Winds of Galactic Capitals express
At the heart of the giant galaxy Hydra-A, there's a hive of activity. A supermassive black hole shoots out two powerful plasma jets. They punch through the surrounding cloud of hot gas and inflate giant bubbles within it. The new Japanese telescope XRISM has, for the first time, allowed astronomers to measure how fast this gas trembles and moves.
The telescope worked in tandem with the veteran Chandra. XRISM provided high resolution in the colors of X-ray light (this is called spectroscopy), while Chandra delivered sharp images. Together, they showed that near the black hole at the center, gas rushes about with a velocity dispersion of about 162 kilometers per second. Each cloudlet tears along at bullet speed. But the most interesting find was slightly to the north. There, where the shock wave from the jets slams into calm gas, the velocity dispersion jumps to 260 kilometers per second — one of the highest ever seen for such objects. It is precisely in this spot that a clump of gas rich in heavy elements was previously found. It may have been pulled upward like a cork from a bottle when old bubbles rose.
Out in the distant outskirts, 95–317 kiloparsecs from the center, things calm down. There, the velocity dispersion drops to 120 kilometers per second and below — almost a dead calm. The jet pushes gas effectively only nearby, and by the time its energy reaches the edges, it is already greatly weakened. Another detail: in the southwestern part of the central region, a bulk motion of gas toward us was detected at about 100 kilometers per second. This could be a slosh — a giant wave traveling across the gas ocean after some past collision.
Original abstract
as written by the authors arXiv:2607.18315
We report on two deep XRISM observations of the central and northern regions of Hydra-A's X-ray atmosphere covering the bubbles inflated by jets from the central galaxy's active galactic nucleus (AGN). We use spatial-spectral mixing that combines Chandra's high spatial resolution with XRISM's high spectral resolution to investigate atmospheric kinematics. The atmospheric velocity dispersion in the northern region, $$\sigma_v = 140^{+30}_{-20}$$ km s$$^{-1}$$, is comparable to that in the central region ($$\sigma_v = 162 \pm 10$$ km s$$^{-1}$$). We show that the motion of the large-scale cocoon shock front could be responsible for the large dispersion toward the north. The velocity dispersion in the northeast quarter of the central pointing, $$\sigma_v = 260 \pm 50$$ km s$$^{-1}$$, is among the highest dispersions measured. This region contains an X-ray-bright feature previously identified as metal-rich, possibly consisting of gas uplifted in the wake of previous-generation cavities. The dispersions in all other quarter regions are low ($$\sigma_v \leq 120$$ km s$$^{-1}$$) and consistent with previous XRISM results from other objects. The kinetic energy at the center is comparable to the enthalpies of the cavities, while in the north, it is roughly an order of magnitude smaller. The jet thus drives gas motion efficiently at smaller scales ($$r < 95$$ kpc) and inefficiently at larger scales ($$95-317$$ kpc toward the north along the jet). A bulk flow toward our line of sight of $$-100 \pm 30$$ km s$$^{-1}$$ in the southwest quarter of the central pointing is also observed, possibly due to sloshing.
The velocity dispersion of gas at the center of Hydra-A (162 km/s) is 30 times faster than the most powerful hurricane on Earth. If such a wind blew in the atmosphere, it would circle the planet in half an hour.
In Vernor Vinge's novel "A Fire Upon the Deep," zones of the galaxy with different speeds of thought are described. The gas turbulence in Hydra-A is a similar contrast: a turbulent center and a calm periphery, only instead of thoughts, it's hot plasma.