7,655 research outputs found
Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle
A suggested solution to the dark energy problem is the void model, where
accelerated expansion is replaced by Hubble-scale inhomogeneity. In these
models, density perturbations grow on a radially inhomogeneous background. This
large scale inhomogeneity distorts the spherical Baryon Acoustic Oscillation
feature into an ellipsoid which implies that the bump in the galaxy correlation
function occurs at different scales in the radial and transverse correlation
functions. We compute these for the first time, under the approximation that
curvature gradients do not couple the scalar modes to vector and tensor modes.
The radial and transverse correlation functions are very different from those
of the concordance model, even when the models have the same average BAO scale.
This implies that if void models are fine-tuned to satisfy average BAO data,
there is enough extra information in the correlation functions to distinguish a
void model from the concordance model. We expect these new features to remain
when the full perturbation equations are solved, which means that the radial
and transverse galaxy correlation functions can be used as a powerful test of
the Copernican Principle.Comment: 12 pages, 8 figures, matches published versio
A Chandra Study of Temperature Substructures in Intermediate-Redshift Galaxy Clusters
By analyzing the gas temperature maps created from the Chandra archive data,
we reveal the prevailing existence of temperature substructures on ~100 kpc
scales in the central regions of nine intermediate-redshift (z~0.1) galaxy
clusters, which resemble those found in the Virgo and Coma Clusters. Each
substructure contains a clump of hot plasma whose temperature is about 2-3 keV
higher than the environment, corresponding to an excess thermal energy of
~1E58-1E60 erg per clump. Since if there were no significant non-gravitational
heating sources, these substructures would have perished in 1E8-1E9 yrs due to
thermal conduction and turbulent flows, whose velocity is found to range from
about 200 to 400 km/s, we conclude that the substructures cannot be created and
sustained by inhomogeneous radiative cooling. We also eliminate the
possibilities that the temperature substructures are caused by supernova
explosions, or by the non-thermal X-ray emission due to the
inverse-Comptonization of the CMB photons. By calculating the rising time of
AGN-induced buoyant bubbles, we speculate that the intermittent AGN outbursts
(~ 1E60 erg per burst) may have played a crucial role in the forming of the
high temperature substructures. Our results are supported by recent study of
McNamara & Nulsen (2007), posing a tight observational constraint on future
theoretical and numerical studies.Comment: 31 pages, 7 figures, ApJ accepte
"Nothing new": responses to the introduction of antiretroviral drugs in South Africa.
Interviews conducted in South Africa found that awareness of antiretroviral therapy was generally poor. Antiretroviral drugs were not perceived as new, but one of many alternative therapies for HIV/AIDS. Respondents had more detailed knowledge of indications, effects and how to access alternative treatments, which is bolstered by the active promotion and legitimization of alternative treatments. Many expressed a lack of excitement about the introduction of antiretroviral therapy, and little change in their attitudes concerning the epidemic
Recalibrating the Wide-field Infrared Survey Explorer (WISE) W4 Filter
We present a revised effective wavelength and photometric calibration for the
Wide-field Infrared Survey Explorer (WISE) W4 band, including tests of
empirically motivated modifications to its pre-launch laboratory-measured
relative system response curve. We derived these by comparing measured W4
photometry with photometry synthesised from spectra of galaxies and planetary
nebulae. The difference between measured and synthesised photometry using the
pre-launch laboratory-measured W4 relative system response can be as large as
0.3 mag for galaxies and 1 mag for planetary nebulae. We find the W4 effective
wavelength should be revised upward by 3.3%, from 22.1 micron to 22.8 micron,
and the W4 AB magnitude of Vega should be revised from m = 6.59 to m = 6.66. In
an attempt to reproduce the observed W4 photometry, we tested three
modifications to the pre-launch laboratory-measured W4 relative system response
curve, all of which have an effective wavelength of 22.8 micron. Of the three
relative system response curve models tested, a model that matches the
laboratory-measured relative system response curve, but has the wavelengths
increased by 3.3% (or 0.73 micron) achieves reasonable agreement between the
measured and synthesised photometry.Comment: Accepted for publication in Publications of the Astronomical Society
of Australia, 6 pages, 4 figures, 1 tabl
A comprehensive study of Modulation effects on CMB Polarization
This article does the most general treatment of modulation in Polarization
fields. We have considered both linear polarization fields Q and U & also
scalar polarization modes E and B. We have shown that any arbitrary modulation
in Q and U is allowed but the same can't be done in case of E and B fields.
This result also gives a mathematical justification that the masking can only
be applied to the Q and U fields and never to E and B fields.Comment: 10 pages, 2 figures, minor corrections, to be submitte
The Herschel view of the environment of the radio galaxy 4C+41.17 at z = 3.8
We present Herschel observations at 70, 160, 250, 350 and 500 μm of the environment of the radio galaxy 4C+41.17 at z = 3.792. About 65 per cent of the extracted sources are securely identified with mid-infrared sources observed with the Spitzer Space Telescope at 3.6, 4.5, 5.8, 8 and 24 μm. We derive simple photometric redshifts, also including existing 850 and 1200 μm data, using templates of active galactic nuclei, starburst-dominated systems and evolved stellar populations. We find that most of the Herschel sources are foreground to the radio galaxy and therefore do not belong to a structure associated with 4C+41.17. We do, however, find that the spectral energy distribution (SED) of the closest (∼25 arcsec offset) source to the radio galaxy is fully consistent with being at the same redshift as 4C+41.17. We show that finding such a bright source that close to the radio galaxy at the same redshift is a very unlikely event, making the environment of 4C+41.17 a special case. We demonstrate that multiwavelength data, in particular on the Rayleigh–Jeans side of the SED, allow us to confirm or rule out the presence of protocluster candidates that were previously selected by single wavelength data setsPeer reviewe
Forming the Moon from terrestrial silicate-rich material
Recent high-precision measurements of the isotopic composition of lunar rocks
demonstrate that the bulk silicate Earth and the Moon show an unexpectedly high
degree of similarity. This is inconsistent with one of the primary results of
classic dynamical simulations of the widely accepted giant impact model for the
formation of the Moon, namely that most of the mass of the Moon originates from
the impactor, not Earth.
