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There's one more item of business before we can explore the dynamics of the intracluster medium — the radiative diagnostics. Unfortunately, we must limit ourselves here to exploring the thermal and relativistic plasmas, with exciting new information from the studies of atoms and molecules in the relatively cooler gas near the massive central cluster galaxies left for another place and time.

Box 2 summarizes the main radiative processes for these two plasmas. Each mechanism both scales with overall cluster parameters and produces a distinct spectral shape (e.g., T-0.5 exp [- hν / kT] for thermal bremsstrahlung and ν for synchrotron radiation, where ν is the observing frequency). These spectral distributions depend both on the single-particle radiation spectra and the energy distributions of the radiating particles — Maxwellian for the thermal plasma and a power law for the cosmic rays. Additional information can be derived from these spectral distributions, such as the temperature of the X-ray emitting gas. Since multiple mechanisms depend on the same cluster physical quantities, we can sometimes combine different measurements to break the degeneracies.

Box 2. ICM: Scaling of Radiative Diagnostics
Thermal plasma Cosmic Rays
Thermal plasma Bremsstrahlung (X-ray)
ne np T0.5
H- and He-like line emission
Pion production (γ-ray)``
np nnCRp
Magnetic field Faraday rotation (radio)
ne B||
Synchrotron (radio)
CMB (t) S-Z effect (mm)
Inverse Compton (X-ray)
γ2 nCRe TCMB4
Notes: Emissivity scalings are “bolometric”, integrated over all frequencies, without showing the spectral dependencies. ne, np, nCRe and nCRp are the densities of electrons, protons in the thermal and relativistic plasmas, respectively. T is the (average) temperature of the thermal plasma and TCMB is the temperature of the CMB at the redshift of the cluster. B is the characteristic strength of the cluster field, while B|| is the vector average of the magnetic field component along the line of sight. γ is the relativistic factor, typically around 1000 for CR electrons observed at radio frequencies.

Basic information about clusters comes from spatially integrated measures of these radiative diagnostics, and how they scale with physical cluster properties such as mass or redshift. Much more value is added when the radiation can be mapped across the cluster. A rich literature of cluster maps in X-ray and radio emission exists, and observations of the S-Z effect (see below) are rapidly increasing. But despite heroic efforts, and great expectations for the Fermi satellite, there is currently no clear detection of cluster γ-rays, which would provide our only window on the CR proton distribution. Similarly, there is not yet a clear detection of the inverse Compton radiation from CR electrons, which could be combined with the synchrotron emissivities to measure the strength of the cluster magnetic fields.

Although we don't have magnetic probes flying through the cluster plasma, the magnetic fields reveal themselves through the synchrotron radiation from CR electrons, and the Faraday rotation of linearly polarized radio emission through the magnetized ICM. The strength and structure of the magnetic fields can illuminate the characteristic turbulent scales in the hot plasma to which they are coupled. Although their energy densities/pressures are low compared to the ICM thermal pressures, the magnetic fields influence the transport of heat in the ICM and the energization of CRs.

One radiation mechanism has a curious spectral signature. It is the inverse Compton scattering of CMB photons to slightly higher energies by the hot ICM electrons. This mechanism is known as the thermal (t) S-Z effect, named for Rashid Sunyaev and Ya. B. Zel'dovich, who first described it in 1972 [5] (and sent the author off on a not sufficiently sensitive search for it at that time). In the Rayleigh-Jeans part of the spectrum (wavelengths longward of ∼ 1.4mm), the shift of the spectrum to higher energies creates a lower brightness compared to looking off the cluster. Subtracting on-off measurements therefore yields a negative signal. By contrast, a positive on-off signal is seen at shorter wavelengths. Today, mapping the (t)S-Z effect is a worldwide industry, producing line-of-sight integrated ICM pressure measurements that can be combined with X-ray data to derive robust physical ICM parameters, identify shocks, etc.

Our current characterization of the ICM based on these diagnostics is shown in Box 3. The thermal plasma is quite different than terrestrial and even other astrophysical plasmas. The extremely large number of electrons within the Debye sphere (∼ 1014) makes the ICM one of the most perfect “collisionless” plasmas in the universe. The effective scattering length is set by plasma waves, for which there are an endless stream of possibilities, perhaps down to scales as small as the ion gyroradius.

107-8 K
102-4 m-3
Sound speed
∼1000 km/s
Particle mfp
∼ few kpc
Plasma scattering length
≳ 108 m
Magnetic field
< 0.1-1 nT
Plasma Beta
∼ 100
Alfven speed
20-100 km/s
Magnetic field typical scale
∼ 10s kpc
CR Diffusion time, ∼ Mpc
1010.5 y @ 1 GeV
Notes: All cluster properties are functions of distance from the center, not shown here.

Finally, there are literal weather vanes in clusters, where low density, usually bipolar jets of plasma are ejected from supermassive black holes at the centers of some galaxies, the so- called Active Galactic Nuclei (AGN). These jets of plasma are deflected and distorted in their relative motion through the ICM - some examples will be seen below.

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