602 research outputs found
Quasar clustering: evidence for an increase with redshift and implications for the nature of AGNs
The evolution of quasar clustering is investigated with a new sample of 388
quasars with 0.3<z<=2.2, B<=20.5 and Mb<-23, selected over an area of 24.6 sq.
deg. in the South Galactic Pole. Assuming a two-point correlation function of
the form xi(r) = (r/r_o)^-1.8, we detect clustering with r_0=6.2 +/- 1.6 h^-1
comoving Mpc at an average redshift of z=1.3. We find a 2 sigma significant
increase of the quasar clustering between z=0.95 and z=1.8, independent of the
quasar absolute magnitude and inconsistent with recent evidence on the
evolution of galaxy clustering. If other quasar samples are added (resulting in
a total data-set of 737 quasars) the increase of the quasar clustering is still
favoured although it becomes less significant. We find epsilon=-2.5.
Evolutionary parameters epsilon>0.0 are excluded at a 0.3% probability level,
to be compared with epsilon=0.8 found for galaxies. The observed clustering
properties appear qualitatively consistent with a scenario of Omega=1 CDM in
which a) the difference between the quasar and the galaxy clustering can be
explained as a difference in the effective bias and redshift distributions, and
b) the quasars, with a lifetime of t~10^8 yr, sparsely sample halos of mass
greater than M_min~10^12-10^13 h^-1 M_sun. We discuss also the possibility that
the observed change in the quasar clustering is due to an increase in the
fraction of early-type galaxies as quasar hosts at high z.Comment: 8 pages including 2 eps figures, LaTeX (AAS v4.0), ApJ in pres
Thousands of Single Nucleotide Polymorphisms in the Critically Endangered Kemp\u27s Ridley Sea Turtle (Lepidochelys kempii) Revealed by Double-Digest Restriction-Associated DNA Sequencing: Opportunities for Previously Elusive Conservation Genetics Research
The velocity distribution of nearby stars from Hipparcos data II. The nature of the low-velocity moving groups
The velocity distribution of nearby stars contains many "moving groups" that
are inconsistent with the standard assumption of an axisymmetric,
time-independent, and steady-state Galaxy. We study the age and metallicity
properties of the low-velocity moving groups based on the reconstruction of the
local velocity distribution in Paper I of this series. We perform stringent,
conservative hypothesis testing to establish for each of these moving groups
whether it could conceivably consist of a coeval population of stars. We
conclude that they do not: the moving groups are not trivially associated with
their eponymous open clusters nor with any other inhomogeneous star formation
event. Concerning a possible dynamical origin of the moving groups, we test
whether any of the moving groups has a higher or lower metallicity than the
background population of thin disk stars, as would generically be the case if
the moving groups are associated with resonances of the bar or spiral
structure. We find clear evidence that the Hyades moving group has higher than
average metallicity and weak evidence that the Sirius moving group has lower
than average metallicity, which could indicate that these two groups are
related to the inner Lindblad resonance of the spiral structure. Further we
find weak evidence that the Hercules moving group has higher than average
metallicity, as would be the case if it is associated with the bar's outer
Lindblad resonance. The Pleiades moving group shows no clear metallicity
anomaly, arguing against a common dynamical origin for the Hyades and Pleiades
groups. Overall, however, the moving groups are barely distinguishable from the
background population of stars, raising the likelihood that the moving groups
are associated with transient perturbations. [abridged
Foregrounds for observations of the cosmological 21 cm line: I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field
We present the first results from a series of observations conducted with the
Westerbork telescope in the 140--160 MHz range with a 2 arcmin resolution aimed
at characterizing the properties of the foregrounds for epoch of reionization
experiments. For the first time we have detected fluctuations in the Galactic
diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low
Galactic latitude area known as Fan region. Those fluctuations have an of
14 K. The total intensity power spectrum shows a power--law behaviour down to
with slope . The detection of
diffuse emission at smaller angular scales is limited by residual point
sources. We measured an confusion noise of 3 mJy beam.
