An ALMA+ACA measurement of the shock in the Bullet Cluster

Abstract

Context. The thermal Sunyaev-Zeldovich (SZ) effect presents a relatively new tool for characterizing galaxy cluster merger shocks, traditionally studied through X-ray observations. Widely regarded as the "textbook example" of a cluster merger bow shock, the western, most-prominent shock front in the Bullet Cluster (1E0657-56) represents the ideal test case for such an SZ study. Aims. We aim to characterize the shock properties using deep, high-resolution interferometric SZ effect observations in combination with priors from an independent X-ray analysis. Methods. Our analysis technique relies on the reconstruction of a parametric model for the SZ signal by directly and jointly fitting data from the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Compact Array (ACA) in Fourier space. Results. The ALMA+ACA data are primarily sensitive to the electron pressure difference across the shock front. To estimate the shock Mach number M, this difference can be combined with the value for the upstream electron pressure derived from an independent Chandra X-ray analysis. In the case of instantaneous electron-ion temperature equilibration, we find M = 2.08 +0.12 −0.12 , in ∼ 2.4σ tension with the independent constraint from Chandra, M X = 2.74 ± 0.25. The assumption of purely adiabatic electron temperature change across the shock leads to M = 2.53 +0.33 −0.25 , in better agreement with the X-ray estimate M X = 2.57 ± 0.23 derived for the same heating scenario. Conclusions. We have demonstrated that interferometric observations of the thermal SZ effect provide constraints on the properties of the shock in the Bullet Cluster that are highly complementary to X-ray observations. The combination of X-ray and SZ data yields a powerful probe of the shock properties, capable of measuring M and addressing the question of electron-ion equilibration in cluster shocks. Our analysis is however limited by systematics related to the overall cluster geometry and the complexity of the post-shock gas distribution. To overcome these limitations, a simultaneous, joint-likelihood analysis of SZ and X-ray data is needed.Key to identifying merging clusters is the detection of shocks in the ICM. A "textbook example of a bow shock" is observed in the X-ray image of the Bullet Cluster (Markevitch et al. 2002). Using 500 ks of Chandra X-ray data, Markevitch (2006) reported a Mach number M = 3.0 ± 0.4 for the western, mostprominent shock in the Bullet Cluster, an estimate largely determined by the density jump conditions. We also note that Shimwell et al. (2015) revealed a second shock, on the eastern (opposite) side of the cluster, which we do not consider here.Here we present deep, continuum Atacama Large Millimeter/submillimeter Array (ALMA) observations, sensitive to the thermal Sunyaev-Zeldovich (tSZ; Sunyaev & Zeldovich 1972) effect, of the main shock in the Bullet Cluster. These observations include data from both the 12-meter array (hereafter "ALMA") and 7-meter Atacama Compact (Morita) Array (ACA). As the tSZ effect is linearly ...

Tempestuous life beyond R500: X-ray view on the Coma cluster with SRG/eROSITA

“…In particular, these include: (i) a faint "bridge" connecting the main cluster and the NGC4839 group; (ii) an extended diffuse region to the west of the main cluster (better seen in the smoothed image; see Sect. 4); (iii) several faint filaments extending to the south barely visible in the image; (iv) a trace of an extended (in the north-south direction) surface brightness edge to the east of the core, which is reminiscent of a Bullet cluster shock (e.g., Markevitch et al 2002;Di Mascolo et al 2019) ahead of a contact discontinuity. We discuss the origin of some of these features in the subsequent sections.…”

Protruding bullet heads indicating dark matter pull

“…The Bullet cluster presents one of the hottest, most X-ray luminous ICMs, harboring one of the strongest known merger shocks, with a Mach number M = 3.0 ± 0.4 (Markevitch et al 2002;Markevitch 2006;Di Mascolo et al 2019). The merger plane appears to approximately coincide with the plane of the sky, and the observation shows the merger only ∼ 0.15 Gyr after nearest approach (Markevitch et al 2002;Barrena et al 2002;Markevitch et al 2002;Mastropietro & Burkert 2008;Wittman et al 2018).…”

Tempestuous life beyond R_500: X-ray view on the Coma cluster with SRG/eROSITA. I. X-ray morphology, recent merger, and radio halo connection

“…In particular, these include a faint "bridge" connecting the main cluster and the NGC4839 group; an extended diffuse region to the west from the main cluster (better seen in the smoothed image, see §4.1); several faint filaments extending to the South barely visible in the image; a trace of an extended (in the North-South direction) surface brightness edge to the East from the core, which is reminiscent of a Bullet cluster shock (e.g. Markevitch et al 2002;Di Mascolo et al 2019) ahead of a contact discontinuity.…”