772 research outputs found
Constraints on the X-ray luminosity function of AGN at z = 5.7â6.4 with the Extragalactic Serendipitous Swift Survey
X-ray luminosity functions (XLFs) of Active Galactic Nuclei (AGN) trace the
growth and evolution of supermassive black hole populations across cosmic time,
however, current XLF models are poorly constrained at redshifts of z>6, with a
lack of spectroscopic constraints at these high redshifts. In this work we
\redit{place limits} on the bright-end of the XLF at z=5.7-6.4 using
high-redshift AGN identified within the Extragalactic Serendipitous Swift
Survey (ExSeSS) catalogue. Within ExSeSS we find one serendipitously X-ray
detected z>6 AGN, ATLAS J025.6821-33.4627, with an X-ray luminosity of
and
making it the highest redshift, spectroscopically confirmed,
serendipitously X-ray detected quasar known to date. We also calculate an upper
limit on the space density at higher luminosities where no additional sources
are found, enabling us to place constraints on the shape of the XLF. Our
results are consistent with the rapid decline in the space densities of
high-luminosity AGN toward high redshift as predicted by extrapolations of
existing parametric models of the XLF. We also find that our X-ray based
measurements are consistent with estimates of the bolometric quasar luminosity
function based on UV measurements at , although they require a large
X-ray to bolometric correction factor at these high luminosities.Comment: 10pages, 6 figures. Resubmitted to MNRAS, following referee comment
The X-ray luminosity function of Active Galactic Nuclei in the redshift interval z=3-5
We combine deep X-ray survey data from the Chandra observatory and the
wide-area/shallow XMM-XXL field to estimate the AGN X-ray luminosity function
in the redshift range z=3-5. The sample consists of nearly 340 sources with
either photometric (212) or spectroscopic (128) redshift in the above range.
The combination of deep and shallow survey fields provides a luminosity
baseline of three orders of magnitude, Lx(2-10keV)~1e43-1e46erg/s at z>3. We
follow a Bayesian approach to determine the binned AGN space density and
explore their evolution in a model-independent way. Our methodology accounts
for Poisson errors in the determination of X-ray fluxes and uncertainties in
photometric redshift estimates. We demonstrate that the latter is essential for
unbiased measurement of space densities. We find that the AGN X-ray luminosity
function evolves strongly between the redshift intervals z=3-4 and z=4-5. There
is also suggestive evidence that the amplitude of this evolution is luminosity
dependent. The space density of AGN with Lx<1e45erg/s drops by a factor of 5
between the redshift intervals above, while the evolution of brighter AGN
appears to be milder. Comparison of our X-ray luminosity function with that of
UV/optical selected QSOs at similar redshifts shows broad agreement at bright
luminosities, Lx>1e45erg/s. The faint-end slope of UV/optical luminosity
functions however, is steeper than for X-ray selected AGN. This implies that
the type-I AGN fraction increases with decreasing luminosity at z>3, opposite
to trends established at lower redshift. We also assess the significance of AGN
in keeping the hydrogen ionised at high redshift. Our X-ray luminosity function
yields ionising photon rate densities that are insufficient to keep the
Universe ionised at redshift z>4. A source of uncertainty in this calculation
is the escape fraction of UV photons for X-ray selected AGN.Comment: MNRAS accepte
Evidence for a mass-dependent AGN Eddington ratio distribution via the flat relationship between SFR and AGN luminosity
The lack of a strong correlation between AGN X-ray luminosity (L X ; a proxy for AGN power) and the star formation rate (SFR) of their host galaxies has recently been attributed to stochastic AGNvariability. Studies using population synthesis models have incorporated this by assuming a broad, universal (i.e. does not depend on the host galaxy properties) probability distribution for AGN specific X-ray luminosities (i.e. the ratio of L X to host stellar mass; a common proxy for Eddington ratio). However, recent studies have demonstrated that this universal Eddington ratio distribution fails to reproduce the observed X-ray luminosity functions beyond z ~ 1.2. Furthermore, empirical studies have recently shown that the Eddington ratio distribution may instead depend upon host galaxy properties, such as SFR and/or stellarmass. To investigate this further, we develop a population synthesis model in which the Eddington ratio distribution is different for star-forming and quiescent host galaxies. We showthat, although this model is able to reproduce the observed X-ray luminosity functions out to z ~ 2, it fails to simultaneously reproduce the observed flat relationship between SFR and X-ray luminosity. We can solve this, however, by incorporating a mass dependency in the AGN Eddington ratio distribution for starforming host galaxies. Overall, our models indicate that a relative suppression of low Eddington ratios (λ Edd < ~0.1) in lower mass galaxies (M * < ~10 10-11 Mâ) is required to reproduce both the observed X-ray luminosity functions and the observed flat SFR/X-ray relationship
Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties
We revisited the problem of the stability of the superconducting state in
RbxWO3 and identified the main causes of the contradictory data previously
published. We have shown that the ordering of the Rb vacancies in the
nonstoichiometric compounds have a major detrimental effect on the
superconducting temperature Tc.The order-disorder transition is first order
only near x = 0.25, where it cannot be quenched effectively and Tc is reduced
below 1K. We found that the high Tc's which were sometimes deduced from
resistivity measurements, and attributed to compounds with .25 < x < .30, are
to be ascribed to interfacial superconductivity which generates spectacular
non-linear effects. We also clarified the effect of acid etching and set more
precisely the low-rubidium-content boundary of the hexagonal phase.This work
makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we
approach this boundary (x = 0.20), if no ordering would take place - as its is
approximately the case in CsxWO3. This behaviour is reminiscent of the
tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism
is responsible for this large increase of Tc despite the considerable
