Kavli Affiliate: Erin Kara
| First 5 Authors: XRISM Collaboration, Marc Audard, Hisamitsu Awaki, Ralf Ballhausen, Aya Bamba
| Summary:
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.
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