3,322 research outputs found
The cosmological co-evolution of supermassive black holes, AGN and galaxies
We model the cosmological co-evolution of galaxies and their central
supermassive black holes (BHs) within a semi-analytical framework developed on
the outputs of the Millennium Simulation (Croton et al., 2006; De Lucia &
Blaizot, 2007). In this work, we analyze the model BH scaling relations,
fundamental plane and mass function, and compare them with the most recent
observational data. Furthermore, we extend the original code developed by
Croton et al. (2006) to follow the evolution of the BH mass accretion and its
conversion into radiation, and compare the derived AGN bolometric luminosity
function with the observed one. We find, for the most part, a very good
agreement between predicted and observed BH properties. Moreover, the model is
in good agreement with the observed AGN number density in 0<z<5, provided it is
assumed that the cold gas fraction accreted by BHs at high redshifts is larger
than at low redshifts (Marulli et al., 2008).Comment: Proceedings of "The Central Kiloparsec: Active Galactic Nuclei and
Their Hosts", Ierapetra, Crete, 4-6 June, 2008. To appear in Volume 79 of the
Memorie della Societa' Astronomica Italiana. 5 pages, 4 figure
Detecting X-ray filaments in the low redshift Universe with XEUS and Constellation-X
We propose a possible way to detect baryons at low redshifts from the
analysis of X-ray absorption spectra of bright AGN pairs. A simple
semi-analytical model to simulate the spectra is presented. We model the
diffuse warm-hot intergalactic medium (WHIM) component, responsible for the
X-ray absorption, using inputs from high-resolution hydro-dynamical simulations
and analytical prescriptions. We show that the number of OVII absorbers per
unit redshift with column density larger than cm -
corresponding to an equivalent width of 1 km/s - which will be possibly
detectable by {\it XEUS}, is \magcir 30 per unit redshift. {\it
Constellation-X} will detect OVII absorptions per unit redshift with
an equivalent width of 10 km/s. Our results show that, in a CDM
Universe, the characteristic size of these absorbers at is
Mpc. The filamentary structure of WHIM can be probed by finding
coincident absorption lines in the spectra of background AGN pairs. We estimate
that at least 20 AGN pairs at separation \mincir 20 arcmin are needed to
detect this filamentary structure at a 3 level. Assuming observations
of distant sources using {\it XEUS} for exposure times of 500 ksec, we find
that the minimum source flux to probe the filamentary structure is erg cm s, in the 0.1-2.4 keV energy band. Thus,
most pairs of these extragalactic X-ray bright sources have already been
identified in the {\it ROSAT} All-Sky Survey. Re-observation of these objects
by future missions could be a powerful way to search for baryons in the low
redshift Universe.Comment: 18 pages, 10 Figures. Two figures added, Sections 2 and 3 expanded.
More optimistic results for Constellation-X. Accepted by MNRA
A numerical study of the effects of primordial non-Gaussianities on weak lensing statistics
While usually cosmological initial conditions are assumed to be Gaussian,
inflationary theories can predict a certain amount of primordial
non-Gaussianity which can have an impact on the statistical properties of the
lensing observables. In order to evaluate this effect, we build a large set of
realistic maps of different lensing quantities starting from light-cones
extracted from large dark-matter only N-body simulations with initial
conditions corresponding to different levels of primordial local
non-Gaussianity strength . Considering various statistical
quantities (PDF, power spectrum, shear in aperture, skewness and bispectrum) we
find that the effect produced by the presence of primordial non-Gaussianity is
relatively small, being of the order of few per cent for values of compatible with the present CMB constraints and reaching at most 10-15
per cent for the most extreme cases with . We also discuss
the degeneracy of this effect with the uncertainties due to the power spectrum
normalization and matter density parameter , finding
that an error in the determination of () of about 3
(10) per cent gives differences comparable with non-Gaussian models having
. These results suggest that the possible presence of an
amount of primordial non-Gaussianity corresponding to is not
