"All things are double, one against another" Ecclesiasticus (xlvii. 24)
"The problem is how much
to stress the sameness of twins and how much to emphasise their
differences.", Baby and Child Care, Benjamin Spock, 1946
Shortly after the discovery of extragalactic radio sources, their
nonthermal spectra and polarisations led to the conclusion that their
emission is produced by synchrotron radiation from a relativistic
plasma. The large radio luminosities together with the lifetimes of the
synchrotron-emitting electrons imply total energies of
1060 erg (e.g.
Miley 1980).
These huge energies,
the collimation on sub-parsec scales and the similarity of orientation
between the compact and overall radio structures led to the conclusion
that the collimated relativistic beams are produced by rotating
supermassive black holes (SMBHs) located at the centres of the host
galaxies. The SMBHs are postulated to have rotation axes aligned with
the radio source axes. The radio sources are powered through
gravitational energy from material accreting onto the SMBHs, that is
converted into kinetic energy of the collimated relativistic jets (See
Section 5.2).
2.1. HzRGS and low-redshift radio galaxies
Extragalactic radio sources can be classified according to their sizes.
2
GHz) are "extended", with
projected linear sizes ranging from several tens of kiloparsec to
several megaparsecs, i.e. much larger than those of their optical galaxy
hosts. Extended extragalactic radio sources have nonthermal spectra,
with typical spectral indices at low-redshifts of
~ -0.7,
with flux proportional to
.
An example is Cygnus A
in Figure 3.
> -0.4).
Galaxies and quasars host extragalactic radio sources of all three classes, with extended radio sources predominantly identified with galaxies and compact sources mainly associated with quasars.
HzRGs generally host extended extragalactic radio sources and differ in several properties from low-redshift radio galaxies. At larger redshifts the typical radio luminosities increase, the typical sizes decrease (Section 2.3) and the typical radio spectra steepen (Section 2.4). The host galaxies of HzRGs differ from the low-redshift radio galaxy hosts in (i) the presence of emission-line halos (Section 3.2, (ii) increased clumpiness of the continuum emission (Section 4.3) and (iii) alignments of the radio sources with both the emission-line halos (Section 3.2.3) and the UV/optical galaxy continuum emission (Section 4.3).
 |
Figure 2. Radio structure of 4C 41.17 at z =
3.8. [From van Breugel and Reuland, private communication]. See also
(Reuland et al. 2003).
Contours obtained at 1.4
GHz with the VLA are superimposed on a Keck narrow-band image in
redshifted Ly |
2.2. Radio sizes and morphologies. Size as an evolutionary clock
The radio structures of HzRGs have been studied by Carilli et al. (1997) and Pentericci et al. (2000). Both articles describe observations of 37 radio galaxies with z > 2 using the VLA at 4.7 and 8.2 GHz, at resolutions down to 0.25 arcec. In accordance with their large radio luminosities, most HzRGs are "Fanaroff-Riley Class II" radio sources (Fanaroff and Riley 1974), with double structures, edge-brightened mophologies and one or more hot-spots located at the extremities of their lobes (Miley 1980, Carilli et al. 1994, Carilli et al. 1997, Pentericci et al. 2000). In general HzRGs do not have appreciable flat-spectrum core components at their nuclei, but the use of the ultra-steep spectrum criterion in searching for HzRGs discriminates against finding HzRGs with flat-spectrum cores. Standard minimum energy assumptions (Miley 1980) give typical minimum pressures in these hotspots of a few x 10-9 dyn cm-2 and corresponding magnetic field strengths of a few hundred µG.
There have been a few VLBI observations of fine-scale structure in the
lobes of extended radio sources associated with HzRGs
(Gurvits et al. 1997,
Cai et al. 2002,
Pérez-Torres and De
Breuck 2005,
Pérez-Torres et
al. 2006).
A component of size ~ 65 pc
has been detected several kpc from the nucleus of 4C 41.17 at z = 3.8
(Pérez-Torres et
al. 2006).
Consideration of the
energetics suggests that the radio component is associated with a gas
clump of mass MB
1.5 × 108
M
. Intriguingly,
this is typical for the masses of "predisrupted clumps" invoked as the
progenitors of globular clusters
(Fall and Rees 1977).
Investigating fine scale radio structure in HzRGs and
using the radio jet interactions to probe the interstellar medium in the
early Universe are likely to be an important field of study for the more
sensitive long-baseline interferometers that are presently under
construction, such as e-Merlin and the e-EVN.
There are several correlations between the sizes of radio sources and
and other properties
(Röttgering et
al. 2000).
First, for 3C sources at lower redshifts, there is
a relation with optical morphology. Smaller radio sources consist of
several bright optical knots aligned along the radio axes, while larger
sources are less lumpy and aligned
(Best et al. 1998).
