Thermal SZ fluctuations in the ICM: probing turbulence and thermodynamics in Coma cluster with Planck

We report the detection of thermal Sunyaev-Zeldovich (SZ) effect fluctuations in the intracluster medium (ICM) of Coma cluster observed with Planck. The SZ data links the maximum observable X-ray scale to the large Mpc scale, extending our knowledge of the power spectrum of ICM fluctuations. Deprojecting the 2D SZ perturbations into 3D pressure fluctuations, we find an amplitude spectrum which peaks at δP/P = 33 ± 12 and 74 ± 19 per cent in the 15 and 40 arcmin radius region, respectively. We perform tests to ensure fluctuations are intrinsic to the cluster and not due to noise contamination. By using high-resolution hydrodynamical models, we improve the ICM turbulence constraints in Coma, finding 3D Mach number Ma_3d = 0.8 ± 0.3 (15 arcmin region), increasing to supersonic values at larger radii (40 arcmin) and an injection scale L_inj ≈ 500 kpc. Such properties are consistent with driving due to mergers, in particular tied to internal galaxy groups. The large pressure fluctuations show that Coma is in adiabatic mode (mediated by sound waves), rather than isobaric mode (mediated by buoyancy waves). As predicted by turbulence models, the distribution of SZ fluctuations is lognormal with mild non-Gaussianities (heavy tails). The substantial non-thermal pressure support implies hydrostatic mass bias b_M = -15 to -45 per cent from the core to the outskirt region, respectively. While total SZ power probes the thermal energy content, the SZ fluctuations constrain the non-thermal deviations important for precision cosmology. The proposed, novel approach can be exploited by multifrequency observations using ground-based interferometers and future space cosmic microwave background missions.