The entropy and energy of intergalactic gas in galaxy clusters

By E. Lloyd-Davies, Trevor J. Ponman, D. B. Cannon. 2000

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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. By 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 have derived gas entropy profiles for 20 galaxy clusters and groups. We show that, when these profiles are scaled such that they should lie on top of one another in the case of self-similarity, the lowest-mass systems have higher-scaled entropy profiles than more massive systems. This appears to be due to a baseline entropy of 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, where it is significant compared with 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 pre-heating temperature of ∼0.3 keV. Since the relationship between entropy and energy injection depends upon gas density, we are able to combine the excesses of 70–140 keV cm2 and 0.3 keV to derive the typical electron density of the gas into which the energy was injected. The resulting value of 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.

Subfields: Astronomy and Astrophysics, Instrumentation, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/0e659de1b9e5d006e4563dee0961c1706117093d

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Citations in the corpus

Modified Entropy Models for the Intracluster Medium (2002, background): “Depending on what is assumed about the unmodified configuration, these excess energies can range from 0.3–1.0 keV for groups (e.g., Lloyd-Davies et al. 2000, 2002) to over 2 keV for clusters (e.g., Wu et al. 2000).”
The Birmingham-CfA cluster scaling project - III. Entropy and similarity in galaxy systems (2003, methodology): “In practice, it is found (Ponman et al. 1999; Lloyd-Davies et al. 2000) that an excess in the entropy, above the self-similar prediction, is apparent in the inner regions of poor clusters and groups, out- c© 2002 RAS side the dense central regions in which cooling is expected to radiate away…”
Pre-Processing of Galaxies before Entering a Cluster (2003, background): “At least for nearby clusters and groups, observations suggest that such non-gravitational heating did happen (Ponman et al. 1999; Lloyd-Davies et al. 2000; Mulchaey 2000; Mulchaey et al. 2003).”
The Birmingham-CfA cluster scaling Project – I. Gas fraction and the M-Tx relation (2003, methodology): “This enables confidence regions to be evaluated, for determining parameter errors (c.f. Lloyd-Davies et al. 2000).”
Near-Infrared Properties of Galaxy Clusters: Luminosity as a Binding Mass Predictor and the State of Cluster Baryons (2003, background): “One interpretation has been that the gas distribution in low mass clusters has been altered relative to the expectations of the standard formation scenario by the addition of entropy or “preheating” by galaxy formation at early times (Kaiser 1991; Evrard & Henry 1991; Cavaliere et al. 1997; Wu et al”