Diffuse polarized emission was also detected for the first time at this
frequency. The polarized signal shows complex structure both spatially and
along the line of sight. The polarization power spectrum shows a power--law
behaviour down to with slope .
The of polarization fluctuations is 7.2 K on 4 arcmin scales. By
extrapolating the measured spectrum of total intensity emission, we find a
contamination on the cosmological signal of K on 5 arcmin scales and a corresponding value
of 18.3 K at the same angular scale. The level of the polarization power
spectrum is K on 5 arcmin scales. Given its exceptionally
bright polarized signal, the Fan region is likely to represent an upper limit
on the sky brightness at moderate and high Galactic latitude.Comment: Minor corrections made to match the final version printed on A&A. A
version with high resolution figures is available at
http://www.astro.rug.nl/~bernardi/FAN/fan.pd
Ring opening polymerisation of lactide with uranium(IV) and cerium(IV) phosphinoaryloxide complexes
A slitless spectroscopic survey for quasars near quasars
We present the results of the Quasars near Quasars (QNQ) survey, a CCD-based
slitless spectroscopic survey for faint V<22 quasars at 1.7<z<3.6 on 18
26.2'x33.5' fields centred on bright quasars at 2.76<z<4.69. In total 169
quasar candidates with emission lines were selected from the extracted
flux-calibrated spectra on the basis of well-defined automatic selection
criteria followed by visual inspection and verification. With follow-up
spectroscopy of 81 candidates that were likely to reside at z>1.7 we were able
to confirm 80 new quasars at 0.580<z<3.586 on 16 of our fields. 64 of the newly
discovered quasars are located at z>1.7. The overall high success rate implies
that most of the remaining 88 candidates are quasars as well, although the
majority of them likely resides at z<1.7 on the basis of the observed line
shapes and strengths. Due to the insufficient depth of the input source
catalogues needed for extraction of the slitless spectra our survey is not well
defined in terms of limiting magnitude for faint 2.5<z<3.6 quasars whose Lyman
alpha emission is detectable well beyond V=22, albeit at a continuum S/N<1.
While not useful for characterising the evolving space density of quasars, our
sample provides many new closely spaced quasar sightlines around intensely
studied quasars for further investigations on the three-dimensional
distribution of the intergalactic medium.Comment: 18 pages, 16 figures, accepted by A&A, includes Table 2 from online
material, full paper with appendices containing additional figures and tables
available at http://www.aip.de/People/gworseck/qnqpaper/qnqpaper_final.pd
Past, present, and future roles of long-term experiments in the LTER Network
Author Posting. © American Institute of Biological Sciences, 2012. This article is posted here by permission of American Institute of Biological Sciences for personal use, not for redistribution. The definitive version was published in BioScience 62 (2012): 377-389, doi:10.1525/bio.2012.62.4.9.The US National Science Foundation—funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after many years of study; (b) influenced management and policy decisions; and (c) evolved into research platforms supporting studies and involving investigators who were not part of the original design. Furthermore, this suite of long-term experiments addresses, at the site level, all of the US National Research Council's Grand Challenges in Environmental Sciences. Despite these contributions, we argue that the scale and scope of global environmental change requires a more-coordinated multisite approach to long-term experiments. Ideally, such an approach would include a network of spatially extensive multifactor experiments, designed in collaboration with ecological modelers that would build on and extend the unique context provided by the LTER Network.2012-10-0
Joint Cosmological Formation of QSOs and Bulge-dominated Galaxies
Older and more recent pieces of observational evidence suggest a strong
connection between QSOs and galaxies; in particular, the recently discovered
correlation between black hole and galactic bulge masses suggests that QSO
activity is directly connected to the formation of galactic bulges. The
cosmological problem of QSO formation is analyzed in the framework of an
analytical model for galaxy formation; for the first time a joint comparison
with galaxy and QSO observables is performed. In this model it is assumed that
the same physical variable which determines galaxy morphology is able to
modulate the mass of the black hole responsible for QSO activity. Both halo
spin and the occurence of a major merger are considered as candidates to this
role. The predictions of the model are compared to available data for the
type-dependent galaxy mass functions, the star-formation history of elliptical
galaxies, the QSO luminosity function and its evolution (including the obscured
objects contributing to the hard-X-ray background), the mass function of
dormant black holes and the distribution of black-hole -- bulge mass ratios. A
good agreement with observations is obtained if the halo spin modulates the
efficiency of black-hole formation, and if the galactic halos at have
shone in an inverted order with respect to the hierarchical one (i.e., stars
and black holes in bigger galactic halos have formed before those in smaller
ones). This inversion of hierarchical order for galaxy formation, which
reconciles galaxy formation with QSO evolution, is consistent with many pieces
of observational evidence.Comment: 20 pages, figures included, mn.sty, in press on MNRAS, fig 6 changed
(new data added at z=4.4
Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector
The inclusive and dijet production cross-sections have been measured for jets
containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass
energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The
measurements use data corresponding to an integrated luminosity of 34 pb^-1.
