6,960 research outputs found
K-Theory for group C^*-algebras
These notes are based on a lecture course given by the first author in the
Sedano Winter School on K-theory held in Sedano, Spain, on January 22-27th of
2007. They aim at introducing K-theory of C^*-algebras, equivariant K-homology
and KK-theory in the context of the Baum-Connes conjecture.Comment: 22 pages, 2 figures, to be published in Springer Lecture Note
Soft X-ray background fluctuations and large-scale structure in the Universe
We have studied the fluctuations of the soft (0.9-2 keV) X-ray background
intensity for ~10 arcmin and ~2 arcmin beam sizes, using 80 high galactic
latitude medium-deep images from the ROSAT position sensitive proportional
counter (PSPC). These fluctuations are dominated (and well reproduced) by
confusion noise produced by sources unresolved with the beam sizes we used. We
find no evidence for any excess fluctuations which could be attributed to
source clustering. The 95 per cent confidence upper limits on excess
fluctuations dIclus are: dIclus/Ixrb_10 arcmin<~ 0.12, dIclus/Ixrb_2 arcmin
<~0.07. We have checked the possibility that low surface brightness extended
objects (like groups or clusters of galaxies) may have a significant
contribution to excess fluctuations, finding that they are not necessary to fit
the distribution of fluctuations, and obtaining an upper limit on the surface
density for this type of source. Standard Cold Dark Matter models would produce
dIclus/Ixrb larger than the above limits for any value of the density of the
Universe Omega=0.1-1, unless the bias parameter of the X-ray emitting matter is
smaller than unity, or an important fraction of the sources of the soft X-ray
background (~30 per cent) is at redshifts z>1. Limits on the 2-10 keV excess
fluctuations are also considered, showing that X-ray sources in that band have
to be at redshifts z>1 unless Omega>0.4. Finally, if the spatial correlation
function of the sources that produce these excess fluctuations is instead a
power law, the density contrast drho/rho implied by the excess fluctuations
reveals that the Universe is smooth and linear on scales of tens of Mpc, while
it can be highly non-linear on scales ~1 Mpc.Comment: 10 pages, LaTeX file, epsf.sty and 7 postscript figures. To appear in
MNRAS. Fig. 7 replaced, some references improved, a few corrections to the
tex
Analysis of Spitzer-IRS spectra of hyperluminous infrared galaxies
Hyperluminous infrared galaxies (HLIRG) are the most luminous persistent
objects in the Universe. They exhibit extremely high star formation rates, and
most of them seem to harbour an AGN. They are unique laboratories to
investigate the most extreme star formation, and its connection to
super-massive black hole growth. The AGN and SB relative contributions to the
total output in these objects is still debated. Our aim is to disentangle the
AGN and SB emission of a sample of thirteen HLIRG. We have studied the MIR low
resolution spectra of a sample of thirteen HLIRG obtained with the IRS on board
Spitzer. The 5-8 {\mu}m range is an optimal window to detect AGN activity even
in a heavily obscured environment. We performed a SB/AGN decomposition of the
continuum using templates, successfully applied for ULIRG in previous works.
The MIR spectra of all sources is largely dominated by AGN emission. Converting
the 6 {\mu}m luminosity into IR luminosity, we found that ~80% of the sample
shows an IR output dominated by the AGN emission. However, the SB activity is
significant in all sources (mean SB contribution ~30%), showing star formation
rates ~300-3000 solar masses per year. Using X-ray and MIR data we estimated
the dust covering factor (CF) of these HLIRG, finding that a significant
fraction presents a CF consistent with unity. Along with the high X-ray
absorption shown by these sources, this suggests that large amounts of dust and
gas enshroud the nucleus of these HLIRG, as also observed in ULIRG. Our results
are in agreement with previous studies of the IR SED of HLIRG using radiative
transfer models, and we find strong evidence that all HLIRG harbour an AGN.
