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

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.

Is the radio emission in the Bullet cluster due to dark matter annihilation?

“…However, the assumption of isothermality can be too inaccurate in this very complex cluster (e.g., Markevitch et al 2002, Prokhorov & Colafrancesco 2012, and also other SZE components can be present, and they can affect the total flux in different ways at different frequencies (see Colafrancesco et al 2011a); iii) an incorrect source subtraction, which is a delicate operation mostly for extended sources, can alter the total radio halo spectrum. Both Liang et al (2000) and Shimwell et al (2014) report a detailed list of sources in the field.…”

“…The combination of the measurements from low to high frequencies allowed us to obtain information on the presence of multiple components in the overall SZE signal, favoring the presence of a second component of non-thermal origin, although a second thermal component with very high temperature cannot be excluded (Colafrancesco et al 2011a). Since the SZE signal seems to be located very close to the MS region (see next subsection for a detailed discussion), we perform here a more refined spectral analysis, by taking into account the properties of the thermal and non-thermal components in the MS as discussed in the previous Sections.…”

“…The SZE can be calculated for both thermal or non-thermal halos when the full relativistic formalism is used (see, e.g., Colafrancesco et al 2003 for details). The SZE from the Bullet cluster has been already studied in Colafrancesco et al (2011a) by using its spectral properties over a wide frequency range from 18 GHz to 850 GHz.…”

“…The same authors provided in their Figure 1 a gamma ray upper limit, extrapolated to E > 0.1 GeV, that is F (> 0.1 GeV) < 5.7 × 10 −9 cm −2 s −1 . Another source of information on the properties of the Bullet Cluster is provided by the SZE observations; the study of its spectral shape, especially at high frequencies (ν > 300 GHz), where the relativistic corrections are more relevant (Colafrancesco et al 2003, Colafrancesco et al 2011a, allows to obtain information on the thermal and possibly the non-thermal electrons in the cluster atmosphere. In the Bullet cluster, the data at high frequencies obtained with Herschel-SPIRE (Zemcov et al 2010) allowed to establish that, in addition to main thermal component, there is a second electron population (more likely of non-thermal origin) that is contributing to the total observed SZE (Colafrancesco et al 2011a), and that the gas temperature distribution along the line of sight is strongly inhomogeneous (Prokhorov & Colafrancesco 2012).…”

Low-resolution spectroscopy of the Sunyaev-Zel’dovich effect and estimates of cluster parameters

“…Itoh et al 1998) and are used to characterize the physical parameters of the cluster atmospheres (see e.g. Colafrancesco et al 2011). b) A Doppler component, caused by the collective motion of the cluster with velocity v in the CMB restframe.…”