904 research outputs found
QSO environments at intermediate redshifts
We have made a survey of quasar environments at 0.5 < z < 0.8, using a sample
of both radio-loud and radio-quiet quasars matched in B-band luminosity. Our
observations include images of background control fields to provide a good
determination of the field galaxy counts. About 10 per cent of the quasars
appear to live in rich clusters, whereas approximately 45 per cent live in
environments similar to that of field galaxies.
The richness of galaxies within a 0.5 Mpc radius around the radio-quiet
quasars is found to be indistinguishable from the richness around the
radio-loud quasars, corresponding on average to groups or poorer clusters of
galaxies. Comparing the galaxy richness in the radio-loud quasar fields with
quasar fields in the literature, we find no evidence of an evolution in the
environment with epoch. Instead, a weak, but significant correlation between
quasar radio luminosity and environmental richness is present. It is thus
possible that the environments of quasars, at least the powerful ones, do not
evolve much between the present epoch and z \approx 0.8.Comment: 6 pages, 4 figures, to appear in the proc. of the workshop `QSO hosts
and their environments', IAA, Granada 10-12 Jan, 200
Asymmetries in the CMB anisotropy field
We report on the results from two independent but complementary statistical
analyses of the WMAP first-year data, based on the power spectrum and N-point
correlation functions. We focus on large and intermediate scales (larger than
about 3 degrees) and compare the observed data against Monte Carlo ensembles
with WMAP-like properties. In both analyses, we measure the amplitudes of the
large-scale fluctuations on opposing hemispheres and study the ratio of the two
amplitudes. The power-spectrum analysis shows that this ratio for WMAP, as
measured along the axis of maximum asymmetry, is high at the 95%-99% level
(depending on the particular multipole range included). The axis of maximum
asymmetry of the WMAP data is weakly dependent on the multipole range under
consideration but tends to lie close to the ecliptic axis. In the N-point
correlation function analysis we focus on the northern and southern hemispheres
defined in ecliptic coordinates, and we find that the ratio of the large-scale
fluctuation amplitudes is high at the 98%-99% level. Furthermore, the results
are stable with respect to choice of Galactic cut and also with respect to
frequency band. A similar asymmetry is found in the COBE-DMR map, and the axis
of maximum asymmetry is close to the one found in the WMAP data.Comment: 6 pages, 5 figures; version to appear in ApJ, textual improvements,
added reference
Testing for Non-Gaussianity in the Wilkinson Microwave Anisotropy Probe Data: Minkowski Functionals and the Length of the Skeleton
The three Minkowski functionals and the recently defined length of the
skeleton are estimated for the co-added first-year Wilkinson Microwave
Anisotropy Probe (WMAP) data and compared with 5000 Monte Carlo simulations,
based on Gaussian fluctuations with the a-priori best-fit running-index power
spectrum and WMAP-like beam and noise properties. Several power
spectrum-dependent quantities, such as the number of stationary points, the
total length of the skeleton, and a spectral parameter, gamma, are also
estimated. While the area and length Minkowski functionals and the length of
the skeleton show no evidence for departures from the Gaussian hypothesis, the
northern hemisphere genus has a chi^2 that is large at the 95% level for all
scales. For the particular smoothing scale of 3.40 degrees FWHM it is larger
than that found in 99.5% of the simulations. In addition, the WMAP genus for
negative thresholds in the northern hemisphere has an amplitude that is larger
than in the simulations with a significance of more than 3 sigma. On the
smallest angular scales considered, the number of extrema in the WMAP data is
high at the 3 sigma level. However, this can probably be attributed to the
effect of point sources. Finally, the spectral parameter gamma is high at the
99% level in the northern Galactic hemisphere, while perfectly acceptable in
the southern hemisphere. The results provide strong evidence for the presence
of both non-Gaussian behavior and an unexpected power asymmetry between the
northern and southern hemispheres in the WMAP data.Comment: 17 pages, 10 figures, accepted for publication in Ap
Increasing evidence for hemispherical power asymmetry in the five-year WMAP data
(Abridged)Motivated by the recent results of Hansen et al. (2008) concerning
a noticeable hemispherical power asymmetry in the WMAP data on small angular
scales, we revisit the dipole modulated signal model introduced by Gordon et
al. (2005). This model assumes that the true CMB signal consists of a Gaussian
isotropic random field modulated by a dipole, and is characterized by an
overall modulation amplitude, A, and a preferred direction, p. Previous
analyses of this model has been restricted to very low resolution due to
computational cost. In this paper, we double the angular resolution, and
compute the full corresponding posterior distribution for the 5-year WMAP data.
