CLUSTERS OF GALAXIES, RADIO OBSERVATIONS
Riccardo Giovanelli
A large fraction of the mass of rich clusters of galaxies that emits
electromagnetic radiation is in the form of a diffuse (density 10-3
atoms cm-3) intracluster medium (ICM), macroscopically at
rest in the
cluster gravitational potential well. This gas is hot (108 K) and
especially conspicuous in x-rays, which are produced by thermal
bremsstrahlung.
Individual galaxies in clusters are, of course, radio emitters, as
are those outside of clusters; however, the location of cluster
galaxies in a high-density environment brings about peculiar
characteristics which are usually attributed to their interaction with
the ICM or to the exceptional character of the objects dwelling in the
cluster cores (e.g., cD galaxies). The radio properties of individual
galaxies, including radio galaxies associated with cDs, are described
elsewhere in this volume. Here we discuss the characteristics of the
radio emission arising from the interaction of radio sources with the
ICM, as well as the global radio properties associated with the ICM
itself.
CLUSTER HALO AND RELIC SOURCES
A handful of clusters of galaxies are known to harbor diffuse
sources of radio emission which cannot be attributed to single
galaxies in the cluster. The best studied of these sources is
associated with the Coma cluster. Cluster halo sources are concentric
with the distribution of galaxies, have very low surface brightness,
and are thus difficult to distinguish from a population of distributed
discrete, weak sources, or from the outskirts of strong, centrally
located radio galaxies. The mean properties of halo sources are
derived from a few clusters where clear identification and distinction
from other cluster radio sources have been made. Halo sources appear
to have steep spectral index (i.e., the radio flux drops with
increasing frequency with a power
larger than -1, typically as
-1.2), a moderately
high radio luminosity (about 1031-1032
erg s-1 Hz-1 at a wavelength of 21 cm or
1041 erg s-1 over the whole
radio spectrum) and large size (diameter of about 1 Mpc). Clusters
that contain radio halo sources have very high x-ray luminosities;
their galaxian population appears compact and dynamically evolved and
is characterized by a high velocity dispersion. These clusters have a
rich, widely distributed ICM and lack a single dominating central cD
galaxy.
Relic sources are also found in clusters. Their properties resemble
those of halo sources: They are extended, diffuse, without an optical
counterpart, and have steep radio spectra. They are, however, not as
extended as halo sources and are not centrally located in the cluster.
It is postulated that relic radio sources are ejecta of now-quiescent
radio galaxies that have moved away from the scene. It has also been
suggested that a cluster halo source is produced by the collective
radio emission of a superposition of relic radio sources in the
cluster.
The steep nonthermal spectra of these radio sources suggests that
the emission process is synchrotron radiation by a cluster population
of relativistic electrons in an intracluster magnetic field that needs
not be stronger than 1 µG.
ACTIVE RADIO GALAXIES AND THE INTRACLUSTER MEDIUM
A ``standard'' radio galaxy has a compact radio source associated with
the active nucleus of an optical galaxy; part of the radio emission is
observed in the form of radio lobes, extended regions of emission
diametrically opposed with respect to and quite distant from the
compact radio source. Narrow jets originating in the central compact
source extend out to the lobes and are the conduits by which energy is
carried from an active region deep in the core of the central source
to the lobes. The energy is carried by a mixture of relativistic and
thermal gas, which outlines the jets. In clusters of galaxies, the
rapid motion of the central source and the interaction of the gas
outflow with the ICM are thought to be responsible for the observed
spectacular departures from alingment of the radio lobes. As the angle
between the direction from the central source to the lobes
progressively departs from 180°, the terminology describing the radio
source morphology varies from wide-angle tails (WATs) to narrow-angle
tails (NATs) to head-tail sources, the latter of striking cometary
appearance. Figure 1 illustrates an archetype of
the NAT category, the
source associated with the elliptical galaxy NGC 1265 in the Perseus cluster. The conventional model to explain
head-tail or NAT sources
pictures them as conventional radio galaxies moving at high velocity
through the ICM. As plasma beams are quasicontinuously ejected from
the active nucleus of the galaxy, they are bent either by direct ram
pressure or, if there is a significant interstellar medium in the
galaxy, the latter forms a cocoon around the plasma beams and the
pressure gradients created by the motion of the galaxy through the ICM
ultimately cause the bending of the plasma beams. The ram pressure
acting on the ejected gas is P = nicm
vg2, where nicm is the ICM
density and vg the velocity of the galaxy relative to
it. While the
basic idea of plasma ejecta interacting with the ICM is still the main
driver in explanations of the peculiar cluster radio source
morphology, many aspects of the interpretation are still quite
uncertain, and are related to the poorly known circumstances
associated with the process of ejection, with the difficulty of
deprojecting two-dimensional images into three-dimensional
representations and with the effects of inhomogeneities in the ICM.
