1,871 research outputs found
Measurement of the Spatial Cross-Correlation Function of Damped Lyman Alpha Systems and Lyman Break Galaxies
We present the first spectroscopic measurement of the spatial
cross-correlation function between damped Lyman alpha systems (DLAs) and Lyman
break galaxies (LBGs). We obtained deep u'BVRI images of nine QSO fields with
11 known z ~ 3 DLAs and spectroscopically confirmed 211 R < 25.5
photometrically selected z > 2 LBGs. We find strong evidence for an overdensity
of LBGs near DLAs versus random, the results of which are similar to that of
LBGs near other LBGs. A maximum likelihood cross-correlation analysis found the
best fit correlation length value of r_0 = 2.9^(+1.4)_(-1.5) h^(-1)Mpc using a
fixed value of gamma = 1.6. The implications of the DLA-LBG clustering
amplitude on the average dark matter halo mass of DLAs are discussed.Comment: 12 pages, 2 figures, accepted for publication in Astrophysical
Journal Letter
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
Matter Power Spectrum from the Lyman-Alpha Forest: Myth or Reality?
We investigate possible systematic errors in the recent measurement of the
matter power spectrum from the Lyman-alpha forest by Croft et al. (2001). We
find that for a large set of prior cosmological models the Croft et al. result
holds quite well, with systematic errors being comparable to random ones, when
a dependence of the recovered matter power spectrum on the cosmological
parameters at z~3 is taken into account. We find that peculiar velocities cause
the flux power spectrum to be smoothed over about 100-300 km/s, dependng on
scale. Consequently, the recovered matter power spectrum is a smoothed version
of the underlying true power spectrum. Uncertainties in the recovered power
spectrum are thus correlated over about 100-300 km/s. As a side effect, we find
that residual fluctuations in the ionizing background, while having almost no
effect on the recovered matter power spectrum, significantly bias estimates of
the baryon density from the Lyman-alpha forest data. We therefore conclude that
the Croft et al. result provides a powerful new constraint on cosmological
parameters and models of structure formation.Comment: accepted for publication in MNRA
Survey for Galaxies Associated with z~3 Damped Lyman alpha Systems II: Galaxy-Absorber Correlation Functions
We use 211 galaxy spectra from our survey for Lyman break galaxies (LBGs)
associated with 11 damped Lyman alpha systems (DLAs) to measure the
three-dimensional LBG auto-correlation and DLA-LBG cross-correlation functions
with the primary goal of inferring the mass of DLAs at z~3. Conventional
binning of the data while varying both r_0 and gamma parameters of the fiducial
model of the correlation function xi(r)=(r/r_0)^{-gamma} resulted in the best
fit values and 1 sigma uncertainties of r_0=2.65+/-0.48, gamma=1.55+/-0.40 for
the LBG auto-correlation and r_0=3.32+/-1.25, gamma=1.74+/-0.36 for DLA-LBG
cross-correlation function. To circumvent shortcomings found in binning small
datasets, we perform a maximum likelihood analysis based on Poisson statistics.
The best fit values and 1 sigma confidence levels from this analysis were
found to be r_0=2.91(+1.0,-1.0), gamma=1.21(+0.6,-0.3) for the LBG
auto-correlation and r_0=2.81(+1.4,-2.0), gamma=2.11(+1.3,-1.4) for the DLA-LBG
cross-correlation function. We report a redshift spike of five LBGs with Delta
z = 0.015 of the z=2.936 DLA in the PSS0808+5215 field and find that the
DLA-LBG clustering signal survives when omitting this field from the analysis.
