Letter to the Editor

Untangling the atmosphere of the Bullet cluster with Sunyaev-Zel’dovich effect observations

¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
e-mail: sergio.colafrancesco@oa-roma.inaf.it
² on leave from: ASI, V.le Liegi 26, Roma, Italy
³ Dipartimento di Fisica, Università di Roma La Sapienza, P.le A. Moro 2, Roma, Italy
e-mail: paolo.marchegiani@oa-roma.inaf.it
⁴ Dipartimento di Fisica, Università di Roma Tor Vergata, Roma, Italy
e-mail: roberto.buonanno@roma2.infn.it

Received: 1 November 2010
Accepted: 25 December 2010

Abstract

Aims. Sunyaev-Zel’dovich effect (SZE) observation of galaxy clusters at high frequency are able to set relevant constraints on intracluster plasma physics because of the strong dependence from the electron distribution function.

Methods. We used the multifrequency SZE observation that are available for the first time up to very high frequencies of  ~850 GHz to set contraints on the structure of the Bullet cluster atmosphere. In this context we explore the predictions of five different plasma models with single or multiple temperatures, as well as a model with the coexistence of a thermal background plasma and an additional nonthermal one.

Results. The statistical analysis of the SZE spectrum for the Bullet cluster excludes single temperature models and instead favors a more complex structure of the cluster atmosphere consisting of either two temperature plasma or – more preferably – a thermal plasma at a temperature of  ~13.9 keV coexisting with a second plasma component, either at higher temperature or, more preferably, of nonthermal origin, confirming the preliminary, but not conclusive, indications of the hard X-ray observations of the Bullet cluster.

Conclusions. The multifrequency study of the SZE signal in the range  ~150−850  GHz observed in the Bullet cluster indicates that there is a complex plasma distribution with a combination of a thermal plus a nonthermal electron distribution consistently with the theoretical expectation for a powerful merging in the Bullet cluster.