On the Influence of X-Ray Galaxy Clusters in the Fluctuations of the Cosmic Microwave Background

The negative evolution found in X--ray clusters of galaxies limits the amount of available hot gas for the inverse Compton scattering of the Cosmic Microwave Background (the Sunyaev--Zel'dovich effect). Using a parametrisation of the X-ray luminosity function and its evolution in terms of a coalescence model (as presented in the analysis of a flux limited X-ray cluster sample by Edge et al. 1990), as well as a simple virialised structure for the clusters (which requires a gas to total mass fraction \approxgt 0.1 in order to reproduce observed properties of nearby clusters) we show that the Compton distortion $y$ parameter is about two orders of magnitude below the current FIRAS upper limits. Concerning the anisotropies imprinted on arcmin scales they are dominated by the hottest undetected objects. We show that they are negligible ({\Delta T\over T}\approxlt 10^{-7}) at wavelengths \lambda\approxgt 1~mm. At shorter wavelengths they become more important (${\Delta T\over T}\sim 10^{-6}$ at $\lambda\sim 0.3$~mm), but in fact most clusters will produce an isolated and detectable feature in sky maps. After removal of these signals, the fluctuations imprinted by the remaining clusters on the residual radiation are still much smaller. The conclusion is that X-ray clusters can be ignored as sources of Cosmic Microwave Background fluctuations.Comment: 20 pages Plain Tex, 7 figures available upon request,UCAST-94-0

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

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

Do nuclear starbursts obscure the X-ray background?

We propose a model for the source of the X-ray background (XRB) in which low luminosity active nuclei (L<10^43 erg/s) are obscured by nuclear starbursts within the inner 100pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion of a few 100 km/s and cloud-cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the starforming cluster itself and the central black hole. A hot (T=10^6-10^7 K) virialised phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of 10^7 solar masses through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.Comment: 5 pages, 2 PS figures included in the text, MNRAS in the press. Also at http://www.ifca.unican.es/~barcons/preprints.htm

The absorption spectra of Q1107+487 and Q1442+295

We present the first moderate resolution (approx. 40--120 km/s) spectroscopic observations of the bright (V<16.7) high-redshift QSOs Q1107+487 (z_{em} = 2.965) and Q1442+295 (z_{em} = 2.669) (Sanduleak & Pesch 1989). The relatively high signal to noise reached in the spectra along with an extensive wavelength coverage of the Lyman alpha and the Lyman beta forest allowed us to obtain, through profile fitting, column densities and Doppler parameters of the Lyman clouds towards these QSOs. The spectral coverage of regions longward of the Lyman alpha emission line of the QSOs at the expected wavelengths of CIV 1548,1550 at the redshifts of the Lyman alpha forest allowed us to identify some heavy element absorption systems (hereafter HEASs) towards these QSOs. We have found no Lyman absorption system or HEAS towards these QSOs for which our data allow a deuterium measurement or to provide an interesting upper limit for the D/H ratio. The reason for this is that the Lyman lines with high column density detected towards these QSOs belong to absorption systems showing velocity structure