Clusters, Jets and X-Ray Emission
Max Camenzind & Martin Krause

Clusters and Jets

The most powerful extragalactic jets are born in the centre of giant ellipticals hosted by clusters of galaxies. These are the oldest galaxies of the Universe, since cluster ellipticals have been assembled in the deep potentials of dark matter forming the clusters, such as Coma cluster .

Their Black Holes have grown to a sizeable mass of billions of solar masses, in contrast to smaller masses found in normal ellipticals and in spirals. The very origin of these seed Black Holes is still unclear, but their formation goes probably back to high redshifts. At redshifts from 100 - 10, the potential wells are not very deep and cool gas is assembled in the density spikes of dark matter fluctuations. This cool gas can accrete onto the primordial cluster potential (a kind of early cooling flow). Supermassive gas clouds are then formed in the cluster potential which become gravitationally unstable and collapse to form a Black Hole. At low redshifts, the densities of the cluster gas are in the range of 0.01 - 0.0001 particles per cc and temperatures in the range of 10 - 100 Mio Kelvin ( Virgo cluster around M 87 , Coma ). This extremely thin plasma can no longer cool and stays hot for times exceeding the Hubble time. The typical cooling time for the hot cluster gas is found to be

tcooling = 8.5 Billion years x (0.01 cm3/n) x (T/100 Mio K)1/2

The hot gas can cool down if either the density n is much higher or the temperature T much lower (as e.g. in the early Universe). Extended cluster gas is also visible at higher redshifts, the radio galaxy 3C 295 is completely embedded into the hot cluster gas.

Cluster Formation and the Gas Density Profiles

An intriguing question in modern astronomy is how the first galaxies and groupings or clusters of galaxies emerged from the primeval gas produced in the Big Bang. Some theories predict that giant galaxies, often found at the centres of rich galaxy clusters, are built up through a step-wise process. Clumps develop in this gas and stars condense out of those clumps to form small galaxies. Finally these small galaxies merge together to form larger units.

An enigmatic class of objects important for investigating such scenarios are galaxies which emit intense radio emission from explosions that occur deep in their nuclei. The explosions are believed to be triggered when material from the merging swarm of smaller galaxies is fed into a rotating black hole located in the central regions. There is strong evidence that these distant radio galaxies are amongst the oldest and most massive galaxies in the early Universe and are often located at the heart of rich clusters of galaxies.

They can therefore help pinpoint regions of the Universe in which large galaxies and clusters of galaxies are being formed.

During the build-up of a cluster , also baryons fall into the gravitational potential and are heated up to X-ray temperatures . This hot cluster gas has extensively been investigated by ROSAT and is presently an important target for Chandra observations. This cluster gas is the real background , against which the jets have to fight.

Jet Propagation

When jets are launched from these central sources inside of less than one parsec, they have to propagate through this hot gas. Inside some characteristic diastance, they move freely and expand due to internal dissipation. At the scale of some 10 kiloparsecs, they have expanded to diameters in the range of one kiloparsec, their beam density is typically fallen below the cluster gas density, depending somewhat on the mass-loss rate in the jets. The most powerful jets loose masses of the order of a fraction of a solar mass per year, micro-jets such as the one in M 87 less than 0.1 percent of a solar mass per year. The plasma in these jets is quite exotic. Due to their propagation at nearly the speed of light, the entire plasma is heated to temperatures in the range of a few 100 billion degrees Kelvin. This menas that the electrons are relativistic with minimal Lorentz factors in the range of 10 - 100. Electrons can easily be accelerated to even higher energies by various processes in the plasma.

The thermal cluster gas is heated by the bow shock to temperature of 100 Mio Kelvin, provided the head of the jet propagates with a speed of a few thousand kilometers through the cluster gas. In thinner clusters, the cluster gas is heated to even higher temperatures, since the bow shocks can propagate with some fraction of the speed of light. This gas cools by the emission of Bremsstrahlung photons.

Cooling of Jet Matter

The hot jet gas itself will cool more efficiently by inverse Compton scattering of background photons from the central source and from the microwave background. The discovery made by Chandra of a handful X-ray emitting kiloparsec-scale jets opens up a new possibility to study the interaction between kiloparsec-scale jets and the ambient cluster gas, as well as the sites of particle acceleration in the jets.

For more details on jet propagation in the cluster medium at high redshifts, see M. Krause