This work provides further support to the idea that AGN and SB are both crucial
to understand the properties of HLIRG. Our study of the CF supports the
hypothesis that HLIRG can be divided in two different populations.Comment: 17 pages, 9 figures, 4 tables. Accepted for publication in A&
X-ray sources as tracers of the large-scale structure in the Universe
We review the current status of studies of large-scale structure in the X-ray
Universe. After motivating the use X-rays for cosmological purposes, we discuss
the various approaches used on different angular scales including X-ray
background multipoles, cross-correlations of the X-ray background with galaxy
catalogues, clustering of X-ray selected sources and small-scale fluctuations
and anisotropies in the X-ray background. We discuss the implications of the
above studies for the bias parameter of X-ray sources, which is likely to be
moderate for X-ray selected AGN and the X-ray background (~1-2). We finally
outline how all-sky X-ray maps at hard X-rays and medium surveys with large sky
coverage could provide important tests for the cosmological models.Comment: Invited review presented at the Workshop X-ray Astronomy'99: Stellar
endpoints, AGN and the diffuse X-ray background (Astrophys Lett and Comm
RETRASO, a code for modeling reactive transport in saturated and unsaturated porous media
The code RETRASO (REactive TRAnsport of SOlutes) simulates reactive transport of dissolved and gaseous species in non-isothermal saturated or unsaturated problems. Possible chemical reactions include aqueous complexation (including redox reactions), sorption, precipitation-dissolution of minerals and gas dissolution. Various models for sorption of solutes on solids are available, from experimental relationships (linear KD, Freundlich and Langmuir isotherms) to cation exchange and surface complexation models (constant capacitance, diffuse layer and triple layer models). Precipitation-dissolution and aqueous complexation can be modelled in equilibrium or according to kinetic laws. For the numerical solution of the reactive transport equations it uses the Direct Substitution Approach. The use of the code is demonstrated by three examples. The first example models various sorption processes in a smectite barrier. The second example models a complex chemical system in a two dimensional cross-section. The last example models pyrite weathering in an unsaturated medium
On the arcmin structure of the X-ray Universe
We present the angular correlation function of the X-ray population of 1063
XMM-Newton observations at high Galactic latitudes, comprising up to ~30000
sources over a sky area of ~125 sq. degrees in the energy bands: soft (0.5-2
keV) and hard (2-10 keV). This is the largest sample of serendipitous X-ray
sources ever used for clustering analysis purposes to date and the results have
been determined with unprecedented accuracy. We detect significant clustering
signals in the soft and hard bands (~10 sigma and ~5 sigma, respectively). We
deproject the angular correlation function via Limber's equation and calculate
the typical spatial lengths. We infer that AGN at redshifts ~1 are embedded in
dark matter halos with typical masses of log M ~ 12.6/h Msol and lifetimes in
the range ~3-5 x 10^8 years, which indicates that AGN activity is a transient
phase in the life of galaxies.Comment: 4 pages, 1 figure. Proc. of the conference "X-ray Astronomy 2009:
Present status, multiwavelength approach and future perspectives", September
2009, Bologna. To appear in AIP Conf. Proc. (editors: A. Comastri, M. Cappi,
L. Angelini)
Relativistic reflection in the average X-ray spectrum of AGN in the V\'eron-Cetty & V\'eron catalogue
The X-ray spectra of active galactic nuclei (AGN) unveil properties of matter
around the super massive black hole (SMBH). We investigate the X-ray spectra of
AGN focusing on Compton reflection and fluorescence, important processes of
interaction between primary radiation and circum-nuclear material. Unresolved
emission lines (most notably the Fe line) in the X-ray spectra of AGN indicate
that this material is located far away from the SMBH. Contributions from the
inner accretion disk, affected by relativistic effects, have also been detected
in several cases.
We studied the average X-ray spectrum of a sample of 263 X-ray unabsorbed AGN
that yield 419023 counts in the 2-12 keV rest-frame band distributed among 388
XMM-Newton spectra. We fitted the average spectrum using a (basically)
unabsorbed power law (primary radiation). From second model that represents the
interaction of the primary radiation with matter located far away from the
SMBH, we found that it was very significantly detected. Finally, we added a
contribution from interaction with neutral material in the accretion disk close
to the central SMBH, which is therefore smeared by relativistic effects, which
improved the fit at 6 sigma. The reflection factors are 0.65 for the accretion
disk and 0.25 for the torus. Replacing the neutral disk-reflection with
low-ionisation disk reflection, also relativistically smeared, fits the data
equally well, suggesting that we do not find evidence for a significant
ionisation of the accretion disk.
We detect distant neutral reflection in the average spectrum of unabsorbed
AGN with z=0.8. Adding the disk-reflection component associated with a
relativistic Fe line improves the data description at 6 sigma confidence level,
suggesting that both reflection components are present. The disk-reflection
component accounts for about 70 % of the total reflected flux.Comment: Accepted by A&A. 10 pages, 7 figure
- …