Secondly,
there is a relation with emission line properties. Smaller radio sources
have generally lower ionisations, higher emission line fluxes and
broader line widths than larger sources
(Best et al. 1999).
Thirdly, there is a connection with
Ly absorption. HzRGs with
smaller radio sizes are more likely to have
Ly absorption than larger
sources (Section 3.3.2)
(van Ojik et al. 1997).
Taken together, these correlations are
consistent with an evolutionary scenario in which radio size can be used
as a "clock" that measures the time elapsed since the start of the radio
activity.
Note that the large sizes of radio sources (usually several tens of kpc) imply that the nucleus of host galaxies has been undergoing activity for at least > 105 y (light travel time across the source) and up to > 108 y (assuming that the jets advance at a few hundred km/s). Hence we know that AGN associated with extended radio sources must be long-lived. This is not the case for radio-quiet or compact radio quasars, that may only have been active quasars for a few × 102 y.
2.3. Radio size vs redshift correlation
It has been known since the nineteen sixties that there is a statistical decrease of the angular size of radio sources with redshift [Miley 1968]. More recent work includes measurement of the angular size-redshift relation for luminous extended radio sources (Nilsson et al. 1993, Neeser et al. 1995, Daly and Guerra 2002), quasars (Buchalter et al. 1998) and compact radio sources (Gurvits et al. 1999).
Over the years there have been many valiant attempts to use the
angular-redshift relation to derive information about the geometry of
the Universe (e.g. determination of q0 and
0) and
even to set constraints on dark energy
(Podariu et al. 2003).
However, there are many observational selection
effects involved. Furthermore, it is difficult to disentangle varying
geometry of the Universe from effects due to physical evolution of the
radio sources, their host galaxies and the surrounding ambient
medium. For example, the sizes of radio sources can be expected to
decrease at larger redshifts due to a systematic increase in density of
the ambient medium. Also the energy density of the cosmic microwave
background increases as (1 + z)4, substantially
enhancing inverse Compton losses. This will tend to extinguish radio
sources earlier in their lives, making them on average smaller.
In summary, the many non-cosmological effects that influence the angular size-redshift relation have made it impossible to draw robust conclusions about cosmology from such considerations.
2.4. Radio spectral index vs redshift correlation
One of the most intriguing properties of the relativistic plasma in
HzRGs is the strong correlation that exists between the steepness of
radio source spectra and the redshift of the associated radio galaxies
(Tielens et al. 1979,
Blumenthal and Miley
1979).
Radio sources with very steep spectral indices at
low frequencies 1
GHz tend to be associated with galaxies at high redshift (e.g.
Figure 3). This empirical correlation between radio
spectral steepness and redshift has proved to be an efficient method for
finding distant radio galaxies. Most known HzRGs
(Table 3) have been
discovered through following up those radio sources with the steepest
ten percentile of radio spectra (spectral index,
~ -1).
The conventional explanation for the z ~
correlation is that it is the result of a concave radio spectrum (see
Cygnus A in Figure 3),
coupled with a radio
K-correction. For higher redshifts, observations of sources at a
fixed observed frequency will sample emission at higher
rest-frame frequencies where the concave spectrum
becomes steeper. The most important mechanisms for making the radio
spectra concave are synchrotron and inverse Compton losses at high
frequencies (e.g.
Klamer et al. 2006)
and synchrotron self-absorption at low frequencies.
Although there is evidence from radio colour-colour plots that
systematic concave curvature is present in the radio spectra (e.g.
Bornancini et
al. 2007),
such an explanation alone is insufficient to explain the observed
correlation. The radio spectra of many distant luminous radio galaxies
are not concave at the relevant frequencies. The radio source
with the most accurately determined spectrum over a wide frequency range
is 4C 41.17 at z = 3.8 (bottom right in
Figure 3).
This source has an extremely straight spectrum between 40
MHz and 5 GHz, the relevant frequency range for the z ~
correlation
(Chambers et al. 1990).
Although the spectrum steepens above 5 GHz, this is
too high a frequency to contribute to the z ~
correlation.
Furthermore, a recent study by
Klamer et al. (2006)
showed that 33 of 37 sources in their SUMSS-NVSS sample have straight
and not concave spectra between 0.8 and 18 GHz.
 |
Figure 3. Left. Plot of radio spectral index versus redshift, showing that more distant sources have steeper spectra. [From De Breuck et al. (2000)]. Above right. Radio spectrum of the luminous radio galaxy Cygnus A at z = 0.05, showing spectral curvature. Bottom right. Radio spectrum of the HzRG 4C 41.17 and its various components. [From Chambers et al. (1990)]. This is still one of the most well determined spectra of a HzRG at low frequencies. Note the absence of significant spectral curvature. |
Two alternative effects have been proposed to explain the observed z ~
relation.