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Figure 1. Total intensity map of the radio
source in NGC 1265 in the Perseus cluster. The image was obtained by
C. O'Dea and F. Owen at a frequency of 1413 MHz with the Very Large
Array radio telescope of the National Radio Astronomy Observatory.
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INTERSTELLAR GAS DEFICIENCY AND CONTINUUM
DISK EMISSION IN CLUSTER SPIRALS
Normal spiral galaxies are characterized by disks containing a fair
fraction of their mass in the form of interstellar gas. The neutral
component of that gas, primarily hydrogen, extends much farther out
from the center of the galaxy than the stellar component. This gas is
endowed with a large specific angular momentum, which supports it in
equilibrium to the outer reaches of the galaxy's potential well. As a
result, this gas is quite vulnerable to external dynamical and thermal
perturbations. As a spiral galaxy travels at high speed through the
denser regions of a cluster, its interaction with the ICM can sweep a
large fraction of its interstellar gas off its outer disk. The
mechanics of the interaction are difficult to model; the sweeping
efficiency depends on the parameters that determine the ICM ram
pressure, the angle between the galaxy's vector of motion and that
defining the inclination of its disk, the degree of clumpiness of the
interstellar gas, the galaxy's total mass, and the degree of thermal
shielding provided by its gaseous corona. Thermal conduction between
the cold interstellar gas and the hot ICM can, in fact, play as
important a role as ram pressure in the sweeping episode.
Interstellar neutral hydrogen is easily detected in external
galaxies by means of its line emission at 21cm. Observations of the HI
line have shown that a spiral galaxy moving through the inner parts of
a dense cluster core can have most of its interstellar neutral
hydrogen removed from the disk. The total neutral hydrogen mass of a
normal spiral galaxy ranges between 1 and 30 billion solar masses. A
single high-velocity passage through the cluster core can free much of
that gas from the galaxy, transferring it to the ICM. A spiral galaxy
is called HI-deficient when its HI mass is at least 2-3 times smaller
than that found on the average in noncluster spirals of the same type
and size. On the other hand, radio observation of the 2.6-mm line of
CO in HI-deficient spirals indicate that the molecular gas content of
those galaxies is little affected by the sweeping events that deplete
its neutral gas reservoir. The explanation of this difference may be
in the different degree of clumpiness and radial distribution in the
galactic disk of the two components of the interstellar medium: The
diffuse, more peripheral HI presents a larger cross-section to ram
pressure forces than the clumpy, more centrally located molecular gas
and it is less tightly bound to the galaxy than the molecular gas. It
has been suggested that the removal of the neutral component of the
interstellar medium has a delayed, quenching effect on the
star-formation rate in the swept galaxies. This ``interruption of
fertility'' does eventually result in an aging of the stellar
population, an obliteration of the spiral pattern, and the conversion
from spiral to lenticular morphology. It has been proposed that the
marked morphological segregation of galaxy types observed in cluster
cores, whereby ellipticals and lenticulars dominate the core
population, may in part be due to this ``secular,'' environment-driven
process.
Disks of spiral galaxies also harbor a diffuse population of
relativistic electrons, which are produced continuously by localized
stellar sources. The interaction of these electrons with a galactic
magnetic field, supported by the interstellar gas, produces
synchrotron radiation, providing the main contribution to the radio
emission of normal spiral galaxies. The ram pressure produced by the
transit of a spiral galaxy through the ICM can produce large-scale
displacements of its interstellar medium, which will carry along field
and relativistic electrons. Thus, images of radio continuum disks can
appear displaced from the stellar disks of galaxies moving through the
ICM. Figure 2 illustrates the Point for the
galaxy NGC 4438, which is
known to be moving at high speed through the core of the Virgo cluster.
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Figure 2. Contours of radio emission of the
displaced disk of NGC 4438, superimposed on an optical image of the
galaxy. Also in the picture (top) is the companion galaxy NGC 4435. The
radio observations where made with the Westerbork Synthesis Radio
Telescope at 1.4 GHz by C. Kotanyi and R. Ekers.
