Vol. 628
In section 3. Cosmology

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

With the advent of the ALMA+ACA array, the thermal Sunyaev-Zeldovich (SZ) effect is becoming one of the most efficient tools for characterizing galaxy cluster merger shocks, in addition to the more traditionally used X-ray observations. The well-known Bullet Cluter, with its spectacular wester shock front, represents the ideal test case for such studies. The authors of this paper characterize the shock properties of the cluster, using deep, high-resolution interferometric SZ effect observations in combination with X-ray analysis. They thus demonstrate that the two sets are highly complementary. In practice, the application of the interferometric modeling technique – using X-ray-motivated priors – allows the authors to place constraints on the electron pressure discontinuity across the shock. The pressure jump implies a Mach number of M = 2.08+- 0.12, which is significantly lower than the one derived from Chandra data alone. The assumption of purely adiabatic electron temperature change across the shock leads to M = 2.5, which is in better agreement with the X-ray estimate. The authors extensively test their modeling choices by varying the geometry, in pre- and post-shock pressure slopes, and by underlying pressure distribution. They conclude that the main limitations are due to the complex morphology observed in the X-ray surface brightness and the uncertainties on the three-dimensional morphology of the cluster. Only a full joint-likelihood analysis of interferometric SZ and X-ray observations, as well as single-dish SZ measurement, would eventually allow us to overcome these limitations.