The entropy and energy of intergalactic gas in galaxy clusters

Abstract

Studies of the X-ray surface brightness profiles of clusters, coupled with theoretical considerations, suggest that the breaking of self-similarity in the hot gas results from an `entropy floor', established by some heating process, which affects the structure of the intracluster gas strongly in lower mass systems. Fitting analytical models for the radial variation in gas density and temperature to X-ray spectral images from the ROSAT PSPC and ASCA GIS, we derive gas entropy profiles for 20 galaxy clusters and groups. Scaling these profiles to coincide in the self-similar case, the lowest mass systems are found to have higher scaled entropy profiles than more massive systems. This appears to be due to a baseline entropy of 70-140 h50^-1/3 keV cm^2, depending on the extent to which shocks have been suppressed in low mass systems. The extra entropy may be present in all systems, but is detectable only in poor clusters, compared to the entropy generated by gravitational collapse. This excess entropy appears to be distributed uniformly with radius outside the central cooling regions. We determine the energy associated with this entropy floor, by studying the net reduction in binding energy of the gas in low mass systems, and find that it corresponds to a preheating temperature of ~0.3 keV. Since the relationship between entropy and energy injection depends upon gas density, we can combine the excesses of 70-140 keV cm^2 and 0.3 keV to derive the typical electron density of the gas into which the energy was injected. The resulting value of 1-3x10^-4 h50^1/2 cm-3, implies that the heating must have happened prior to cluster collapse but after a redshift z~7-10. The energy requirement is well matched to the energy from supernova explosions responsible for the metals which now pollute the intracluster gas.Comment: 15 pages, 10 figures, accepted for publication in MNRA

SNe heating and the chemical evolution of the intra-cluster medium

“…However, all of these estimated energies for SNe are based on the assumption of thermal winds. If the wind velocity were ∼ 3 times larger than the escape velocity more energy could be gained by the ICM (see Lloyd-Davies et al 2000). …”

Gas entropy in a representative sample of nearby X-ray galaxy clusters (REXCESS): relationship to gas mass fraction

“…Early measurements of the entropy based primarily on ROSAT and ASCA data indicated that groups had flatter entropy profiles than cluster scale objects (David et al 1996), and measurements of the entropy at 0.1 R 200 revealed an entropytemperature (K − T ) relation that was shallower than expected (Ponman et al 1999;Lloyd-Davies et al 2000;Ponman et al 2003). These data also afforded the first indications for excess entropy above that expected from gravitational collapse even at large radius in group-scale objects (Finoguenov et al 2002;Ponman et al 2003).…”

XMM-Newton observation of the distant ($\vec{z=0.6}$) galaxy cluster RX J1120.1+4318

“…A definitive evidence of breaking of self-similarity is the entropy excess (the "entropy floor") detected in the core of cool systems with a baseline entropy of about 70−140 keV cm 2 (Ponman et al 1999;Lloyd-Davies et al 2000). The origin of this break of similarity is not yet understood.…”