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COOLING FLOWS IN CLUSTER ELLIPTICALS
The ICM radiates mainly in the x-ray energy band, via thermal
bremsstrahlung. This process provides the primary means of cooling for
the intracluster gas. The cooling time of the ICM, that is, the time
necessary for the temperature of the gas to drop significantly from
its equilibrium value, can be approximated by
tcool = 8.5 x 1010 [nicm
/ 10-3 cm-3]-1 [Ticm
/ 108 K]1/2 yr,
where nicm and Ticm, are the density
and temperature of the ICM. In
most of the ICM, cool is comparable with or longer
than the age of the
universe, and therefore no substantial cooling is expected to have
occurred. However, in the central parts of the cluster, where the
densities are higher, cooling may be important. The cooling gas will
then flow towards the center of the cluster and coalesce onto massive
galaxies sitting at the bottom of the gravitational potential
well. Once in the cooling flow, the gas may rapidly cool to
temperatures where hydrogen recombination can occur, and the 21-cm
line of the neutral hydrogen may then be detected in emission, or in
absorption if the cluster core harbors a strong radio continuum
source. The likelihood of the presence of a cooling flow in a cluster
is estimated by the inspection of the observed x-ray parameters of the
ICM at the cluster core, which yield an indication of
nicm and of
Ticm. Early searches for cooling flows in the 21-cm
line led to
negative results. More recent measurements with the Arecibo telescope,
however, have provided very encouraging evidence of this interesting
phenomenon.
THE COSMIC MICROWAVE BACKGROUND RADIATION AND THE ICM
The ICM is not completely transparent to radio waves. In fact, the
optical depth for scattering of microwave photons of a gas with
electron density ne is
= T ne dl,
where the integration is performed along the line
of sight through the ICM and T is the Thompson electron scattering
cross-section, which is on the order of 10-28
m2. For a typical
cluster, is between 0.001 and
0.01. Thus, a small fraction of the
photons from any radio source behind a cluster will be scattered off
the line of sight by the cluster's ICM. The cosmic microwave
background radiation (MBR) is a nearly isotropic bath of radio photons
well described by a blackbody temperature of 2.7 K, a ``fossil'' relic of
the Big Bang. Because of its nearly isotropic character, MBR photons
can be scattered into the line of sight from any direction. Because
they are much ``cooler'' than the ICM electrons (2.7 versus 108 K),
they can gain energy, that is, be ``heated'' by the interaction with the
ICM electrons (the process is known as inverse Compton
scattering). The net result of the transit of the MBR photons through
a cluster is that the blackbody curve that describes their spectrum is
slightly shifted. The shift of the spectral energy density of the MBR
F () at the frequency is described by
F () = d / d {4 d / d
[-3
Fbb ()]},
where =
(kTicm /
me c2), k is the
Boltzmann constant, me the electron's mass, c
the speed of light, and
Fbb is the blackbody curve at 2.7 K. Because the
number of MBR photons
is conserved in transit through the ICM, the heating of the MBR by
this process translates into a decrease of the number of photons on
the low-frequency side of the blackbody curve, and an increase on the
high-frequency side. Because the measurements are generally done on
the low-frequency side (the Rayleigh-Jeans domain) of the blackbody
curve, the effect of the cluster on that spectral region is an at
first sight paradoxical apparent cooling of the MBR: The MBR flux in
the line of sight to the cluster is somewhat lower than in the
directions around it. However, if the energy budget is estimated over
all frequencies, on both sides of the peak of the energy distribution
curve, it is found that the total energy carried by MBR photons after
transit through the cluster has increased.
This effect was first proposed by Ya.B.Zeldovich and Rashid A.
Sunyaev in 1972. The shift for the densest and hottest clusters can be
described as a temperature shift in the Rayleigh-Jeans part of the MBR
spectrum of about -0.1 mK. This is a very tiny effect, a T / Tmbr on
the order of -10-4. It has however been successfully measured
in a few
clusters at frequencies of about 20 GHz. Testing the Sunyaev-Zeldovich
effect has important cosmological applications. Among them is the
possibility of obtaining an estimate of the Hubble constant in a
manner completely independent from that of more traditional methods
and the uncertainties associated with local calibrators. Although
these measurements are difficult and still relatively inaccurate, the
potential of this line of radio astronomical work is exceptionally attractive.
Additional Reading
Fabian, A.C., ed. (1987). Cooling Flows in Clusters and Galaxies.
Reidel, Dordrecht.
Haynes, M.P., Giovanelli, R., and Chincarini, G.L. (1984). Enviromental
effects on the HI content of galaxies. Ann. Rev.
Astron. Ap. 22 445.
O'Dea, C. and Owen, F.N. (1986). Astrophysical implications of
the multifrequency VLA observations of NGC 1265. Ap. J. 301 841
O'Dea, C. and Uson, J.M., eds. (1986). Radio Continuum Processes
in Cluster of Galaxies. NRAO, Green Bank.
Sarazin, C.L. (1986). X-ray emission from clusters of galaxies.
Rev. Mod. Phys. 58 1.
Uson, J.M. and Wilkinson, D.T. (1988). The microwave background
radiation. In Galactic and Extragalactic Radio Astronomy,
G.L. Verschuur and K.I. Kellermann, eds. Springer-Verlag, Berlin.
See also Background Radiation, Microwave; Clusters of Galaxies;
Clusters of Galaxies, X-Ray Observations; Galaxies, Radio Emission;
Intracluster Medium.