Measuring turbulence and gas motions in galaxy clusters via synthetic Athena X-IFU observations

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via Gobetti 93, 40127 Bologna, Italy
e-mail: mauro.roncarelli@unibo.it
² Istituto Nazionale di Astrofisica (INAF) – Osservatorio di Astrofisica e Scienza dello Spazio (OAS), via Gobetti 93/3, 40127 Bologna, Italy
³ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ, 08544-1001 USA
⁴ Istituto Nazionale di Fisica Nucleare (INFN) – Sezione di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ Dipartimento di Fisica dell’Università di Trieste, Sezione di Astronomia, via Tiepolo 11, 34131 Trieste, Italy
⁶ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138 USA
⁸ IRAP, Université de Toulouse, CNRS, UPS, CNES, 31042 Toulouse, France

Received: 5 May 2018
Accepted: 2 July 2018

Abstract

Context. The X-ray Integral Field Unit (X-IFU) that will be on board the Athena telescope will provide an unprecedented view of the intracluster medium (ICM) kinematics through the observation of gas velocity, ν, and velocity dispersion, w, via centroid-shift and broadening of emission lines, respectively.

Aims. The improvement of data quality and quantity requires an assessment of the systematics associated with this new data analysis, namely biases, statistical and systematic errors, and possible correlations between the different measured quantities.

Methods. We have developed an end-to-end X-IFU simulator that mimics a full X-ray spectral fitting analysis on a set of mock event lists, obtained using SIXTE. We have applied it to three hydrodynamical simulations of a Coma-like cluster that include the injection of turbulence. This allowed us to assess the ability of X-IFU to map five physical quantities in the cluster core: emission measure, temperature, metal abundance, velocity, and velocity dispersion. Finally, starting from our measurements maps, we computed the 2D structure function (SF) of emission measure fluctuations, ν and w, and compared them with those derived directly from the simulations.

Results. All quantities match with the input projected values without bias; the systematic errors were below 5%, except for velocity dispersion whose error reaches about 15%. Moreover, all measurements prove to be statistically independent, indicating the robustness of the fitting method. Most importantly, we recover the slope of the SFs in the inertial regime with excellent accuracy, but we observe a systematic excess in the normalization of both SF_ν and SF_w ascribed to the simplistic assumption of uniform and (bi-)Gaussian measurement errors.

Conclusions. Our work highlights the excellent capabilities of Athena X-IFU in probing the thermodynamic and kinematic properties of the ICM. This will allow us to access the physics of its turbulent motions with unprecedented precision.