The first possibility is that the z ~
relation is an indirect
manifestation of a luminosity, L ~
effect
(Chambers et al. 1990,
Blundell and Rawlings
1999).
Classical synchrotron theory predicts that continuous particle injection
will result in a spectrum with a low-frequency cut-off,
l, whose frequency
depends on the source luminosity, L according to
l ~ L-6/7.
For a flux-limited sample, Malmquist bias will cause sources at higher
redshift to have preferentially larger radio luminosities. Over the
relevant frequency range, the L-6/7 vs
l effect would
therefore result in the observation
of an z ~ relation.
However, it is unlikely that this luminosity - spectrum relation is the
correct explanation, or at least the whole story.
Athreya and Kapahi
(1998)
showed that a
z ~ correlation still
persists even when samples are restricted to a limited range of L.
A second explanation is that some physical effect causes the spectral
index to steepen with higher ambient density and that the ambient
density increases with redshift
(Athreya and Kapahi
1998,
Klamer et al. 2006).
For example, putting the radio source in a denser
environment would cause the upstream fluid velocity of the relativistic
particles to decrease and a first-order Fermi acceleration process would
then produce a steeper synchrotron spectrum. Recently
Klamer et al. (2006)
pointed out that such a mechanism would (i) result in both z ~
and L ~
correlations and (ii)
provide a natural physical link between high-redshift radio galaxies and
nearby cluster halos.
However, it is difficult to produce the observed z ~
relation from such a
simple density-dependent effect alone.
The clumpy UV/optical morphologies (e.g
Section 6.1) indicate that the density of
gas around HzRGs is highly non-uniform and that the density is larger
close to the nucleus than in the outer regions. Furthermore, the
internal spatial variations of spectral index within individual
source is observed to be smaller than the source to source
variations of the integrated spectral indices (e.g.
Carilli et al. 1994).
If the ambient medium is highly non-uniform,
how can one side of a radio source "know"
that the other side has an uncommon ultra-steep spectrum?
It is therefore likelier that the ultra-steep spectra are produced by
some mechanism in which the spectral index is determined within the
galaxy
nucleus rather than by the environment at the locations of the radio lobes.
In summary, the origin of the z ~
effect is still
unclear and more detailed information is needed about the dependence of
the radio spectrum on redshift. In the near future accurate measurements
of low-frequency radio spectra of HzRGs with new facilities such as
LOFAR will be important for such studies.
2.5. Nonthermal X-ray emission
Although X-ray measurements of HzRGs are sensitivity-limited, significant progress in the field has been made in the last decade using the Chandra and XMM X-ray telescopes. Extended X-ray emission has been detected from about a dozen high-redshift radio galaxies and radio-loud quasars (Carilli et al. 2002, Fabian et al. 2003, Fabian et al. 2003, Scharf et al. 2003, Yuan et al. 2003, Belsole et al. 2004, Overzier et al. 2005, Blundell et al. 2006, Erlund et al. 2006, Johnson et al. 2007). The extended X-ray emission is typically elongated in the direction of the radio source, indicating that there is some physical link between the X-ray emission and the relativistic plasma.
Several mechanisms have been proposed for producing the extended X-ray
emission. The one that is most widely suggested inverse Compton
scattering (IC) of the cosmic microwave background
(Fabian et al. 2003,
Scharf et al. 2003,
Belsole et al. 2004,
Erlund et al. 2006,
Johnson et al. 2007).
Because the density of CMB photons increase as (1 +
z)4, IC scattering of the CMB becomes increasingly
important at high redshift. Under the assumption that the X-ray emission
is due to this process, comparison of the radio and X-ray luminosities
(Felten and Rees 1969)
yields magnetic field strengths consistent with equipartition
(Belsole et al. 2004,
Overzier et al. 2005,
Johnson et al. 2007).
Because the radiative lifetimes of radio synchrotron-emitting electrons
are shorter than the lifetimes of the X-ray emitting IC electrons, the
IC emission traces older particles. In 4C 23.56 at z = 2.48, the X-ray
emission is observed to extend by ~ 500 kpc
(Johnson et al. 2007),
implying an energy in both relativistic and IC-emitting electrons of
1059 erg, an
energy reservoir equivalent to ~ 108 supernovae.
Other processes that have been invoked to produce the extended X-ray emission include inverse-Compton up-scattering of synchrotron photons in the jet (synchrotron self-Compton emission - SSC) (Scharf et al. 2003) and thermal emission from shocks (Carilli et al. 2002, Belsole et al. 2004) (see Section 3.1).