Clusters and the Cosmic Web

We discuss the intimate relationship between the filamentary features and the rare dense compact cluster nodes in this network, via the large scale tidal field going along with them, following the cosmic web theory developed Bond et al. The Megaparsec scale tidal shear pattern is responsible for the contraction of matter into filaments, and its link with the cluster locations can be understood through the implied quadrupolar mass distribution in which the clusters are to be found at the sites of the overdense patches. We present a new technique for tracing the cosmic web, identifying planar walls, elongated filaments and cluster nodes in the galaxy distribution. This will allow the practical exploitation of the concept of the cosmic web towards identifying and tracing the locations of the gaseous WHIM. These methods, the Delaunay Tessellation Field Estimator (DTFE) and the Morphology Multiscale Filter (MMF) find their basis in computational geometry and visualization.Comment: 13 pages, 6 figures, appeared in proceedings workshop "Measuring the Diffuse Intergalactic Medium", eds. J-W. den Herder and N. Yamasaki, Hayama, Japan, October 2005. For version with high-res figures see http://www.astro.rug.nl/~weygaert/weywhim05.pd

ROSAT PSPC observations of Cygnus-A : X-ray spectra of the cooling flow and hot spots

We present a {\it ROSAT} Position Sensitive Proportional Counter (PSPC) observation of the powerful radio galaxy Cygnus-A. The X-ray emission in the {\it ROSAT} band is dominated by thermal emission from the hot intracluster medium of the associated cluster. Image deprojection confirms the existence of a significant cluster cooling flow with total mass deposition rate of $\sim 250\,{\rm M}_\odot\,{\rm yr}^{-1}$ and a (Hubble time) cooling radius of $\sim 180$\,kpc. Spectral data show the gradient in the emission-weighted mean temperature with the temperature decreasing towards the centre of the cluster. We also find signatures of the radio source: in particular, we detect the X-ray emission from the western radio hot spot previously found by the {\it ROSAT} High Resolution Imager (HRI). We find the emission from the hot spot to be hard and discuss the physical implications of this result.Comment: uuencoded compressed postscript. The preprint is also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm

Multiwavelength Analysis of Dark Matter Annihilation and RX-DMFIT

Dark matter (DM) particles are predicted by several well motivated models to yield Standard Model particles through self-annihilation that can potentially be detected by astrophysical observations. In particular, the production of charged particles from DM annihilation in astrophysical systems that contain magnetic fields yields radio emission through synchrotron radiation and X-ray emission through inverse Compton scattering of ambient photons. We introduce RX-DMFIT, a tool used for calculating the expected secondary emission from DM annihilation. RX-DMFIT includes a wide range of customizable astrophysical and particle parameters and incorporates important astrophysics including the diffusion of charged particles, relevant radiative energy losses, and magnetic field modelling. We demonstrate the use and versatility of RX-DMFIT by analyzing the potential radio and X-ray signals for a variety of DM particle models and astrophysical environments including galaxy clusters, dwarf spheroidal galaxies and normal galaxies. We then apply RX-DMFIT to a concrete example using Segue I radio data to place constraints for a range of assumed DM annihilation channels. For WIMP models with $M_{\chi} \leq 100$ GeV and assuming weak diffusion, we find that the the leptonic $\mu^+\mu^-$ and $\tau^+\tau^-$ final states provide the strongest constraints, placing limits on the DM particle cross-section well below the thermal relic cross-section, while even for the $b\bar{b}$ channel we find limits close to the thermal relic cross-section. Our analysis shows that radio emission provides a highly competitive avenue for dark matter searches.Comment: 21 pages, 9 figures, 2 tables, corrections to figures, additional text, accepted to JCA

Constraining H0 from Chandra Observations of Q0957+561

We report the detection of the lens cluster of the gravitational lens (GL) system Q0957+561 from a deep observation with the Advanced CCD Imaging Spectrometer on-board the Chandra X-ray Observatory. Intracluster X-ray emission is found to be centered 4.3 +/- 1.3 arcsec east and 3.5(-0.6,+1.3) arcsec north of image B, nearer than previous estimates. Its spectrum can be modeled well with a thermal plasma model consistent with the emission originating from a cluster at a redshift of 0.36. Our best-fit estimates of the cluster temperature of T_e = 2.09(-0.54,+0.83) keV (90 percent confidence) and mass distribution of the cluster are used to derive the convergence parameter kappa, the ratio of the cluster surface mass density to the critical density required for lensing. We estimate the convergence parameter at the location of the lensed images A and B to be kappa_A = 0.22(+0.14,-0.07) and kappa_B = 0.21(+0.12,-0.07), respectively (90 percent confidence levels). The observed cluster center, mass distribution and convergence parameter kappa provide additional constraints to lens models of this system. Our new results break a mass-sheet degeneracy in GL models of this system and provide better constraints of ~ 29 percent (90 percent confidence levels) on the Hubble constant. We also present results from the detection of the most distant X-ray jet (z = 1.41) detected to date. The jet extends approximately 8 arcsec NE of image A and three knots are resolved along the X-ray jet with flux densities decreasing with distance from the core. The observed radio and optical flux densities of the knots are fitted well with a synchrotron model and the X-ray emission is modeled well with inverse Compton scattering of Cosmic Microwave Background photons by synchrotron-emitting electrons in the jet.Comment: 18 pages, includes 7 figures, Accepted for publication in Ap

Dark Matter in Modern Cosmology

The presence of Dark Matter (DM) is required in the universe regulated by the standard general relativistic theory of gravitation. The nature of DM is however still elusive to any experimental search. We discuss here the process of accumulation of evidence for the presence of DM in the universe, the astrophysical probes for the leading DM scenarios that can be obtained through a multi-frequency analysis of cosmic structures on large scales, and the strategies related to the multi-messenger and multi-experiment astrophysical search for the nature of the DM.Comment: 25 pages, 9 figures. Updated version of the review included in ASTROPHYSICS AND COSMOLOGY AFTER GAMOW: Proceedings of the 4th Gamow International Conference on Astrophysics and Cosmology After Gamow and the 9th Gamow Summer School "Astronomy and Beyond: Astrophysics, Cosmology, Radio Astronomy, High Energy Physics and Astrobiology". AIP Conference Proceedings, Volume 1206, p.

Tracing the Warm Hot Intergalactic Medium in the local Universe

We present a simple method for tracing the spatial distribution and predicting the physical properties of the Warm-Hot Intergalactic Medium (WHIM), from the map of galaxy light in the local universe. Under the assumption that biasing is local and monotonic we map the ~ 2 Mpc/h smoothed density field of galaxy light into the mass density field from which we infer the spatial distribution of the WHIM in the local supercluster. Taking into account the scatter in the WHIM density-temperature and density-metallicity relation, extracted from the z=0 outputs of high-resolution and large box size hydro-dynamical cosmological simulations, we are able to quantify the probability of detecting WHIM signatures in the form of absorption features in the X-ray spectra, along arbitrary directions in the sky. To illustrate the usefulness of this semi-analytical method we focus on the WHIM properties in the Virgo Cluster region.Comment: 16 pages 11 Figures. Discussion clarified, alternative methods proposed. Results unchanged. MNRAS in pres