Galaxy clusters grow through mergers that inject up to 10^64 erg of energy into the hot intergalactic gas (the intracluster medium). Some of this energy goes into turbulence and bulk flows, and some into accelerating relativistic particles and amplifying magnetic fields. But directly measuring the gas velocity field was nearly impossible: X-ray CCD detectors could not resolve the fine structure of lines. This work joins a series of studies of cluster dynamics using X-ray iron lines, but stands out by being the first to apply the XRISM/Resolve calorimeter spectrometer to the brightest core of Coma, where strong inhomogeneities are expected. Neighboring papers have studied X-ray iron signatures, quasar breathing, winds, and magnetic fields in clusters, but no one had mapped line profiles with such detail at the center of a merging cluster.
Observations were made with the Japanese X-ray satellite XRISM during its performance verification phase: two pointings of 3×3 arcminutes each, one at the X-ray peak (Center, exposure 398,000 seconds) and one 6 arcminutes to the south (South, 158,000 seconds total). The key instrument is the Resolve microcalorimeter, which measures the energy of each photon with a precision of about 4.5 eV (full width at half maximum) at 6 keV, enough to resolve Doppler shifts of order hundreds of km/s. Spectra in the 2–10 keV range were modeled with thermal plasma emission from bapec with a Gaussian velocity spread, and then deviations from a single Gaussian were sought. For a systematic search for components, a blind fit was used: 51 bapec components spaced in energy at 3 eV intervals (slightly less than the resolution), with zero velocity dispersion and linked temperature and abundance. To assess the significance of the detected peaks, hundreds of synthetic spectra were generated from a model without additional components, and the frequency with which random fluctuations produced a similar signal was examined.
In the spectrum of the northwest quadrant of the central field, besides the main peak and the already known redshifted component (Δcz ≈ −40 km/s), a third, blueshifted component was found at Δcz = −1274 ± 100 km/s. Its significance from simulations is about 96%. The full central-field dataset shows a second component at Δcz = −6 (+229/−151) km/s, significant at the 99% level in simulations but only weakly favored by information criteria (AIC slightly in favor, BIC against). The blind fit with 3 eV spacing revealed a blueshifted peak near cz ≈ 5850 km/s (Δcz ≈ −1150 km/s) in the center with a false detection probability of 3%, and a redshifted peak near cz ≈ 8250 km/s (Δcz ≈ +1250 km/s) in the southern field with a probability of 5%. The main velocity components coincide with optically identified galaxy groups: the blueshifted peak is close to group S5 (Δcz = −1131 km/s), and the redshifted peak in the southern field is close to group S4 (Δcz = +982 km/s). The energy density of turbulence (1.9–2.2 eV/cm³) and bulk motions (up to 8.5 eV/cm³) is several times greater than the magnetic field energy (0.42–0.55 eV/cm³ at B ≈ 5 μG), and the turbulence dissipation time (0.14 billion years at a scale of 85 kpc) is comparable to the synchrotron cooling time of the radio halo electrons (0.25 billion years).
This work is the first to directly show that the iron line profile at the center of a merging cluster is not Gaussian but consists of several Doppler components. This changes the approach to velocity measurements: the standard single-Gaussian fit overestimates the velocity dispersion and masks real flows. Unlike neighboring studies that examined average gas properties or large-scale gradients, this one achieves line-of-sight resolution within a single instrument field of view and detects structures tens of kiloparsecs in size. The link between X-ray flows and optical galaxy groups is confirmed, providing a new tool for reconstructing the merger geometry.
The next step will be deeper XRISM observations of the same and neighboring fields to confirm the marginal components and build a three-dimensional velocity map. It may become possible to separate the contributions of thermal broadening and turbulence using lines from different ions (for example, Fe Lyα). Joint analysis with LOFAR radio observations and optical group catalogs will clarify how the energy of bulk motions is transferred to particle acceleration and magnetic fields. The blind fitting method with 3 eV spacing can be applied to other clusters and even to Galactic supernova remnants.
The results will influence studies of cluster mergers, the origin of radio halos and relics, and the calibration of cluster mass measurement methods based on X-ray spectra. They are also important for understanding turbulent gas heating and magnetic field generation in cosmic plasmas.
Conduct longer XRISM observations of the Coma center and southern field, and obtain spectra from other position angles to separate rotation from radial flows. Complement the analysis with optical data on galaxy velocities and high-resolution radio maps.
For cluster astrophysics, the question of exactly how merger energy is distributed among heat, turbulence, magnetic fields, and cosmic rays remains unresolved. This work provides the first direct estimates of the energy densities of turbulence and bulk motions in the Coma core and shows that the magnetic field is energetically subdominant to gas motions, which supports the dynamo mechanism for its amplification. The nature of the radio halo also remains an open question: the results suggest that turbulence can sustain them for a billion years, but the exact mechanism of electron acceleration has yet to be determined.
The Coma cluster contains about 1000 galaxies, and its X-ray halo extends for millions of light-years. The gas temperature in the core reaches 100 million kelvins, but its density is so low that a teaspoon of this matter would weigh less than a billionth of a gram.
Isaac Asimov's novel The Gods Themselves describes energy transfer between parallel universes, and in reality galaxy clusters transfer energy from gravitational collapse to turbulence and magnetic fields. The idea that the motions of invisible gas can be read from faint lines in a spectrum echoes the methods of the astrophysicists in Liu Cixin's The Three-Body Problem.