The results from our analysis are the following: The best-fit modulation
amplitude for l <= 64 and the ILC data with the WMAP KQ85 sky cut is A=0.072
+/- 0.022, non-zero at 3.3sigma, and the preferred direction points toward
Galactic coordinates (l,b) = (224 degree, -22 degree) +/- 24 degree. The
corresponding results for l <~ 40 from earlier analyses was A = 0.11 +/- 0.04
and (l,b) = (225 degree,-27 degree). The statistical significance of a non-zero
amplitude thus increases from 2.8sigma to 3.3sigma when increasing l_max from
40 to 64, and all results are consistent to within 1sigma. Similarly, the
Bayesian log-evidence difference with respect to the isotropic model increases
from Delta ln E = 1.8 to Delta ln E = 2.6, ranking as "strong evidence" on the
Jeffreys' scale. The raw best-fit log-likelihood difference increases from
Delta ln L = 6.1 to Delta ln L = 7.3. Similar, and often slightly stronger,
results are found for other data combinations. Thus, we find that the evidence
for a dipole power distribution in the WMAP data increases with l in the 5-year
WMAP data set, in agreement with the reports of Hansen et al. (2008).Comment: 6 pages, 2 figures; added references and minor comments. Accepted for
publication in Ap
Radio-quiet quasar environments at 0.5 < z < 0.8
We have quantified the galaxy environments around a sample of 0.5 < z < 0.8
radio-quiet quasars using the amplitude of the spatial galaxy--quasar
correlation function, B_gq. The quasars exist in a wide variety of
environments, some sources are located in clusters as rich as Abell class 1--2
clusters, whereas others exist in environments comparable to the field. We find
that on average, the quasars prefer poorer clusters of approximately Abell
class 0, which suggests that quasars are biased tracers of mass compared to
galaxies.
The mean B_gq for the sample is found to be indistinguishable from the mean
amplitude for a sample of radio-loud quasars matched in redshift and optical
luminosity. These observations are consistent with recent studies of the hosts
of radio-quiet quasars at low to intermediate redshifts, and suggest that the
mechanism for the production of powerful radio jets in radio-loud quasars is
controlled by processes deep within the active galactic nucleus itself, and is
unrelated to the nature of the hosts or their environments.Comment: 19 pages, 8 figures, accepted for publication in MNRA
Estimating N-Point Correlation Functions from Pixelized Sky Maps
We develop, implement and test a set of algorithms for estimating N-point
correlation functions from pixelized sky maps. These algorithms are slow, in
the sense that they do not break the O(N_pix^N) barrier, and yet, they are fast
enough for efficient analysis of data sets up to several hundred thousand
pixels. The typical application of these methods is Monte Carlo analysis using
several thousand realizations, and therefore we organize our programs so that
the initialization cost is paid only once. The effective cost is then reduced
to a few additions per pixel multiplet (pair, triplet etc.). Further, the
algorithms waste no CPU time on computing undesired geometric configurations,
and, finally, the computations are naturally divided into independent parts,
allowing for trivial (i.e., optimal) parallelization.Comment: 11 pages, 8 figures, accepted for publication in ApJS; textual
improvements, references update
Surface Superconductivity in Niobium for Superconducting RF Cavities
A systematic study is presented on the superconductivity (sc) parameters of
the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz
cavities for the linear collider project TESLA. Cylindrical Nb samples have
been subjected to the same surface treatments that are applied to the TESLA
cavities: buffered chemical polishing (BCP), electrolytic polishing (EP),
low-temperature bakeout (LTB). The magnetization curves and the complex
magnetic susceptibility have been measured over a wide range of temperatures
and dc magnetic fields, and also for di erent frequencies of the applied ac
magnetic field. The bulk superconductivity parameters such as the critical
temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found
to be in good agreement with previous data. Evidence for surface
superconductivity at fields above Bc2 is found in all samples. The critical
surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in
BCP-treated samples and increases even further if EP or LTB are applied. From
the field dependence of the susceptibility and a power-law analysis of the
complex ac conductivity and resistivity the existence of two different phases
of surface superconductivity can be established which resemble the Meissner and
Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing
sc shielding currents flowing around the entire cylindrical sample, for
external fields B in the range between Bc2 and Bcohc3, and (2) incoherent
surface superconductivity with disconnected sc domains between Bcohc3 and Bc3.
The coherent critical surface field separating the two phases is found to be
Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are
different for BCP and EP samples, pointing to different surface topologies.Comment: 15 pages, 21 figures, DESY-Report 2004-02
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