The gas keeps losing energy
A galaxy cluster contains more than its galaxies. Between them lies a thin, extremely hot atmosphere that shines in X-rays. The dense central gas radiates especially strongly. As it loses energy, some can cool, become denser and eventually supply star formation or the central black hole.
The simplest uninterrupted-cooling picture predicts more cold material arriving at the centre than astronomers observe. Something must replenish much of the lost energy. Accretion near a black hole can power jets, but that leaves a practical question: how does energy released near a tiny central engine reach a much larger atmosphere?
Reading: Fabian et al. (2000) · Rafferty et al. (2006)
The radio lobes occupy the X-ray holes
ROSAT had already revealed holes associated with the radio lobes. In 2000, Fabian and colleagues used Chandra to resolve the surroundings in much greater detail. The cavities contain jet-supplied particles and magnetic fields, with ordinary X-ray-emitting gas displaced around them.
Inflating a light bubble in a surrounding medium takes work. Energy must go into its contents and into making room for them. Its size and the pressure around it therefore provide an estimate of the mechanical energy involved. Enthalpy is the name for the internal energy plus this displacement work.
The darkness of a hole also depends on geometry. A long column of normal gas in front of or behind a cavity partially fills in the missing X-rays. The same cavity can look much less conspicuous along a different sightline.
Reading: Fabian et al. (2000) · Rafferty et al. (2006)
A bright rim need not be shock-heated
The rims are a tempting place to picture gas being violently heated. Yet the inner rims in the 2000 Chandra study were cooler than their surroundings. Fabian and colleagues identified this as evidence against a simple shock-heated shell. Brightness counts emitted X-rays; a spectrum is needed to infer temperature.
A deeper observation reported in 2003 revealed faint ripples beyond the cavities. The researchers connected these features to expanding bubbles and discussed pressure waves as another way of distributing energy. A cavity can move through the gas while a pressure disturbance travels ahead of it.
Reading: Fabian et al. (2000) · Fabian et al. (2003)
Transport comes before dissipation
A light cavity feels buoyancy and rises. Falling external pressure lets it expand, reducing the enthalpy it still carries. Some released energy first becomes motion in the surrounding gas or its wake. Dissipation converts that motion into heat later.
A passing pressure wave also compresses gas and then allows it to expand again. Nearly reversible compression can produce a temporary temperature rise. How much heat remains after the wave passes depends on dissipation.
Cavity images trace the route taken by energy. Modern X-ray spectra also measure gas motions along that route. Together, the energy, positions and timescales reveal how a small central engine can influence a vast atmosphere through repeated outbursts.
Reading: Rafferty et al. (2006) · XRISM Collaboration (2026)