from C.L.Sarazin paper.
from C.L.Sarazin paper.
Clusters of galaxies generally form by the gravitational merger of smaller clusters and groups. Major cluster mergers are the most energetic events in the Universe since the Big Bang. Mergers drive shocks into the intracluster gas, and these shocks heat the intracluster gas, and should also accelerate nonthermal relativistic particles. The X-ray signatures of the thermal effects of merger shocks will be discussed. X-ray observations of shocks can be used to determine the geometry and kinematics of the merger. As a result of particle acceleration in shocks, clusters of galaxies should contain very large populations of relativistic electrons and ions. Electrons with Lorentz factors gamma sim 300 (energies E = gamma m_e c^2 sim 150 MeV) are expected to be particularly common. Observations and models for the radio, extreme ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles accelerated in these shocks will also be described. The predicted gamma-ray fluxes of clusters should make them easily observable with GLAST.
We derive the atomic hydrogen content for 1900 spirals in the fields of eighteen nearby clusters. By comparing the HI deficiency distributions of the galaxies inside and outside one Abell radius (R_A) of the center of each region, we find that two thirds of the clusters in our sample show a dearth of neutral gas in their interiors. Possible connections between the gaseous deficiency and the characteristics of both the underlying galaxies and their environment are investigated in order to gain insight into the cause of HI depletion. In the clusters in which neutral gas deficiency is pronounced, we see clear indications that the amount of depletion is related to the morphology of the galaxies: early-type and, probably, dwarf spirals are more easily emptied of gas than the intermediate Sbc--Sc types. Gas contents below one tenth, and even one hundredth, of the expected value have been measured, implying that gas removal must be very efficient. Our 21-cm data also show that in the HI-deficient clusters the proportion of gas-poor spirals decreases continuously towards the outskirts of these systems, the zone of significant deficiency reaching as far out as 2R_A. In an independent analysis of the Virgo cluster, we find suggestive indications that gas losses are driven by the interaction of the disks with the inner hot intracluster gas around M87. We also report evidence that gas-poor spirals follow more radial orbits than those of the gas-rich objects. We conclude that ISM-IGM hydrodynamic effects appear as the most plausible cause of HI removal.
RICHARD LIEU, MASSIMILIANO BONAMENTE and JONATHAN P.D. MITTAZ .ps .pdf
The Virgo and A1795 clusters of galaxies were re-observed by EUVE with in situ background measurements by pointing at small offsets. Earlier, a similar re-observationa l strategy applied to the cluster A2199 revealed that the background radial profile was consistent with a flat distribution, and therefore the original method of extracting cluster EUV signals by the subtraction of an aymptotically determined background was valid. It is shown here that the same conclusions hold for the current sample. In particular, for A1795, the in situ background was observed with equal exposure as tha t for the cluster (each 78 ksec), no tangible inner enhancement of the background profile (which may mimic cluster emission) is apparent. A model of the background was obtained from its known properties and the in situ measurements. The subtracted cluster fluxes remain in agreement with those reported in our discovery papers. They are also consistent with results from the most conservative procedure of direct point-to-point subtraction of the in situ background and proper error propagation, which still preserves the existence of the EUV excess and its rising radial trend. We present evidence which argues against the soft excess as due to peculiarities in the line-of-sight Galactic absorption. The data appear to favor a thermal origin of the emission.
The number of diffuse radio halos in clusters of galaxies has grown in recent years, making it possible to derive statistical properties of these sources and of the hosting clusters. We show that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes. The radio and X-ray structures are often similar, and correlations are found between radio and X-ray parameters. A model for the formation and maintenance of these sources is suggested, including a first phase of relativistic particle injection by past major merger events, starbust and AGN activity and a second phase of reacceleration by the energy supplied from shocks and turbulence in a recent merger event.
Galaxy clusters are astrophysical systems which host different physical phenomena of thermal and non-thermal origin. Here we discuss a few arguments which relate the presence of Cold dark Matter with the existence of non-thermal phenomena in these cosmic structures. Specifically, we discuss here the origin of extended cluster radio halos in a model in which neutralinos, chi, are assumed to be the best candidates for the Cold Dark Matter. We finally show that the next generation gamma-ray experiments have the power to discriminate among different models for the origin of non-thermal phenomena in galaxy clusters.
MASSIMILIANO BONAMENTE, RICHARD LIEU and JONATHAN P.D. MITTAZ .ps .pdf
The discovery of EUV and soft X--ray excess emission in clusters of galaxies (the cluster soft--excess phenomenon) challenged the notion of the hot (sim 10({7-8) K) gas as the only dominant thermal component of the intracluster medium (ICM). The spatial analysis of ROSAT PSPC 1/4 keV images presented here reveals compelling evidence for substantial amounts of cold gas (HI) in the ICM of the Coma and Virgo clusters. This finding bolsters the original interpretation of the soft excess as emission from a `warm' (sim 10^6 K) gas and points to the scenario of a multiphase ICM where the hot component co--exists with gases at sub--Virial temperatures (the `warm' and `cold' phases).
The recent discovery in some clusters of galaxies of soft and hard excesses with respect to the thermal bremsstrahlung emission by the hot intracluster medium has opened a new window in the study of this class of objects. For the hard excess, the spectral capabilities of BeppoSAX and RXTE have led to a significant breakthrough in the discovery of this new spectral component predicted in clusters showing radio halos. Various interpretations have been proposed since the discovery of nonthermal emission in the Coma cluster. We discuss future observations to search for hard X-ray tails in clusters of galaxies.
EVAN SCANNAPIECO AND TOM BROADHURST .ps .pdf
The recent discovery in some clusters of galaxies of soft and hard excesses with respect to the thermal bremsstrahlung emission by the hot intracluster medium has opened a new window in the study of this class of objects. For the hard excess, the spectral capabilities of BeppoSAX and RXTE have led to a significant breakthrough in the discovery of this new spectral component predicted in clusters showing radio halos. Various interpretations have been proposed since the discovery of nonthermal emission in the Coma cluster. We discuss future observations to search for hard X-ray tails in clusters of galaxies.
We have investigated evolution of non-thermal emission from relativistic electrons accelerated at around the shock fronts during merger of clusters of galaxies. We estimate synchrotron radio emission and inverse Compton scattering of cosmic microwave background photons from extreme ultraviolet (EUV) to hard X-ray range. The hard X-ray emission is most luminous in the later stage of merger. Both hard X-ray and radio emissions are luminous only while signatures of merging events are clearly seen in thermal intracluster medium (ICM). On the other hand, EUV radiation is still luminous after the system has relaxed. Propagation of shock waves and bulk-flow motion of ICM play crucial roles to extend radio halos. In the contracting phase, radio halos are located at the hot region of ICM, or between two substructures. In the expanding phase, on the other hand, radio halos are located between two ICM hot regions and shows rather diffuse distribution.