hampering a robust determination of the main cosmological parameters in present
and future weak lensing surveys, while a positive detection of deviations from
the Gaussian hypothesis is possible only breaking the degeneracy with other
cosmological parameters and using data from deep surveys covering a large
fraction of the sky.Comment: accepted by MNRA
Density Contrast-Peculiar Velocity Relation in the Newtonian Gauge
In general relativistic framework of the large scale structure formation
theory in the universe, we investigate the relation between density contrast
and peculiar velocity in the Newtonian gauge. According to the gauge-invariant
property of the energy-momentum tensor in the Newtonian gauge, we consider the
perturbation of velocity in the energy-momentum tensor behaves as the Newtonian
peculiar velocity. It is shown that in the relativistic framework, the relation
between peculiar velocity and density contrast has an extra correction term
with respect to the Newtonian Peebles formula which in small scales, can be
ignorable . The relativistic correction of peculiar velocity for the structures
with the extension of few hundred mega parsec is about few percent which is
smaller than the accuracy of the recent observations for measuring peculiar
velocity. The peculiar velocity in the general relativistic framework also
changes the contribution of Doppler effect on the anisotropy of CMB.Comment: 9 pages, 1 figure, accepted in Int. J. Mod. Phys
Measuring the Nonlinear Biasing Function from a Galaxy Redshift Survey
We present a simple method for evaluating the nonlinear biasing function of
galaxies from a redshift survey. The nonlinear biasing is characterized by the
conditional mean of the galaxy density fluctuation given the underlying mass
density fluctuation, or by the associated parameters of mean biasing and
nonlinearity (following Dekel & Lahav 1999). Using the distribution of galaxies
in cosmological simulations, at smoothing of a few Mpc, we find that the mean
biasing can be recovered to a good accuracy from the cumulative distribution
functions (CDFs) of galaxies and mass, despite the biasing scatter. Then, using
a suite of simulations of different cosmological models, we demonstrate that
the matter CDF is robust compared to the difference between it and the galaxy
CDF, and can be approximated for our purpose by a cumulative log-normal
distribution of 1+\delta with a single parameter \sigma. Finally, we show how
the nonlinear biasing function can be obtained with adequate accuracy directly
from the observed galaxy CDF in redshift space. Thus, the biasing function can
be obtained from counts in cells once the rms mass fluctuation at the
appropriate scale is assumed a priori. The relative biasing function between
different galaxy types is measurable in a similar way. The main source of error
is sparse sampling, which requires that the mean galaxy separation be smaller
than the smoothing scale. Once applied to redshift surveys such as PSCz, 2dF,
SDSS, or DEEP, the biasing function can provide valuable constraints on galaxy
formation and structure evolution.Comment: 23 pages, 7 figures, revised version, accepted for publication in Ap
Choroidal Imaging Using Spectral-Domain Optical Coherence Tomography
Author Manuscript received 2012 June 22.Background: A structurally and functionally normal choroidal vasculature is essential for retinal function. Therefore, a precise clinical understanding of choroidal morphology should be important for understanding many retinal and choroidal diseases.
Methods: PUBMED ( http://www.ncbi.nlm.nih.gov/site...) was used for most of the literature search for this article. The criterion for inclusion of an article in the references for this review was that it included materials about both the clinical and the basic properties of choroidal imaging using spectral-domain optical coherence tomography.
Results: Recent reports show successful examination and accurate measurement of choroidal thickness in normal and pathologic states using spectral-domain optical coherence tomography systems. This review focuses on the principles of the new technology that make choroidal imaging using optical coherence tomography possible and on the changes that subsequently have been documented to occur in the choroid in various diseases. Additionally, it outlines future directions in choroidal imaging.
