The extreme ultraviolet excess emission in five clusters of galaxies revisited

Evidence for excess extreme ultraviolet (EUV) emission over a tail of X-ray gas bremsstrahlung emission has been building up recently, but in some cases remains controversial, mostly due to the moderate quality of the EUV data. In order to improve the signal to noise ratio in the EUV, we have performed the wavelet analysis and image reconstructions for five clusters of galaxies observed both at EUV and X-ray energies with the EUVE and ROSAT satellites respectively. The profiles of the EUV and X-ray reconstructed images all differ at a very large confidence level and an EUV excess over a thermal bremsstrahlung tail is detected in all five clusters (Abell 1795, Abell 2199, Abell 4059, Coma and Virgo) up to large radii. These results, coupled with recent XMM-Newton observations, suggest that the EUV excess is probably non thermal in origin.Comment: accepted for publication in Astronomy & Astrophysics, final versio

XMM-Newton discovery of O VII emission from warm gas in clusters of galaxies

XMM-Newton recently discovered O VII line emission from ~2 million K gas near the outer parts of several clusters of galaxies. This emission is attributed to the Warm-Hot Intergalactic Medium. The original sample of clusters studied for this purpose has been extended and two more clusters with a soft X-ray excess have been found. We discuss the physical properties of the warm gas, in particular the density, spatial extent, abundances and temperature.Comment: 8 pages, 3 figures, conference "Soft X-ray emission from clusters of galaxies and related phenomena", ed. R. Lieu, Kluwer, in pres

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

Controlling the onset of turbulence by streamwise traveling waves. Part 2. Direct numerical simulations

This work builds on and confirms the theoretical findings of Part 1 of this paper, Moarref & Jovanovi\'c (2010). We use direct numerical simulations of the Navier-Stokes equations to assess the efficacy of blowing and suction in the form of streamwise traveling waves for controlling the onset of turbulence in a channel flow. We highlight the effects of the modified base flow on the dynamics of velocity fluctuations and net power balance. Our simulations verify the theoretical predictions of Part 1 that the upstream traveling waves promote turbulence even when the uncontrolled flow stays laminar. On the other hand, the downstream traveling waves with parameters selected in Part 1 are capable of reducing the fluctuations' kinetic energy, thereby maintaining the laminar flow. In flows driven by a fixed pressure gradient, a positive net efficiency as large as 25 % relative to the uncontrolled turbulent flow can be achieved with downstream waves. Furthermore, we show that these waves can also relaminarize fully developed turbulent flows at low Reynolds numbers. We conclude that the theory developed in Part 1 for the linearized flow equations with uncertainty has considerable ability to predict full-scale phenomena.Comment: To appear in J. Fluid Mec

Extreme Ultraviolet Emission from Clusters of Galaxies: Inverse Compton Radiation from a Relic Population of Cosmic Ray Electrons?

We suggest that the luminous extreme ultraviolet (EUV) emission which has been detected recently from clusters of galaxies is Inverse Compton (IC) scattering of Cosmic Microwave Background (CMB) radiation by low energy cosmic ray electrons in the intracluster medium. The cosmic ray electrons would have Lorentz factors of gamma ~ 300, and would lose energy primarily by emitting EUV radiation. These particles have lifetimes comparable to the Hubble time; thus, the electrons might represent a relic population of cosmic rays produced by nonthermal activity over the history of the cluster. The IC model naturally explains the observed increase in the ratio of EUV to X-ray emission with radius in clusters. The required energy in cosmic ray electrons is typically 1--10% of the thermal energy content of the intracluster gas. We suggest that the cosmic ray electrons might have been produced by supernovae in galaxies, by radio galaxies, or by particle acceleration in intracluster shocks.Comment: ApJ Letters, in press, 4 pages with 1 embedded figure, Latex in emulateapj styl