#paperday (submitted to A&A) on #astroph:
"On the interpretation of XRISM X-ray measurements of turbulence in the intracluster medium: a comparison with cosmological simulations" by myself and G. Brunetti.
https://arxiv.org/abs/2507.04727
It's short reaction paper to the latest (cool) XRISM measurements of gas motions in the Coma cluster (and others), which were interpreted as evidence of weak and much-steeper-than-Kolomogorov turbulence.
We think this is wrong..
#science #astrodon #astronomy
In a nutshell, we used a standard hydro-MHD simulation of a Coma-like cluster of galaxies (from 2018) and analyse the 3D dimensional properties of gas motions in this system, and compared them with the ones one would infer with the same approach used by the recent XRISM Collaboration paper (https://arxiv.org/abs/2504.20928).
We argue that X-ray spectroscopic observations bias the measurements of turbulence and move them away from the ground truth.
The XRISM Resolve microcalorimeter array measured the velocities of hot intracluster gas at two positions in the Coma galaxy cluster: 3'x3' squares at the center and at 6' (170 kpc) to the south. We find the line-of-sight velocity dispersions in those regions to be sigma_z=208+-12 km/s and 202+-24 km/s, respectively. The central value corresponds to a 3D Mach number of M=0.24+-0.015 and the ratio of the kinetic pressure of small-scale motions to thermal pressure in the intracluster plasma of only 3.1+-0.4%, at the lower end of predictions from cosmological simulations for merging clusters like Coma, and similar to that observed in the cool core of the relaxed cluster A2029. Meanwhile, the gas in both regions exhibits high line-of-sight velocity differences from the mean velocity of the cluster galaxies, Delta v_z=450+-15 km/s and 730+-30 km/s, respectively. A small contribution from an additional gas velocity component, consistent with the cluster optical mean, is detected along a sightline near the cluster center. The combination of the observed velocity dispersions and bulk velocities is not described by a Kolmogorov velocity power spectrum of steady-state turbulence; instead, the data imply a much steeper effective slope (i.e., relatively more power at larger linear scales). This may indicate either a very large dissipation scale resulting in the suppression of small-scale motions, or a transient dynamic state of the cluster, where large-scale gas flows generated by an ongoing merger have not yet cascaded down to small scales.
By simulating many possible measurements of the turbulence along the line of sight of potential XRIMS-like integrations, we find that systematically these only get ~60% of the true (energy-weighted) turbulence along the line of sight.
Why? because X-ray weights more dense regions along the line of sight, which are systematically closer to the centre, where clusters tend to have (at least in the world of simulation) lower budgets of turbulence
The grey are are XRISM measurements- which we can reproduce in the mock observation, while the true velocities are the green ones.
So through X-rays, one can only probe a fraction of the turbulence within the same area been observed. Which is fine, except one needs to take this into account when interpreting gas motions in the grand scheme of things.
The XRISM analysis had a limited amount of observed fields of view (integrations are expensive!!) and tried to reconstruct the "velocity structure function" (VSF), i.e. a standard way to study turbulence in many different environments.
Textbook turbulence (isotropic, incompressible) has a very distinct trend of VSF, first derived by genius A. #Kolmogorov in 1941.
The XRISM paper reported that the VSF in the Coma cluster is way different than this, hence turbulence must be weird there.
It's the magnetic field to cause particles to stick together like a fluid, but not the simple one we have in mind! this gives rise to many kinetic plasma effects (@jaztrophysicist being the local master) which it would be nice to observe.
However, the expectation was that these effects should be affecting global gas dynamics only on <<10kpc, so it would be very cool to have surprisingly detected them already on the big scales observed by XRISM!
But we argue we did not, as their VSF is biased 😩
franco_vazza