2,441 research outputs found
Impact of foregrounds on Cosmic Microwave Background maps
We discuss the possible impact of astrophysical foregrounds on three recent
exciting results of Cosmic Microwave Background (CMB) experiments: the WMAP
measurements of the temperature-polarization (TE) correlation power spectrum,
the detection of CMB polarization fluctuations on degree scales by the DASI
experiment, and the excess power on arcminute scales reported by the CBI and
BIMA groups. A big contribution from the Galactic synchrotron emission to the
TE power spectrum on large angular scales is indeed expected, in the lower
frequency WMAP channels, based on current, albeit very uncertain, models; at
higher frequencies the rapid decrease of the synchrotron signal may be, to some
extent, compensated by polarized dust emission. Recent measurements of
polarization properties of extragalactic radio sources at high radio frequency
indicate that their contamination of the CMB polarization on degree scales at
30 GHz is substantially below the expected CMB E-mode amplitude. Adding the
synchrotron contribution, we estimate that the overall foreground contamination
of the signal detected by DASI may be significant but not dominant. The excess
power on arc-min scales detected by the BIMA experiment may be due to
galactic-scale Sunyaev-Zeldovich effects, if the proto-galactic gas is heated
to its virial temperature and its cooling time is comparable to the Hubble time
at the epoch of galaxy formation. A substantial contamination by radio sources
of the signal reported by the CBI group on scales somewhat larger than BIMA's
cannot be easily ruled out.Comment: 10 pages, 5 figures, to appear in proc. int. conf. "Thinking,
Observing and Mining the Universe", Sorrento, Sept. 200
Polarization fluctuations due to extragalactic sources
We have derived the relationship between polarization and intensity
fluctuations due to point sources. In the case of a Poisson distribution of a
population with uniform evolution properties and constant polarization degree,
polarization fluctuations are simply equal to intensity fluctuations times the
average polarization degree. Conservative estimates of the polarization degree
of the classes of extragalactic sources contributing to fluctuations in the
frequency ranges covered by the forthcoming space missions MAP and Planck
Surveyor indicate that extragalactic sources will not be a strong limiting
factor to measurements of the polarization of the Cosmic Microwave Background.Comment: 15 pages LaTeX file, 3 postscript figures. Uses elsart.sty and
elsart.cls Accepted for publication in New Astronom
Theoretical study of the charge transport through C60-based single-molecule junctions
We present a theoretical study of the conductance and thermopower of
single-molecule junctions based on C60 and C60-terminated molecules. We first
analyze the transport properties of gold-C60-gold junctions and show that these
junctions can be highly conductive (with conductances above 0.1G0, where G0 is
the quantum of conductance). Moreover, we find that the thermopower in these
junctions is negative due to the fact that the LUMO dominates the charge
transport, and its magnitude can reach several tens of micro-V/K, depending on
the contact geometry. On the other hand, we study the suitability of C60 as an
anchoring group in single-molecule junctions. For this purpose, we analyze the
transport through several dumbbell derivatives using C60 as anchors, and we
compare the results with those obtained with thiol and amine groups. Our
results show that the conductance of C60-terminated molecules is rather
sensitive to the binding geometry. Moreover, the conductance of the molecules
is typically reduced by the presence of the C60 anchors, which in turn makes
the junctions more sensitive to the functionalization of the molecular core
with appropriate side groups.Comment: 9 pages, 7 figure
Average fractional polarization of extragalactic sources at Planck frequencies
Recent detailed simulations have shown that an insufficiently accurate
characterization of the contamination of unresolved polarized extragalactic
sources can seriously bias measurements of the primordial cosmic microwave
background (CMB) power spectrum if the tensor-to-scalar ratio as
predicted by models currently of special interest (e.g., Starobinsky's
and Higgs inflation). This has motivated a reanalysis of the median
polarization fraction of extragalactic sources (radio-loud AGNs and dusty
galaxies) using data from the \textit{Planck} polarization maps. Our approach,
exploiting the intensity distribution analysis, mitigates or overcomes the most
delicate aspects of earlier analyses based on stacking techniques. By means of
simulations, we have shown that the residual noise bias on the median
polarization fraction, , of extragalactic sources is
generally \simlt 0.1\%. For radio sources, we have found , with no significant dependence on either frequency or flux
density, in good agreement with the earlier estimate and with high-sensitivity
measurements in the frequency range 5--40\,GHz. No polarization signal is
detected in the case of dusty galaxies, implying 90\% confidence upper limits
of \Pi_{\rm dusty}\simlt 2.2\% at 353\,GHz and of \simlt 3.9\% at 217\,GHz.