Using the correlation functions measurements and uncertainties, we compute the
z~3 LBG galaxy bias b_LBG to be 1.5<b_LBG<3 corresponding to an average halo
mass of 10^(9.7)<M_LBG<10^(11.6) M_odot and the z~3 DLA galaxy bias b_DLA to be
1.3<b_DLA<4 corresponding to an average halo mass of 10^(9)<M_LBG<10^(12)
M_odot. Lastly, two of the six QSOs discovered in this survey were found to lie
within Delta z = 0.0125 of two of the survey DLAs. We estimate the probability
of this occurring by chance is 1 in 940 and may indicate a possible
relationship between the distribution of QSOs and DLAs at z~3.Comment: 44 pages, 12 figures, accepted to The Astrophysical Journa
The phase-diagram of cosmological baryons
We investigate the behaviour of cosmological baryons at low redshifts
after reionization through analytic means. In particular, we study the
density-temperature phase-diagram which describes the history of the gas. We
show how the location of the matter in this diagram expresses the
various constraints implied by usual hierarchical scenarios. This yields robust
model-independent results which agree with numerical simulations. The IGM is
seen to be formed via two phases: a ``cool'' photo-ionized component and a
``warm'' component governed by shock-heating. We also briefly describe how the
remainder of the matter is distributed over galaxies, groups and clusters. We
recover the fraction of matter and the spatial clustering computed by numerical
simulations. We also check that the soft X-ray background due to the ``warm''
IGM component is consistent with observations. We find in the present universe
a baryon fraction of 7% in hot gas, 24% in the warm IGM, 38% in the cool IGM,
9% within star-like objects and, as a still un-observed component, 22% of dark
baryons associated with collapsed structures, with a relative uncertainty no
larger than 30% on these numbers.Comment: 17 pages, accepted by A&A. This final version contains a more
detailed discussion of the physics of the IGM and of the properties of the
Warm IG
Temporal Networks
A great variety of systems in nature, society and technology -- from the web
of sexual contacts to the Internet, from the nervous system to power grids --
can be modeled as graphs of vertices coupled by edges. The network structure,
describing how the graph is wired, helps us understand, predict and optimize
the behavior of dynamical systems. In many cases, however, the edges are not
continuously active. As an example, in networks of communication via email,
text messages, or phone calls, edges represent sequences of instantaneous or
practically instantaneous contacts. In some cases, edges are active for
non-negligible periods of time: e.g., the proximity patterns of inpatients at
hospitals can be represented by a graph where an edge between two individuals
is on throughout the time they are at the same ward. Like network topology, the
temporal structure of edge activations can affect dynamics of systems
interacting through the network, from disease contagion on the network of
patients to information diffusion over an e-mail network. In this review, we
present the emergent field of temporal networks, and discuss methods for
analyzing topological and temporal structure and models for elucidating their
relation to the behavior of dynamical systems. In the light of traditional
network theory, one can see this framework as moving the information of when
things happen from the dynamical system on the network, to the network itself.
Since fundamental properties, such as the transitivity of edges, do not
necessarily hold in temporal networks, many of these methods need to be quite
different from those for static networks
The line-of-sight proximity effect in individual quasar spectra
We exploit a set of high signal-to-noise (~70), low-resolution (R~800) quasar
spectra to search for the signature of the so-called proximity effect in the HI
Ly alpha forest. Our sample consists of 17 bright quasars in the redshift range
2.7<z<4.1. Analysing the spectra with the flux transmission technique, we
detect the proximity effect in the sample at high significance. We use this to
estimate the average intensity of the metagalactic UV background, assuming it
to be constant over this redshift range. We obtain a value of J =
(9+-4)x10^{-22}ergcm^{-2}s^{-1}Hz^{-1}sr^{-1}, in good agreement with previous
measurements at similar z. We then apply the same procedure to individual lines
of sight, finding a significant deficit in the effective optical depth close to
the emission redshift in every single object except one (which by a different
line of evidence does nevertheless show a noticeable proximity effect). Thus,
we clearly see the proximity effect as a universal phenomenon associated with
individual quasars. Using extensive Monte-Carlo simulations to quantify the
error budget, we assess the expected statistical scatter in the strength of the
proximity effect due to shot noise (cosmic variance). The observed scatter is
larger than the predicted one, so that additional sources of scatter are
required. We rule out a dispersion of spectral slopes as a significant
contributor. Possible effects are long time-scale variability of the quasars
and/or gravitational clustering of Ly alpha forest lines. We speculate on the
possibility of using the proximity effect as a tool to constrain individual
quasar ages, finding that ages between ~10^6 and ~10^8 yrs might produce a
characteristic signature in the optical depth profile towards the QSO. We
identify one possible candidate for this effect in our sample.Comment: A&A accepted for publication, 14 pages, 24 figures (including 17
online figures
What is the evidence for the management of patients along the pathway from the emergency department to acute admission to reduce unplanned attendance and admission? An evidence synthesis
Background
Globally, the rate of emergency hospital admissions is increasing. However, little evidence exists to inform the development of interventions to reduce unplanned Emergency Department (ED) attendances and hospital admissions. The objective of this evidence synthesis was to review the evidence for interventions, conducted during the patient’s journey through the ED or acute care setting, to manage people with an exacerbation of a medical condition to reduce unplanned emergency hospital attendance and admissions.
