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

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)

On evolutionarily stable strategies and replicator dynamics in asymmetric two-population games

We analyze the main dynamical properties of the evolutionarily stable strategy ESS for asymmetric two-population games of finite size in its corresponding replicator dynamics. We introduce a defnition of ESS for two-population asymmetric games and a method of symmetrizing such an asymmetric game. Then, we show that every strategy profile of the asymmetric game corresponds to a strategy in the symmetric game, and that every Nash equilibrium (NE) of the asymmetric game corresponds to a (symmetric) NE of the symmetric version game. So, we study (standard) replicator dynamics for the asymmetric game and define corresponding (non-standard) dynamics of the symmetric game.Asymmetric game; Evolutionary games; ESS; Replicator dynamics.

The evolutionary sequence of active galactic nuclei and galaxy formation revealed

Today, almost every galaxy spheroid contains a massive black hole: a remnant of, and testament to, a period in its evolution when it contained an active galactic nucleus (AGN). However, the sequence and timescales of the formation of the black hole and surrounding spheroid of stars are completely unknown, leaving a large gap in our knowledge of how the universe attained its present appearance. Here we present submillimeter observations of matched samples of X-ray absorbed and unabsorbed AGNs that have luminosities and redshifts characteristic of the sources responsible for most of the mass in present-day black holes. Strong submillimeter emission, an isotropic signature of copious star formation, is found only in the X-ray–absorbed sample, ruling out orientation effects as the cause of the absorption. The space density and luminosity range of the X-ray–absorbed AGNs indicate that they are undergoing the transition between a hidden growth phase and an unabsorbed AGN phase and imply that the X-ray–absorbed period in the AGN's evolution coincides with the formation of the galaxy spheroid

X-ray absorbed QSOs and the QSO evolutionary sequence

Unexpected in the AGN unified scheme, there exists a population of broad-line z~2 QSOs which have heavily absorbed X-ray spectra. These objects constitute 10% of the population at luminosities and redshifts characteristic of the main producers of QSO luminosity in the Universe. Our follow up observations in the submm show that these QSOs are often embedded in ultraluminous starburst galaxies, unlike most QSOs at the same redshifts and luminosities. The radically different star formation properties between the absorbed and unabsorbed QSOs implies that the X-ray absorption is unrelated to the torus invoked in AGN unification schemes. Instead, these results suggest that the objects represent a transitional phase in an evolutionary sequence relating the growth of massive black holes to the formation of galaxies. The most puzzling question about these objects has always been the nature of the X-ray absorber. We present our study of the X-ray absorbers based on deep (50-100ks) XMM-Newton spectroscopy. We show that the absorption is most likely due to a dense ionised wind driven by the QSO. This wind could be the mechanism by which the QSO terminates the star formation in the host galaxy, and ends the supply of accretion material, to produce the present day black hole/spheroid mass ratio.Comment: 4 pages, to appear in conference proceedings "Studying Galaxy Evolution with Spitzer and Herschel

Some results on thermal stress of layered plates and shells by using Unified Formulation

This work presents some results on two-dimensional modelling of thermal stress problems in multilayered structures. The governing equations are written by referring to the Unified Formulation (UF) introduced by the first author. These equations are obtained in a compact form, that doesn&#039;t depend on the order of expansion of variables in the thickness direction or the variable description (layer-wise models and equivalent single layers models). Classical and refined theories based on the Principle of Virtual Displacements (PVD) and advanced mixed theories based on the Reissner Mixed Variational Theorem (RMVT) are both considered. As a result, a large variety of theories are derived and compared. The temperature profile along the thickness of the plate/shell is calculated by solving the Fourier&#039;s heat conduction equation. Alternatively, thermo-mechanical coupling problems can be considered, in which the thermal variation is influenced by mechanical loading. Exact closed-form solutions are provided for plates and shells, but also the applications of the Ritz method and the Finite Element Method (FEM) are presented

The Evolutionary Processes for the Populations of Firms and Workers

This paper analyzes the cultural evolution of firms and workers. Following an imitation rule, each firm and worker decides whether to be innovative (or not) and skilled (or unskilled). We apply evolutionary game theory to find the system of replicator dynamics, and characterize the low-level and high-level equilibria as Evolutionarily Stable Strategies (ESS) “against the field.” Hence, we study how a persistent state of underdevelopment can arise in strategic environments in which players are imitative rather than rational maximizers. We show that when the current state of the economy is in the basin of attraction of the poverty trap, players should play against the field if they want to change their status quo. The threshold level to overcome the poverty trap can be lowered if there is an appropriate policy using income taxes, education costs and skill premia. Hence, we study the replicator dynamics with a subsidy and payoff taxation to overcome the poverty trap.Imitative behavior, conformism, poverty traps, skill premium, strategic complementarities

Cosmology in the Solar System: Pioneer effect is not cosmological

Does the Solar System and, more generally, a gravitationally bound system follow the cosmic expansion law ? Is there a cosmological influence on the dynamics or optics in such systems ? The general relativity theory provides an unique and unambiguous answer, as a solution of Einstein equations with a local source in addition to the cosmic fluid, and obeying the correct (cosmological) limiting conditions. This solution has no analytic expression. A Taylor development of its metric allows a complete treatment of dynamics and optics in gravitationally bound systems, up to the size of galaxy clusters, taking into account both local and cosmological effects. In the solar System, this provides an estimation of the (non zero) cosmological influence on the Pioneer probe: it fails to account for the " Pioneer effect " by about 10 orders of magnitude. We criticize contradictory claims on this topic

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