254 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
The BeppoSAX High Energy Large Area Survey. IV. On the soft X-ray properties of the hard X-ray-selected HELLAS sources
We present a comprehensive study of the soft X-ray properties of the BeppoSAX
High-Energy Large Area Survey (HELLAS) sources. A large fraction (about 2/3) of
the hard X-ray selected sources is detected by ROSAT. The soft X-ray colors for
many of these objects, along with the 0.5-2 keV flux upper limits for those
undetected in the ROSAT band, do imply the presence of absorption. The
comparison with the ROSAT Deep Survey sources indicates that a larger fraction
of absorbed objects among the HELLAS sources is present, in agreement with
their hard X-ray selection and the predictions of the X-ray background
synthesis models. Another striking result is the presence of a soft
(additional) X-ray component in a significant fraction of absorbed objects.Comment: 11 pages, LateX, 6 figures (included), aa.cls, graphicx.sty and
lscape.sty macros. Accepted by A&
The physical oceanography of the transport of floating marine debris
Marine plastic debris floating on the ocean surface is a major environmental problem. However, its distribution in the ocean is poorly mapped, and most of the plastic waste estimated to have entered the ocean from land is unaccounted for. Better understanding of how plastic debris is transported from coastal and marine sources is crucial to quantify and close the global inventory of marine plastics, which in turn represents critical information for mitigation or policy strategies. At the same time, plastic is a unique tracer that provides an opportunity to learn more about the physics and dynamics of our ocean across multiple scales, from the Ekman convergence in basin-scale gyres to individual waves in the surfzone. In this review, we comprehensively discuss what is known about the different processes that govern the transport of floating marine plastic debris in both the open ocean and the coastal zones, based on the published literature and referring to insights from neighbouring fields such as oil spill dispersion, marine safety recovery, plankton connectivity, and others. We discuss how measurements of marine plastics (both in situ and in the laboratory), remote sensing, and numerical simulations can elucidate these processes and their interactions across spatio-temporal scales
High precision X-ray logN-logS distributions: implications for the obscured AGN population
We have constrained the extragalactic source count distributions over a broad
range of X-ray fluxes and in various energy bands to test whether the
predictions from X-ray background synthesis models agree with the observational
constraints provided by our measurements. We have used 1129 XMM-Newton
observations at |b|>20 deg covering a sky area of 132.3 deg^2 to compile the
largest complete samples of X-ray objects to date in the 0.5-1 keV, 1-2 keV,
2-4.5 keV, 4.5-10 keV, 0.5-2 keV and 2-10 keV energy bands. Our survey includes
in excess of 30,000 sources down to ~10^-15 erg/cm^2/s below 2 keV and down to
~10^{-14} erg/cm^2/s above 2 keV. A break in the source count distributions was
detected in all energy bands except the 4.5-10 keV band. An analytical model
comprising 2 power-law components cannot adequately describe the curvature seen
in the source count distributions. The shape of the logN(>S)-logS is strongly
dependent on the energy band with a general steepening apparent as we move to
higher energies. This is due to non-AGN populations, comprised mainly of stars
and clusters of galaxies, contribute up to 30% of the source population at
energies 10^{-13} erg/cm^2/s, and these populations of
objects have significantly flatter source count distributions than AGN. We find
a substantial increase in the relative fraction of hard X-ray sources at higher
energies, from >55% below 2 keV to >77% above 2 keV. However the majority of
sources detected above 4.5 keV still have significant flux below 2 keV.
Comparison with predictions from the synthesis models suggest that the models
might be overpredicting the number of faint absorbed AGN, which would call for
fine adjustment of some model parameters such as the obscured to unobscured AGN
ratio and/or the distribution of column densities at intermediate obscuration.Comment: Accepted for publication in Astronomy and Astrophysics. Abridged
Abstract. 23 pages, 47 figures, 8 table
Constraints on Cosmological and Biasing models using AGN clustering
We attempt to put constraints on different cosmological and biasing models by
combining the recent clustering results of X-ray sources in the local () and distant universe (). To this end we compare the measured
angular correlation function for bright (Akylas et al. 2000) and faint
(Vikhlinin & Forman 1995) {\em ROSAT} X-ray sources respectively with those
expected in three spatially flat cosmological models. Taking into account the
different functional forms of the bias evolution, we find that there are two
cosmological models which performs well the data. In particular,
low- cosmological models
() which contain either (i) high
value with galaxy merging bias, or (ii) low with non-bias, best reproduce the AGN clustering results. While CDM models
with different bias behaviour are ruled out at a high significance level.Comment: 7 pages, including 4 figures, accepted for publication in MNRA
Author Correction: A consensus-based transparency checklist.
An amendment to this paper has been published and can be accessed via a link at the top of the paper
Using the large scale quasar clustering to constrain flat quintessential universes
We search for the most suitable set of cosmological parameters that describes
the observable universe. The search includes the possibility of quintessential
flat universes, i.e., the analysis is restricted to the determination of the
dimensionless matter density and the quintessential parameters, and , respectively. Our study is focused on comparing the
position of features at large scales in the density fluctuation field at
different redshifts by analysing the evolution of the quasar two-point
correlation function. We trace the density field fluctuations at large scales
using a large and homogeneous sample of quasars ( 38000 objects with 0.3
z 2.4 and a median ) drawn from the Sloan Digital Sky
Survey Data Release Six. The analysis relies on the assumption that, in the
linear regime, the length scale of a particular feature should remain fixed at
different times of the universe for the proper cosmological model. Our study
does not assume any particular comoving length scale at which a feature should
be found, but intends to perform a comparison for a wide range of scales
instead. This is done by quantifying the amount of overlap among the quasar
correlation functions at different times using a cross-correlation technique.