associated reduction of the electron density of state ? By reviewing the other
available data on these bronzes we conclude that the theoretical models which
are able to answer this question are probably those where the instability of
the lattice plays a major role and, particularly, the model which call upon
local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review
The galaxyâs gas content regulated by the dark matter halo mass results in a superlinear M BHâM â Relation
Supermassive black holes (SMBHs) are tightly correlated with their hosts, but the origin of such connection remains elusive. To explore the cosmic buildup of this scaling relation, we present an empirically motivated model that tracks galaxy and SMBH growth down to z = 0. Starting from a random mass seed distribution at z = 10, we assume that each galaxy evolves on the star-forming "main sequence" (MS) and each BH follows the recently derived stellar mass (M sstarf) dependent ratio between BH accretion rate and star formation rate, going as . Our simple recipe naturally describes the BHâgalaxy buildup in two stages. At first, the SMBH lags behind the host that evolves along the MS. Later, as the galaxy grows in M sstarf, our M sstarf-dependent BHAR/SFR induces a superlinear BH growth, as . According to this formalism, smaller BH seeds increase their relative mass faster and earlier than bigger BH seeds, at fixed M sstarf, thus setting along a gradually tighter M BHâM sstarf locus toward higher M sstarf. Assuming reasonable values of the radiative efficiency epsilon ~ 0.1, our empirical trend agrees with both high-redshift model predictions and intrinsic M BHâM sstarf relations of local BHs. We speculate that the observed nonlinear BHâgalaxy buildup is reflected in a twofold behavior with dark matter halo mass (M DM), displaying a clear turnover at M DM ~ 2 Ă 1012 M â. While supernovae-driven feedback suppresses BH growth in smaller halos (), above the M DM threshold cold gas inflows possibly fuel both BH accretion and star formation in a similar fashion ()
AEGIS-X: The Chandra Deep Survey of the Extended Groth Strip
We present the AEGIS-X survey, a series of deep Chandra ACIS-I observations
of the Extended Groth Strip. The survey comprises pointings at 8 separate
positions, each with nominal exposure 200ks, covering a total area of
approximately 0.67 deg2 in a strip of length 2 degrees. We describe in detail
an updated version of our data reduction and point source detection algorithms
used to analyze these data. A total of 1325 band-merged sources have been found
to a Poisson probability limit of 4e-6, with limiting fluxes of 5.3e-17
erg/cm2/s in the soft (0.5-2 keV) band and 3.8e-16 erg/cm2/s in the hard (2-10
keV) band. We present simulations verifying the validity of our source
detection procedure and showing a very small, <1.5%, contamination rate from
spurious sources. Optical/NIR counterparts have been identified from the DEEP2,
CFHTLS, and Spitzer/IRAC surveys of the same region. Using a likelihood ratio
method, we find optical counterparts for 76% of our sources, complete to
R(AB)=24.1, and, of the 66% of the sources that have IRAC coverage, 94% have a
counterpart to a limit of 0.9 microJy at 3.6 microns (m(AB)=23.8). After
accounting for (small) positional offsets in the 8 Chandra fields, the
astrometric accuracy of the Chandra positions is found to be 0.8 arcsec RMS,
however this number depends both on the off-axis angle and the number of
detected counts for a given source. All the data products described in this
paper are made available via a public website.Comment: 17 pages, 9 figures. Accepted for publication in ApJS. Data products
are available at http://astro.imperial.ac.uk/research/aegis
Sunyaev-Zel'dovich Cluster Profiles Measured with the South Pole Telescope
We present Sunyaev-Zel'dovich measurements of 15 massive X-ray selected
galaxy clusters obtained with the South Pole Telescope. The Sunyaev-Zel'dovich
(SZ) cluster signals are measured at 150 GHz, and concurrent 220 GHz data are
used to reduce astrophysical contamination. Radial profiles are computed using
a technique that takes into account the effects of the beams and filtering. In
several clusters, significant SZ decrements are detected out to a substantial
fraction of the virial radius. The profiles are fit to the beta model and to a
generalized NFW pressure profile, and are scaled and stacked to probe their
average behavior. We find model parameters that are consistent with previous
studies: beta=0.86 and r_core/r_500 = 0.20 for the beta model, and (alpha,
beta, gamma, c_500)=(1.0,5.5,0.5,1.0) for the generalized NFW model. Both
models fit the SPT data comparably well, and both are consistent with the
average SZ profile out to the virial radius. The integrated Compton-y parameter
Y_SZ is computed for each cluster using both model-dependent and
model-independent techniques, and the results are compared to X-ray estimates
of cluster parameters. We find that Y_SZ scales with Y_X and gas mass with low
scatter. Since these observables have been found to scale with total mass, our
results point to a tight mass-observable relation for the SPT cluster survey.Comment: 21 pages, 24 figures, updated to published versio
A CMB lensing mass map and its correlation with the cosmic infrared background
We use a temperature map of the cosmic microwave background (CMB) obtained
using the South Pole Telescope at 150 GHz to construct a map of the
gravitational convergence to z ~ 1100, revealing the fluctuations in the
projected mass density. This map shows individual features that are significant
at the ~ 4 sigma level, providing the first image of CMB lensing convergence.
We cross-correlate this map with Herschel/SPIRE maps covering 90 square degrees
at wavelengths of 500, 350, and 250 microns. We show that these
submillimeter-wavelength (submm) maps are strongly correlated with the lensing
convergence map, with detection significances in each of the three submm bands
ranging from 6.7 to 8.8 sigma. We fit the measurement of the cross power
spectrum assuming a simple constant bias model and infer bias factors of
b=1.3-1.8, with a statistical uncertainty of 15%, depending on the assumed
model for the redshift distribution of the dusty galaxies that are contributing
to the Herschel/SPIRE maps.Comment: 5 pages, 3 figures, to be submitted to ApJ
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