Conclusion: Optical coherence tomography is now proven to be an effective noninvasive tool to evaluate the choroid and to detect choroidal changes in pathologic states. Additionally, choroidal evaluation using optical coherence tomography can be used as a parameter for diagnosis and follow-up.Research to Prevent Blindness, Inc. (United States) (Unrestricted Grant)National Institutes of Health (U.S.) (Contract RO1-EY11289-25)National Institutes of Health (U.S.) (Contract R01-EY13178-10)National Institutes of Health (U.S.) (Contract R01-EY013516-07)National Institutes of Health (U.S.) (Contract R01-EY019029-02)United States. Air Force Office of Scientific Research (Grant FA9550-10-1-0551)United States. Air Force Office of Scientific Research (FA9550-10-1-0063
PO-0940: Feasibility of planning CT to MVCT deformable registration for ĂŹdose of the dayĂź calculation in helical Tomotherapy
NĂșm. a Art pĂșblic: 1135Digitalitzat per Tecnodo
Tracing the Warm Hot Intergalactic Medium in the local Universe
We present a simple method for tracing the spatial distribution and
predicting the physical properties of the Warm-Hot Intergalactic Medium (WHIM),
from the map of galaxy light in the local universe. Under the assumption that
biasing is local and monotonic we map the ~ 2 Mpc/h smoothed density field of
galaxy light into the mass density field from which we infer the spatial
distribution of the WHIM in the local supercluster. Taking into account the
scatter in the WHIM density-temperature and density-metallicity relation,
extracted from the z=0 outputs of high-resolution and large box size
hydro-dynamical cosmological simulations, we are able to quantify the
probability of detecting WHIM signatures in the form of absorption features in
the X-ray spectra, along arbitrary directions in the sky. To illustrate the
usefulness of this semi-analytical method we focus on the WHIM properties in
the Virgo Cluster region.Comment: 16 pages 11 Figures. Discussion clarified, alternative methods
proposed. Results unchanged. MNRAS in pres
Analysis of Normal Peripapillary Choroidal Thickness via Spectral Domain Optical Coherence Tomography
Purpose
To analyze the normal peripapillary choroidal thickness utilizing a commercial spectral domain optical coherence tomography (OCT) device and determine the intergrader reproducibility of this method.
Design
Retrospective, noncomparative, noninterventional case series.
Participants
Thirty-six eyes of 36 normal patients seen at the New England Eye Center between April and September 2010.
Methods
All patients underwent high-definition scanning with the Cirrus HD-OCT. Two raster scans were obtained per eye, a horizontal and a vertical scan, both of which were centered at the optic nerve. Two independent graders individually measured the choroidal thickness. Choroidal thickness was measured from the posterior edge of the retinal pigment epithelium to the choroidâscleral junction at 500-ÎŒm intervals away from the optic nerve in the superior, inferior, nasal, and temporal quadrants. Statistical analysis was conducted to compare mean choroidal thicknesses. Intergrader reproducibility was assessed by intraclass correlation coefficient and Pearson's correlation coefficient. Average choroidal thickness in each quadrant was compared with retinal nerve fiber layer (RNFL) thickness in their respective quadrants.
Main Outcome Measures
Peripapillary choroidal thickness, intraclass coefficient, and Pearson's correlation coefficient.
Results
The peripapillary choroid in the inferior quadrant was significantly thinner compared with all other quadrants (P<0.001). None of the other quadrants were significantly different from each other in terms of thickness. The inferior peripapillary choroid was significantly thinner compared with all other quadrants at all distances measured away from the optic nerve (P<0.001). Generally, the peripapillary choroid increases in thickness the farther it was away from the optic nerve and eventually approaching a plateau. The intraclass correlation coefficient ranged from 0.62 to 0.93 and Pearson's correlation coefficient ranged from 0.74 to 0.95 (P<0.001). Neither RNFL thickness nor average age was significantly correlated with average choroidal thickness.
Conclusions
Manual segmentation of the peripapillary choroidal thickness is reproducible between graders, suggesting that this method is accurate. The inferior peripapillary choroid was significantly thinner than all other quadrants (P<0.001).Research to Prevent Blindness, Inc. (United States)National Institutes of Health (U.S.) (RO1-EY11289-24)National Institutes of Health (U.S.) (R01-EY13178-10)National Institutes of Health (U.S.) (R01-EY013516-07)United States. Air Force Office of Scientific Research (FA9550-07-1-0101)United States. Air Force Office of Scientific Research (FA9550-07-1-0014
Galaxy and Cluster Biasing from Local Group Dynamics
Comparing the gravitational acceleration induced on the Local Group of
galaxies by different tracers of the underline density field we estimate,
within the linear gravitational instability theory and the linear biasing
ansatz, their relative bias factors. Using optical SSRS2 galaxies, IRAS (PSCz)
galaxies and Abell/ACO clusters, we find b_{O,I} ~ 1.21 +- 0.06 and b_{C,I} ~
4.3 +- 0.8, in agreement with other recent studies. Finally, there is an
excellent one-to-one correspondence of the PSCz and Abell/ACO cluster dipole
profiles, once the latter is rescaled by b_{C,I}, out to at least ~150 h^{-1}
Mpc.Comment: 7 pages, 5 figures, accepted for publication in MNRA
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