Methods
A rapid evidence synthesis, using a systematic literature search, was undertaken in the electronic data bases of MEDLINE, EMBASE, CINAHL, the Cochrane Library and Web of Science, for the years 2000–2014. Evidence included in this review was restricted to Randomised Controlled Trials (RCTs) and observational studies (with a control arm) reported in peer-reviewed journals. Studies evaluating interventions for patients with an acute exacerbation of a medical condition in the ED or acute care setting which reported at least one outcome related to ED attendance or unplanned admission were included.
Results
Thirty papers met our inclusion criteria: 19 intervention studies (14 RCTs) and 11 controlled observational studies. Sixteen studies were set in the ED and 14 were conducted in an acute setting. Two studies (one RCT), set in the ED were effective in reducing ED attendance and hospital admission. Both of these interventions were initiated in the ED and included a post-discharge community component. Paradoxically 3 ED initiated interventions showed an increase in ED re-attendance. Six studies (1 RCT) set in acute care settings were effective in reducing: hospital admission, ED re-attendance or re-admission (two in an observation ward, one in an ED assessment unit and three in which the intervention was conducted within 72 h of admission).
Conclusions
There is no clear evidence that specific interventions along the patient journey from ED arrival to 72 h after admission benefit ED re-attendance or readmission. Interventions targeted at high-risk patients, particularly the elderly, may reduce ED utilization and warrant future research. Some interventions showing effectiveness in reducing unplanned ED attendances and admissions are delivered by appropriately trained personnel in an environment that allows sufficient time to assess and manage patients
Measurements of fiducial and differential cross sections for Higgs boson production in the diphoton decay channel at s√=8 TeV with ATLAS
Measurements of fiducial and differential cross sections are presented for Higgs boson production in proton-proton collisions at a centre-of-mass energy of s√=8 TeV. The analysis is performed in the H → γγ decay channel using 20.3 fb−1 of data recorded by the ATLAS experiment at the CERN Large Hadron Collider. The signal is extracted using a fit to the diphoton invariant mass spectrum assuming that the width of the resonance is much smaller than the experimental resolution. The signal yields are corrected for the effects of detector inefficiency and resolution. The pp → H → γγ fiducial cross section is measured to be 43.2 ±9.4(stat.) − 2.9 + 3.2 (syst.) ±1.2(lumi)fb for a Higgs boson of mass 125.4GeV decaying to two isolated photons that have transverse momentum greater than 35% and 25% of the diphoton invariant mass and each with absolute pseudorapidity less than 2.37. Four additional fiducial cross sections and two cross-section limits are presented in phase space regions that test the theoretical modelling of different Higgs boson production mechanisms, or are sensitive to physics beyond the Standard Model. Differential cross sections are also presented, as a function of variables related to the diphoton kinematics and the jet activity produced in the Higgs boson events. The observed spectra are statistically limited but broadly in line with the theoretical expectations
Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at s√=8 TeV with the ATLAS detector
The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy s√=8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb−1. No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c) of approximately 30
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