The contamination of CMB polarization maps by unresolved point sources is
discussed.Comment: 10 pages, 3 figures, 7 tables; revised version. In press on Astronomy
and Astrophysic
WMAP 3yr data with the CCA: anomalous emission and impact of component separation on the CMB power spectrum
The Correlated Component Analysis (CCA) allows us to estimate how the
different diffuse emissions mix in CMB experiments, exploiting also
complementary information from other surveys. It is especially useful to deal
with possible additional components. An application of CCA to WMAP maps
assuming that only the canonical Galactic emissions are present, highlights the
widespread presence of a spectrally flat "synchrotron" component, largely
uncorrelated with the synchrotron template, suggesting that an additional
foreground is indeed required. We have tested various spectral shapes for such
component, namely a power law as expected if it is flat synchrotron, and two
spectral shapes that may fit the spinning dust emission: a parabola in the logS
- log(frequency) plane, and a grey body. Quality tests applied to the
reconstructed CMB maps clearly disfavour two of the models. The CMB power
spectra, estimated from CMB maps reconstructed exploiting the three surviving
foreground models, are generally consistent with the WMAP ones, although at
least one of them gives a significantly higher quadrupole moment than found by
the WMAP team. Taking foreground modeling uncertainties into account, we find
that the mean quadrupole amplitude for the three "good" models is less than 1
sigma below the expectation from the standard LambdaCDM model. Also the other
reported deviations from model predictions are found not to be statistically
significant, except for the excess power at l~40. We confirm the evidence for a
marked North-South asymmetry in the large scale (l < 20) CMB anisotropies. We
also present a first, albeit preliminary, all-sky map of the "anomalous"
component.Comment: 14 pages, 17 figures, submitted to MNRAS, references adde
Polarization Properties of Extragalactic Radio Sources and Their Contribution to Microwave Polarization Fluctuations
We investigate the statistical properties of the polarized emission of
extragalactic radio sources and estimate their contribution to the power
spectrum of polarization fluctuations in the microwave region. The basic
ingredients of our analysis are the NVSS polarization data, the multifrequency
study of polarization properties of the B3-VLA sample (Mack et al. 2002) which
has allowed us to quantify Faraday depolarization effects, and the 15 GHz
survey by Taylor et al. (2001), which has provided strong constraints on the
high-frequency spectral indices of sources. The polarization degree of both
steep- and flat-spectrum at 1.4 GHz is found to be anti-correlated with the
flux density. The median polarization degree at 1.4 GHz of both steep- and
flat-spectrum sources brighter than mJy is . The data by Mack et al. (2002) indicate a substantial mean Faraday
depolarization at 1.4 GHz for steep spectrum sources, while the depolarization
is undetermined for most flat/inverted-spectrum sources. Exploiting this
complex of information we have estimated the power spectrum of polarization
fluctuations due to extragalactic radio sources at microwave frequencies. We
confirm that extragalactic sources are expected to be the main contaminant of
Cosmic Microwave Background (CMB) polarization maps on small angular scales. At
frequencies GHz the amplitude of their power spectrum is expected to be
comparable to that of the -mode of the CMB. At higher frequencies, however,
the CMB dominates.Comment: 10 pages, A&A in pres
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