A Hot Cocktail in the Heart of Perseus express
A giant cauldron of simmering soup stretches across millions of light-years. This is the atmosphere of the Perseus galaxy cluster — a cloud of scorching gas held together by gravity. Until recently, astronomers thought that in the center of this cauldron, the gas was calmly cooling, ready to condense into new stars. But a fresh look from Japan's XRISM telescope XRISM revealed that things are far more interesting.
The gas at the heart of Perseus is not a homogeneous broth. The telescope discerned two distinct layers. The main one, extremely hot (about 40 million degrees), behaves relatively quietly: its particles move at about 140 km/s. But hidden within this vapor is a second layer, roughly half as cool (about 20 million degrees). This gas churns and dashes about: its velocity relative to the central galaxy reaches 200 km/s, and the velocity dispersion is nearly 400 km/s. In a calm sea, a swift underwater current suddenly appears.
What stirs this cosmic cocktail? Scientists propose two scenarios that may act together. The first is gravitational 'whirlpools' from mergers with other galaxy clusters. Such collisions create giant spiral waves that mix the gas. The second is the activity of the central black hole. It ejects powerful jets of plasma that, like a giant mixer, froth the surrounding medium, preventing it from cooling uniformly.
This discovery is important because it explains a long-standing puzzle: why gas in the centers of many clusters doesn't cool completely and form stars at the predicted rate. It turns out that the turbulent motion of the cooler gas, accelerated to nearly the speed of sound (Mach number about 0.8), acts as an additional source of pressure that impedes compression. Roughly a quarter of the gas's total energy is the energy of its chaotic motion. Thus, nature, unintentionally, puts a powerful brake on star formation. And in the giant cauldron of the Perseus cluster, stellar fuel never quite reaches readiness.
Original abstract
as written by the authors arXiv:2604.22975
We present an analysis of the temperature structure of the Perseus cluster atmosphere using XRISM Resolve observations. The average temperature rises from 3.3 keV near the nucleus of NGC 1275 to 8 keV at 10 arcmin (210 kpc), which is consistent with Chandra and XMM measurements. The velocity and velocity dispersion profiles are broadly consistent with those in arXiv:2509.04421. While the gas at altitudes beyond $$\sim60$$ kpc can be modeled as a single temperature plasma, we find evidence for more than one gas phase in the inner $$\sim60$$ kpc. The hotter gas component, traced primarily by the Fe He$$\alpha$$ line, has a velocity dispersion of $$\lesssim140$$ km s$$^{-1}$$. We detect a cooler, $$\sim1.87-2.43$$ keV, gas component with a velocity dispersion of $$\sim300-400$$ km s$$^{-1}$$ and a bulk velocity of $$\sim 21-213$$ km s$$^{-1}$$ with respect to the central galaxy. These ranges reflect large systematic uncertainties associated with modeling spatial-spectral mixing and the bright central point source. Potential low energy gain variations may add additional uncertainties. The cooler component is identified by broad wings in prominent emission lines, most notably S Ly$$\alpha$$ and Fe He$$\alpha$$. This cooler component's Mach number $$\sim0.73-0.96$$ and non-thermal pressure fraction of $$\sim22.9-33.7\%$$ are much higher than found for the hotter gas. The cooler gas may be associated with merging halos along the line of sight which formed the cool, sloshing spiral and/or cooling gas being disturbed by the radio jets and lobes.
A temperature of 40 million degrees is nearly three times hotter than the center of our Sun. But this gas is so tenuous that a space probe would fly through it without even heating up.
A similar image of 'living' and inhomogeneous cosmic gas, churning around black holes, often appears in Alastair Reynolds's novels set in the 'Revelation Space' universe.