Origin of the steep ICM velocity power spectrum in Coma

Determine whether the steep effective velocity power spectrum inferred for the intracluster medium in the Coma cluster core is caused by (i) rapid dissipation of gas motions on intermediate spatial scales that suppresses power at small scales—implying a dissipation scale ℓ_dis ≳ 240 kpc and a correspondingly high effective viscosity—or (ii) a transient dynamical state in which large-scale merger-driven flows have not yet cascaded to small scales; establish tighter constraints on the shape of the three-dimensional velocity power spectrum P(k) to discriminate between these scenarios.

Background

Using XRISM Resolve, the authors measured low line-of-sight velocity dispersions (≈200 km s⁻¹) yet large bulk velocity offsets relative to the cluster galaxy mean in two pointings within the Coma cluster core. When interpreted via a velocity structure function and modeled with a three-dimensional power spectrum P(k), these measurements are inconsistent with a Kolmogorov slope for steady-state turbulence and instead imply a much steeper effective spectrum or, equivalently, a very large dissipation scale suppressing small-scale motions.

The paper identifies two physical interpretations for the steep spectrum: (a) fast dissipation on intermediate scales (ℓ_dis ≳ 240 kpc), which would entail a very high effective viscosity of the intracluster medium, or (b) a transient state where large-scale merger-driven flows have not yet cascaded, despite the galaxy velocity distribution not strongly indicating an ongoing major merger. The authors conclude that distinguishing between these possibilities requires improved constraints on the shape of the velocity power spectrum.