ASCA observations of groups at radii of low overdensity. Implications for the cosmic preheating

Through a three-dimensional modeling of ASCA observations, we performed a spatially resolved X-ray spectroscopic study, extending to radii exceeding 150 kpc, for a sample of nine groups of galaxies. Combined with published ROSAT results, we conclude that these systems generally exhibit a strong temperature decline at outer radii. In our best case, NGC 3268, this corresponds to a flattening of the entropy pro le at a level of similar to400 keV cm(2). This value is high compared both to the observed entropy floor of similar to100 keV cm(2) and to the expected value from gravitational heating. We suggest that the observed entropy profile in most groups at densities exceeding 500 times the critical is driven purely by nongravitational heating processes. After comparison with a larger sample of groups and clusters, we conclude that there is a variation in the level of nongravitational heating between similar to100 and similar to400 keV cm(2) within every system. Using models of cluster formation as a reference frame, we established that the accreted gas reaches an entropy level of 400 keV cm(2) by redshift 2.0-2.5, while such high entropies where not present at redshifts higher than 2.8-3.5, favoring nearly instantaneous preheating. Adopting galactic winds as a source of preheating and scaling the released energy by the observed metal abundance, the variation in the preheating could be ascribed mostly to variation in the typical overdensity of the energy injection, from 30 for an entropy floor of 100 keV cm(2) to similar to5 for an entropy of 400 keV cm(2).