A direct test of quasar feedback
Astronomers have long treated feedback from actively feeding supermassive black holes as an essential ingredient in galaxy-formation models. Without a process capable of depositing large amounts of energy into surrounding gas, simulations tend to form overly massive galaxies and allow too much gas to cool into stars. The difficulty has been observational: a bright quasar can show that a black hole is accreting rapidly, but demonstrating how effectively its energy reaches gas far beyond its host galaxy has been much harder.
A new study using the X-Ray Imaging and Spectroscopy Mission, XRISM, addresses that question in H1821+643, a galaxy cluster whose central galaxy contains a luminous quasar. The system lies at a redshift of 0.297 and is particularly useful because its quasar is close enough, in cosmological terms, for the surrounding intracluster medium to be measured in detail.
The researchers report exceptionally broad X-ray emission lines from highly ionised iron in the cluster’s hot plasma. The broadening corresponds to a line-of-sight velocity dispersion of roughly 280–300 kilometres per second. Crucially, modelling indicates that more than 90% of the relevant iron-line emission comes from gas about 20 to 100 kiloparsecs from the centre, rather than from the immediate vicinity of the quasar. The observation therefore traces motions on scales beyond the central galaxy itself.
Why broad iron lines matter
Galaxy clusters contain vast amounts of diffuse plasma, heated to millions of degrees and visible primarily in X-rays. Iron ions in this gas produce emission at well-defined energies. If the plasma contains turbulent motions, coherent flows or both, Doppler shifts broaden those lines. Measuring such subtle changes requires much finer spectral resolution than conventional X-ray imaging can provide.
XRISM’s Resolve microcalorimeter was built for this kind of measurement. Its high-resolution spectroscopy separates the narrow energy features of hot gas sufficiently well to estimate gas velocities directly. The technique builds on the brief but influential Hitomi mission, which made the first direct measurement of relatively modest gas turbulence in the Perseus cluster core before the spacecraft was lost in 2016.
In H1821+643, the measured gas motions are substantially stronger than those typically seen in comparable central regions of other clusters observed by Hitomi and XRISM. The authors estimate that non-thermal motions account for about 8% of the thermal energy in the gas between 20 and 100 kiloparsecs. That is an important fraction: it means the cluster atmosphere cannot be considered a nearly static reservoir of hot gas.
A quasar, rather than only a jet
Black holes can affect their surroundings through more than one mode. In nearby galaxy clusters, attention has often focused on radio jets. These narrow outflows can inflate cavities or bubbles in the X-ray-emitting gas, transferring mechanical energy into the cluster core. Such jet-driven feedback is well established in many lower-accretion systems.
H1821+643 presents a different case. Its central object is a radio-quiet but highly X-ray-luminous quasar. The study finds that the energy associated with the observed non-thermal gas motions is around an order of magnitude greater than the mechanical energy inferred from the system’s known X-ray cavities. That comparison does not exclude a role for jets, but it makes jets alone an insufficient explanation for the scale of the detected disturbance.
The proposed interpretation is that a powerful quasar wind expanded into the intracluster medium and drove a weak shock. Behind that shock, the gas would acquire turbulence and velocity shear, leaving the broadened iron lines that XRISM detects. A weak shock can be difficult to identify in X-ray images because its surface-brightness contrast is small, especially at large radii; gas kinematics may therefore preserve a clearer record of the event than imaging alone.
The distinction matters because quasar-mode feedback is usually invoked to explain an intense phase of black-hole growth in the early Universe. It has been more difficult to establish whether such radiative activity can efficiently influence gas on cluster-core scales. H1821+643 suggests that it can.
An energetics result with significant caveats
The authors estimate a non-thermal energy of approximately 4 × 10^60 erg in the 20–100 kiloparsec region. Depending on how large the putative shock is assumed to be, they infer that the quasar may have coupled at least about 1%, and potentially around 10%, of its available radiative energy into gas beyond galactic scales. Those values are much larger than some earlier estimates based on ionised quasar winds, and they overlap with the efficiencies adopted in several contemporary cosmological simulations.
However, the result is not a direct measurement of a feedback efficiency. It is an inference that depends on the interpretation of the line widths, the duration and geometry of the outburst, and especially the assumed radius of the shock. The researchers also note that their spectrum cannot fully separate isotropic turbulence from organised velocity shear, radial flows or sloshing motions. The firm observation is vigorous gas motion; identifying the precise mixture of physical motions will require deeper observations and spatially resolved spectroscopy.
That caution strengthens rather than weakens the wider significance of the work. The study does not claim that every cluster-core disturbance is driven by a quasar. XRISM measurements in Perseus have already shown that cluster growth and central black-hole activity can produce distinct kinematic signatures. H1821+643 instead offers an unusually clean example of a luminous quasar coinciding with an extreme level of gas motion at tens of kiloparsecs.
Implications for galaxy and cluster evolution
The hot atmosphere in a cluster core is a shared fuel reservoir. If it cools rapidly, gas can condense and promote star formation in the central galaxy. Energy injected by a black hole can offset that cooling, alter pressure support and redistribute material through the core. The new measurement adds a dynamical constraint to this picture: it shows that a luminous quasar can disturb a massive volume of intracluster gas rather than merely its immediate galactic environment.
The observation also highlights a broader change in X-ray astronomy. Measurements of brightness and temperature have long revealed where cluster gas is located and how hot it is. High-resolution line spectroscopy now makes it possible to measure how that gas moves. Expanding such observations to more clusters will be necessary to determine whether H1821+643 is exceptional or a visible example of a feedback process that was more common when quasars were abundant.
For now, the central conclusion is measured rather than speculative: the gas around H1821+643 is moving far more vigorously than in many other cluster cores, and the system’s luminous quasar is the leading explanation for the energy behind those motions.