Resolution of this discrepancy without changing the main premises of the
giant impact model requires total isotopic homogenisation of Earth and impactor
material after the impact for a wide range of elements including O, Si, K, Ti,
Nd and W. Even if this process could explain the O isotope similarity, it is
unlikely to work for the much heavier, refractory elements. Given the
increasing uncertainty surrounding the giant impact model in light of these
geochemical data, alternative hypotheses for lunar formation should be
explored. In this paper, we revisit the hypothesis that the Moon was formed
directly from terrestrial mantle material. We show that the dynamics of this
scenario requires a large amount of energy, almost instantaneously generated
additional energy. The only known source for this additional energy is nuclear
fission. We show that it is feasible to form the Moon through the ejection of
terrestrial silicate material triggered by a nuclear explosion at Earths
core-mantle boundary (CMB), causing a shock wave propagating through the Earth.
Hydrodynamic modelling of this scenario shows that a shock wave created by
rapidly expanding plasma resulting from the explosion disrupts and expels
overlying mantle and crust material.Comment: 26 pages, 5 figures, 1 tabl
Evolution of faint radio sources in the VIDEO-XMM3 field
© 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical SocietyIt has been speculated that low-luminosity radio-loud active galactic nuclei (AGN) have the potential to serve as an important source of AGN feedback, and may be responsible for suppressing star formation activity in massive elliptical galaxies at late times. As such the cosmic evolution of these sources is vitally important to understand the significance of such AGN feedback processes and their influence on the global star formation history of the Universe. In this paper, we present a new investigation of the evolution of faint radio sources out to z ~ 2.5. We combine a 1 square degree Very Large Array radio survey, complete to a depth of 100 μJy, with accurate 10 band photometric redshifts from the following surveys: Visible and Infrared Survey Telescope for Astronomy Deep Extragalactic Observations and Canada-France-Hawaii Telescope Legacy Survey. The results indicate that the radio population experiences mild positive evolution out to z ~ 1.2 increasing their space density by a factor of ~3, consistent with results of several previous studies. Beyond z = 1.2, there is evidence of a slowing down of this evolution. Star-forming galaxies drive the more rapid evolution at low redshifts, z 1.2. The evolution is best fitted by pure luminosity evolution with star-forming galaxies evolving as (1 + z)2.47 ± 0.12 and AGN as (1 + z)1.18 ± 0.21M.Peer reviewe
The QUIET Instrument
The Q/U Imaging ExperimenT (QUIET) is designed to measure polarization in the
Cosmic Microwave Background, targeting the imprint of inflationary
gravitational waves at large angular scales (~ 1 degree). Between 2008 October
and 2010 December, two independent receiver arrays were deployed sequentially
on a 1.4 m side-fed Dragonian telescope. The polarimeters which form the focal
planes use a highly compact design based on High Electron Mobility Transistors
(HEMTs) that provides simultaneous measurements of the Stokes parameters Q, U,
and I in a single module. The 17-element Q-band polarimeter array, with a
central frequency of 43.1 GHz, has the best sensitivity (69 uK sqrt(s)) and the
lowest instrumental systematic errors ever achieved in this band, contributing
to the tensor-to-scalar ratio at r < 0.1. The 84-element W-band polarimeter
array has a sensitivity of 87 uK sqrt(s) at a central frequency of 94.5 GHz. It
has the lowest systematic errors to date, contributing at r < 0.01. The two
arrays together cover multipoles in the range l= 25-975. These are the largest
HEMT-based arrays deployed to date. This article describes the design,
calibration, performance of, and sources of systematic error for the
instrument
Extending BEAMS to incorporate correlated systematic uncertainties
New supernova surveys such as the Dark Energy Survey, Pan-STARRS and the LSST
will produce an unprecedented number of photometric supernova candidates, most
with no spectroscopic data. Avoiding biases in cosmological parameters due to
the resulting inevitable contamination from non-Ia supernovae can be achieved
with the BEAMS formalism, allowing for fully photometric supernova cosmology
studies. Here we extend BEAMS to deal with the case in which the supernovae are
correlated by systematic uncertainties. The analytical form of the full BEAMS
posterior requires evaluating 2^N terms, where N is the number of supernova
candidates. This `exponential catastrophe' is computationally unfeasible even
for N of order 100. We circumvent the exponential catastrophe by marginalising
numerically instead of analytically over the possible supernova types: we
augment the cosmological parameters with nuisance parameters describing the
covariance matrix and the types of all the supernovae, \tau_i, that we include
in our MCMC analysis. We show that this method deals well even with large,
unknown systematic uncertainties without a major increase in computational
time, whereas ignoring the correlations can lead to significant biases and
incorrect credible contours. We then compare the numerical marginalisation
technique with a perturbative expansion of the posterior based on the insight
that future surveys will have exquisite light curves and hence the probability
that a given candidate is a Type Ia will be close to unity or zero, for most
objects. Although this perturbative approach changes computation of the
posterior from a 2^N problem into an N^2 or N^3 one, we show that it leads to
biases in general through a small number of misclassifications, implying that
numerical marginalisation is superior.Comment: Resubmitted under married name Lochner (formally Knights). Version 3:
major changes, including a large scale analysis with thousands of MCMC
chains. Matches version published in JCAP. 23 pages, 8 figure
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