284 research outputs found

    The angular correlation function of the ROSAT All Sky Survey Bright Source Catalogue

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    We have derived the angular correlation function of a sample of 2096 sources detected in the ROSAT All Sky Survey Bright Source Catalogue, in order to investigate the clustering properties of AGN in the local Universe. Our sample is constructed by rejecting all known stars, as well as extended X-ray sources. Areas with |b|<30 deg. and declination <-30 deg. are also rejected due to the high or uncertain neutral hydrogen absorption. Cross-correlation of our sample with the Hamburg/RASS optical identification catalogue, suggests that the vast majority of our sources are indeed AGN. A 4.1 sigma correlation signal between 0 and 8 degrees was detected with w(theta<8 deg.)=0.025 +- 0.006. Assuming the usual power-law form of the 2-point correlation function we find an angular correlationlength of 0.062 degrees. Deprojection on 3 dimensions, using the Limber's equation, yields a spatial correlation length of 6.0+- 1.6 h^-1 Mpc. This is consistent with the AGN clustering results derived at higher redshifts in optical surveys and suggests a comoving model for the clustering evolution.Comment: 5 pages, revised version accepted in MNRA

    The X-ray luminosity function of local galaxies

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    We present an estimate of the local X-ray luminosity function and emissivity for different subsamples of galaxies namely Seyferts, LINERS, star-forming and passive (no-emission-line) galaxies. This is performed by convolving their optical luminosity function, as derived from the Ho et al. spectroscopic sample of nearby galaxies with the corresponding L_x/L_B relation. The local galaxy emissivity is about 1.6 X 10^{39} h erg/sec Mpc^3 in agreement with the results of Lahav et al. derived from cross-correlation techniques of the X-ray background with optical and infrared galaxy catalogues. From our analysis, it becomes evident that the largest fraction of the galaxy emissivity comes from galaxies associated with AGN (Seyferts but also LINERS) while the contribution of star-forming and passive galaxies is small. This independently supports the view that most of the yet unidentified X-ray sources in deep \rosat fields which are associated with faint optical galaxies, do harbour an AGN.Comment: 4 pages, 2 figures, MNRAS Pink pages (in press

    The Clustering of XMM-Newton Hard X-ray Sources

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    We present the clustering properties of hard (2-8 keV) X-ray selected sources detected in a wide field (~2 deg^{2}) shallow [f(2-8 keV)~ 10^{-14} erg cm^{-2} s^{-1}] and contiguous XMM-Newton survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect a ~ 4\sigma correlation signal out to 300 arcsec with w(theta < 300^{''}) ~ 0.13 +- 0.03. Modeling the two point correlation function as a power law of the usual form we find: theta_o=48.9^{+15.8}_{-24.5} arcsec and gamma=2.2 +- 0.30. Fixing the correlation function slope to gamma=1.8 we obtain theta_o=22.2^{+9.4}_{-8.6} arcsec. Using Limber's integral equation and a variety of possible luminosity functions of the hard X-ray population, we find a relatively large correlation length, ranging from r_o ~ 9 to 19 h^{-1} Mpc (for gamma=1.8 and the concordance cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics.Comment: In "Multiwavelength AGN Surveys" (Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujica, Singapore: World Scientific, 200

    Cluster versus POTENT Density and Velocity Fields: Cluster Biasing and Omega

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    The density and velocity fields as extracted from the Abell/ACO clusters are compared to the corresponding fields recovered by the POTENT method from the Mark~III peculiar velocities of galaxies. In order to minimize non-linear effects and to deal with ill-sampled regions we smooth both fields using a Gaussian window with radii ranging between 12 - 20\hmpc. The density and velocity fields within 70\hmpc exhibit similarities, qualitatively consistent with gravitational instability theory and a linear biasing relation between clusters and mass. The random and systematic errors are evaluated with the help of mock catalogs. Quantitative comparisons within a volume containing âˆŒâ€‰âŁ12\sim\!12 independent samples yield \betac\equiv\Omega^{0.6}/b_c=0.22\pm0.08, where bcb_c is the cluster biasing parameter at 15\hmpc. If bc∌4.5b_c \sim 4.5, as indicated by the cluster correlation function, our result is consistent with Ω∌1\Omega \sim 1.Comment: 18 pages, latex, 2 ps figures 6 gif figures. Accepted for pubblications in MNRA