The b-jets are identified using either a lifetime-based method, where secondary
decay vertices of b-hadrons in jets are reconstructed using information from
the tracking detectors, or a muon-based method where the presence of a muon is
used to identify semileptonic decays of b-hadrons inside jets. The inclusive
b-jet cross-section is measured as a function of transverse momentum in the
range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet
cross-section is measured as a function of the dijet invariant mass in the
range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets
and the angular variable chi in two dijet mass regions. The results are
compared with next-to-leading-order QCD predictions. Good agreement is observed
between the measured cross-sections and the predictions obtained using POWHEG +
Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet
cross-section. However, it does not reproduce the measured inclusive
cross-section well, particularly for central b-jets with large transverse
momenta.Comment: 10 pages plus author list (21 pages total), 8 figures, 1 table, final
version published in European Physical Journal
Jet energy measurement with the ATLAS detector in proton-proton collisions at root s=7 TeV
The jet energy scale and its systematic uncertainty are determined for jets measured with the ATLAS detector at the LHC in proton-proton collision data at a centre-of-mass energy of √s = 7TeV corresponding to an integrated luminosity of 38 pb-1. Jets are reconstructed with the anti-kt algorithm with distance parameters R=0. 4 or R=0. 6. Jet energy and angle corrections are determined from Monte Carlo simulations to calibrate jets with transverse momenta pT≥20 GeV and pseudorapidities {pipe}η{pipe}<4. 5. The jet energy systematic uncertainty is estimated using the single isolated hadron response measured in situ and in test-beams, exploiting the transverse momentum balance between central and forward jets in events with dijet topologies and studying systematic variations in Monte Carlo simulations. The jet energy uncertainty is less than 2. 5 % in the central calorimeter region ({pipe}η{pipe}<0. 8) for jets with 60≤pT<800 GeV, and is maximally 14 % for pT<30 GeV in the most forward region 3. 2≤{pipe}η{pipe}<4. 5. The jet energy is validated for jet transverse momenta up to 1 TeV to the level of a few percent using several in situ techniques by comparing a well-known reference such as the recoiling photon pT, the sum of the transverse momenta of tracks associated to the jet, or a system of low-pT jets recoiling against a high-pT jet. More sophisticated jet calibration schemes are presented based on calorimeter cell energy density weighting or hadronic properties of jets, aiming for an improved jet energy resolution and a reduced flavour dependence of the jet response. The systematic uncertainty of the jet energy determined from a combination of in situ techniques is consistent with the one derived from single hadron response measurements over a wide kinematic range. The nominal corrections and uncertainties are derived for isolated jets in an inclusive sample of high-pT jets. Special cases such as event topologies with close-by jets, or selections of samples with an enhanced content of jets originating from light quarks, heavy quarks or gluons are also discussed and the corresponding uncertainties are determined. © 2013 CERN for the benefit of the ATLAS collaboration
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