The most likely cosmological model is and , in agreement with previous studies. These constraints are the
result of a good overall agreement of the correlation function at different
redshifts over scales \sim 100-300\mpc. Under the assumption of a flat
cosmological model, our results indicate that we are living in a low density
universe with a quintessential parameter greater than the one corresponding to
a cosmological constant.Comment: 10 pages, 8 figures, accepted for publication in Astronomy &
Astrophysic
Clustering of Low-Redshift (z <= 2.2) Quasars from the Sloan Digital Sky Survey
We present measurements of the quasar two-point correlation function,
\xi_{Q}, over the redshift range z=0.3-2.2 based upon data from the SDSS. Using
a homogeneous sample of 30,239 quasars with spectroscopic redshifts from the
DR5 Quasar Catalogue, our study represents the largest sample used for this
type of investigation to date. With this redshift range and an areal coverage
of approx 4,000 deg^2, we sample over 25 h^-3 Gpc^3 (comoving) assuming the
current LCDM cosmology. Over this redshift range, we find that the
redshift-space correlation function, xi(s), is adequately fit by a single
power-law, with s_{0}=5.95+/-0.45 h^-1 Mpc and \gamma_{s}=1.16+0.11-0.16 when
fit over s=1-25 h^-1 Mpc. Using the projected correlation function we calculate
the real-space correlation length, r_{0}=5.45+0.35-0.45 h^-1 Mpc and
\gamma=1.90+0.04-0.03, over scales of rp=1-130 h^-1 Mpc. Dividing the sample
into redshift slices, we find very little, if any, evidence for the evolution
of quasar clustering, with the redshift-space correlation length staying
roughly constant at s_{0} ~ 6-7 h^-1 Mpc at z<2.2 (and only increasing at
redshifts greater than this). Comparing our clustering measurements to those
reported for X-ray selected AGN at z=0.5-1, we find reasonable agreement in
some cases but significantly lower correlation lengths in others. We find that
the linear bias evolves from b~1.4 at z=0.5 to b~3 at z=2.2, with
b(z=1.27)=2.06+/-0.03 for the full sample. We compare our data to analytical
models and infer that quasars inhabit dark matter haloes of constant mass M ~2
x 10^12 h^-1 M_Sol from redshifts z~2.5 (the peak of quasar activity) to z~0.
[ABRIDGED]Comment: 28 pages, 26 figures, ApJ accepted. Online materials (including
source code, catalogues and high-resolution figures) can be found at
http://www.astro.psu.edu/users/npr/DR5
A possible dose–response association between distance to farmers’ markets and roadside produce stands, frequency of shopping, fruit and vegetable consumption, and body mass index among customers in the Southern United States
Background: The association between farmers’ market characteristics and consumer shopping habits remains
unclear. Our objective was to examine associations among distance to farmers’ markets, amenities within farmers’
markets, frequency of farmers’ market shopping, fruit and vegetable consumption, and body mass index (BMI). We
hypothesized that the relationship between frequency of farmers’ market shopping and BMI would be mediated by
fruit and vegetable consumption.
Methods: In 15 farmers’ markets in northeastern North Carolina, July–September 2015, we conducted a crosssectional
survey among 263 farmers’ market customers (199 provided complete address data) and conducted
farmers’ market audits. To participate, customers had to be over 18 years of age, and English speaking. Dependent
variables included farmers’ market shopping frequency, fruit and vegetable consumption, and BMI. Analysis of
variance, adjusted multinomial logistic regression, Poisson regression, and linear regression models, adjusted for
age, race, sex, and education, were used to examine associations between distance to farmers’ markets, amenities
within farmers’ markets, frequency of farmers’ market shopping, fruit and vegetable consumption, and BMI.
Results: Those who reported shopping at farmers’ markets a few times per year or less reported consuming 4.4
(standard deviation = 1.7) daily servings of fruits and vegetables, and those who reported shopping 2 or more times
per week reported consuming 5.5 (2.2) daily servings. There was no association between farmers’ market amenities,
and shopping frequency or fruit and vegetable consumption. Those who shopped 2 or more times per week had a
statistically significantly lower BMI than those who shopped less frequently. There was no evidence of mediation of
the relationship between frequency of shopping and BMI by fruit and vegetable consumption.
Conclusions: More work should be done to understand factors within farmers’ markets that encourage fruit and
vegetable purchases.ECU Open Access Publishing Support Fun
The Enchantment of the Archaeological Record
Empirical studies increasingly testify to the capacity for archaeological and cultural heritage sites to engender wonder, transformation, attachment, and community bonding among diverse individuals. Following political theorist Jane Bennett, these sites have the power to ‘enchant’ and, in so doing, they are seedbeds of human generosity, ethical mindfulness, and care for the world at large. However, the means by which such enchantment is created, and the extent to which these intimate encounters with the prehistoric or historic record can be deliberately crafted, are little understood. Worsening the predicament, professional practices commonly thwart the potential for archaeology to provoke ethical action amongst humans. Here, I propose a multi-stranded conceptual model for generating enchantment with the archaeological record across both professional audiences and broader publics. With reference to the European Commission-funded EMOTIVE Project, I articulate one particular strand of this model: facilitated dialogue. Alongside exploring the role of digital culture in its advancement, I argue that an enchantment-led approach is imperative for achieving a truly socially-beneficial archaeological discipline
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