their radio emission aligned with the minor axis. Remarkably the
strength of the correlation of the radio major axis with the optical minor axis
depends on radio loudness. Those objects with a low ratio of FIRST radio flux
density to total stellar light show a strong minor axis correlation while the
stronger radio sources do not. This may reflect different formation histories
for the different objects and we suggest we may be seeing the different
behaviour of rationally supported and non-rotationally supported ellipticals.Comment: Version to appear in MNRA
On the origin of dark matter axions
We discuss the possible sources of dark matter axions in the early universe.
In the standard thermal scenario, an axion string network forms at the
Peccei-Quinn phase transition T\sim \fa and then radiatively decays into a
cosmological background of axions; to be the dark matter, these axions must
have a mass \ma \sim 100 \mu eV with specified large uncertainties. An
inflationary phase with a reheat temperature below the PQ-scale T_{reh} \lapp
\fa can also produce axion strings through quantum fluctuations, provided that
the Hubble parameter during inflation is large H_1 \gapp \fa; this case again
implies a dark matter axion mass \ma \sim 100 \mu eV. For a smaller Hubble
parameter during inflation H_1 \lapp \fa, `anthropic tuning' allows dark
matter axions to have any mass in a huge range below \ma\lapp 1 meV.Comment: to be published in the proceedings of the 5th IFT Workshop on Axion
Cosmological constraints from Sunyaev-Zeldovich cluster counts: an approach to account for missing redshifts
The accumulation of redshifts provides a significant observational bottleneck
when using galaxy cluster surveys to constrain cosmological parameters. We
propose a simple method to allow the use of samples where there is a fraction
of the redshifts that are not known. The simplest assumption is that the
missing redshifts are randomly extracted from the catalogue, but the method
also allows one to take into account known selection effects in the
accumulation of redshifts. We quantify the reduction in statistical precision
of cosmological parameter constraints as a function of the fraction of missing
redshifts for simulated surveys, and also investigate the impact of making an
incorrect assumption for the distribution of missing redshifts.Comment: 6 pages, 5 figures, accepted by Ap
Regularized braneworlds of arbitrary codimension
We consider a thick p-brane embedded in an n-dimensional spacetime possessing
radial symmetry in the directions orthogonal to the brane. We first consider a
static brane, and find a general fine tuning relationship between the brane and
bulk parameters required for the brane to be flat. We then consider the
cosmology of a time dependent brane in a static bulk, and find the Friedmann
equation for the brane scale factor a(t). The singularities that would
ordinarily arise when considering arbitrary codimensions are avoided by
regularizing the brane, giving it a finite profile in the transverse
dimensions. However, since we consider the brane to be a strictly local defect,
we find that the transverse dimensions must have infinite volume, and hence
gravity cannot be localized on the brane without resorting to some infra-red
cutoff.Comment: 21 page
f(R) as a dark energy fluid
We study the equations for the evolution of cosmological perturbations in
and conclude that this modified gravity model can
be expressed as a dark energy fluid at background and linearised perturbation
order. By eliminating the extra scalar degree of freedom known to be present in
such theories, we are able to characterise the evolution of the perturbations
in the scalar sector in terms of equations of state for the entropy
perturbation and anisotropic stress which are written in terms of the density
and velocity perturbations of the dark energy fluid and those in the matter, or
the metric perturbations. We also do the same in the much simpler vector and
tensor sectors. In order to illustrate the simplicity of this formulation, we
numerically evolve perturbations in a small number of cases.Comment: 12 pages, 5 figure
The Cosmic Microwave Background and the Ionization History of the Universe
Details of how the primordial plasma recombined and how the universe later
reionized are currently somewhat uncertain. This uncertainty can restrict the
accuracy of cosmological parameter measurements from the Cosmic Microwave
Background (CMB). More positively, future CMB data can be used to constrain the
ionization history using observations. We first discuss how current
uncertainties in the recombination history impact parameter constraints, and
show how suitable parameterizations can be used to obtain unbiased parameter
estimates from future data. Some parameters can be constrained robustly,
however there is clear motivation to model recombination more accurately with
quantified errors. We then discuss constraints on the ionization fraction
binned in redshift during reionization. Perfect CMB polarization data could in
principle distinguish different histories that have the same optical depth. We
discuss how well the Planck satellite may be able to constrain the ionization
history, and show the currently very weak constraints from WMAP three-year
data.Comment: Changes to match MNRAS accepted versio
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