Does magnetic pressure affect the ICM dynamics?

A possible discrepancy found in the determination of mass from gravitational lensing data, and from X-rays observations, has been largely discussed in the latest years (for instance, Miralda-Escude & Babul (1995)). Another important discrepancy related to these data is that the dark matter is more centrally condensed than the X-ray-emitting gas, and also with respect to the galaxy distribution (Eyles et al. 1991). Could these discrepancies be consequence of the standard description of the ICM, in which it is assumed hydrostatic equilibrium maintained by thermal pressure? We follow the evolution of the ICM, considering a term of magnetic pressure, aiming at answering the question whether or not these discrepancies can be explained via non-thermal terms of pressure. Our results suggest that the magnetic pressure could only affect the dynamics of the ICM on scales as small as < 1kpc. Our models are constrained by the observations of large and small scale fields and we are successful at reproducing available data, for both Faraday rotation limits and inverse Compton limits for the magnetic fields. In our calculations the radius (from the cluster center) in which magnetic pressure reaches equipartition is smaller than radii derived in previous works, as a consequence of the more realistic treatment of the magnetic field geometry and the consideration of a sink term in the cooling flow.Comment: 8 pages with 7 figures included. MNRAS accepted. Minor changes in the section of discussions and conclusions. Also available at http://www.iac.es/publicaciones/preprints.htm

On the absence of gravitational lensing of the cosmic microwave background

The magnification of distant sources by mass clumps at lower ($z \leq 1$) redshifts is calculated analytically. The clumps are initially assumed to be galaxy group isothermal spheres with properties inferred from an extensive survey. The average effect, which includes strong lensing, is exactly counteracted by the beam divergence in between clumps (more precisely, the average reciprocal magnification cancels the inverse Dyer-Roeder demagnification). This conclusion is in fact independent of the matter density function within each clump, and remains valid for arbitrary densities of matter and dark energy. When tested against the CMB, a rather large lensing induced {\it dispersion} in the angular size of the primary acoustic peaks of the TT power spectrum is inconsistent with WMAP observations. The situation is unchanged by the use of NFW profiles for the density distribution of groups. Finally, our formulae are applied to an ensemble of NFW mass clumps or isothermal spheres having the parameters of galaxy {\it clusters}. The acoustic peak size dispersion remains unobservably large, and is also excluded by WMAP. For galaxy groups, two possible ways of reconciling with the data are proposed, both exploiting maximally the uncertainties in our knowledge of group properties. The same escape routes are not available in the case of clusters, however, because their properties are well understood. Here we have a more robust conclusion: neither of the widely accepted models are good description of clusters, or important elements of physics responsible for shaping zero curvature space are missing from the standard cosmological model. When all the effects are accrued, it is difficult to understand how WMAP could reveal no evidence whatsoever of lensing by groups and clusters.Comment: ApJ v628, pp. 583-593 (August 1, 2005

A massive warm baryonic halo in the Coma cluster

Several deep PSPC observations of the Coma cluster reveal a very large-scale halo of soft X-ray emission, substantially in excess of the well known radiation from the hot intra-cluster medium. The excess emission, previously reported in the central region of the cluster using lower-sensitivity EUVE and ROSAT data, is now evident out to a radius of 2.6 Mpc, demonstrating that the soft excess radiation from clusters is a phenomenon of cosmological significance. The X-ray spectrum at these large radii cannot be modeled non-thermally, but is consistent with the original scenario of thermal emission from warm gas at ~ 10^6 K. The mass of the warm gas is on par with that of the hot X-ray emitting plasma, and significantly more massive if the warm gas resides in low-density filamentary structures. Thus the data lend vital support to current theories of cosmic evolution, which predict that at low redshift \~30-40 % of the baryons reside in warm filaments converging at clusters of galaxies.Comment: Astrophysical Journal, in pres