For several decades, extragalactic radio surveys remained the most powerful tool to probe the distant universe. Even `shallow' radio surveys, those of limited radio sensitivity, reach sources with redshifts predominantly above 0.5. Since the 1960s, the most effective method for finding high-z galaxies has been the optical identification of radio sources, a situation persisting until the mid-1990's, when the arrival of the new generation of 8-10 m class optical/infrared telescopes, the refurbishment of the Hubble Space Telescope, the Lyman-break technique (Steidel et al. 1996) and the Sloan Digital Sky Survey (York et al. 2000) produced an explosion of data on high-redshift galaxies.
This is not a historical account (see Sullivan III 2009); but listing the revolutions in astrophysics and cosmology wrought by radio surveys serves to set out concepts and terminology. On the astrophysics side we note the following:
On the cosmology side we note the following:
r-2, so that
N
S-3/2.)
Bondi
& Gold (1948)
together with
Hoyle
(1948)
were uncompromising proponents of the new Steady-State theory. Ryle et
al. interpreted the 2C apparent excess of faint sources in terms of the
radio sources having far greater space density at earlier epochs of the
Universe. Confusion, the blending of weak sources to produce a continuum
of strong sources, was then shown to have disastrous effects on the
early Cambridge source counts. From an independent survey in the South,
Mills
et al. (1958)
found an initial slope of -1.65 after corrections for
instrumental effects, significantly lower than that found for 2C.
Scheuer
(1957)
developed the P(D) technique, circumventing confusion and showing that
the interferometer results of 2C were consistent with the findings of
Mills et al. But the damage had been done: cosmologists, led by Hoyle,
believed that radio astronomers did not know how to interpret their
data.
In 1965 the `source-count controversy' became irrelevant in one sense. Penzias & Wilson (1965) found what was immediately interpreted (Dicke et al. 1965) as the relic radiation from a hot dense phase of the Universe. The Big Bang was confirmed.
Ryle was right all the time. Integral source-count slopes of -1.8 or even as shallow as -1.5 were nowhere near what the known redshifts plus Steady State cosmology - or even any standard Friedman cosmology - predicted. These all come out at -1.2 or -1.3, shallower than the asymptotic -1.5 as sources of infinite flux density are not observed, and nobody has ever claimed the initial source count slope at any frequency to be as flat as this. The discovery of the fossil radiation (see Peebles et al. 2009) may indeed have shown that a Big Bang took place; but the source counts demonstrated further that objects in the Universe evolve either individually or as a population - a concept not fully accepted by the astronomy community until both galaxy sizes and star-formation rates were shown to change with epoch.
Source counts from radio and mm surveys - with errors and biases now understood - are currently recognized as essential data in delineating the different radio-source populations and in defining the cosmology of AGNs. These counts are dominated down to milli-Jansky (mJy) levels by the canonical radio sources, believed to be powered by supermassive black-holes (e.g. Begelman et al. 1984) in AGNs. At fainter flux-density levels, a flattening of slope in the Euclidean normalized differential counts (i.e. counts of sources with flux density S, within dS, multiplied by S2.5, see Section 2) was found (Windhorst et al. 1984, Fomalont et al. 1984, Condon & Mitchell 1984), interpreted at the time as the appearance of a new population whose radio emission is, to some still-debated extent, associated with star-forming galaxies.
Radio-source spectra are usually described as power laws
(S
-
)
1; the early low-frequency
meter-wavelength (e.g. 178 MHz) surveys found radio sources with spectra
almost exclusively of steep power-law form, with
~ 0.8. Later
surveys at cm-wavelengths (higher frequencies, e.g. 5 GHz) found objects
of diverse spectral types, some with spectra rising to the high
frequencies, some with steep low-frequency portions flattening and
rising to the high frequencies, and yet others with a hump in the radio
regime, or indeed two or more humps. In general, anything which was not
`steep-spectrum' in form was called `flat-spectrum', an inaccurate
nomenclature: very few truly flat-spectrum sources have been found and
even then the flatness persists over only a limited frequency range.
Nevertheless AGN-powered radio sources are traditionally classified in
two main categories: steep-
( > 0.5) and
flat-spectrum (
< 0.5). Broadly speaking, to radio telescopes the steep-spectrum
objects showed extended double-lobed structures, while the flat-spectrum
objects were point sources, unresolved until the Very-Long-Baseline
Interferometry (VLBI) technique provided sub-arc-second mapping. The
compact nature of flat-spectrum sources led to the conventional
interpretation of synchrotron self-absorption at frequencies below the
bump(s), implying brightness temperatures of ~ 1011 K
for the estimated magnetic field strengths.
From a physical point of view, it is appropriate to consider the integrated spectra as composites, built of the combination of different components of radio sources. Unified models provide a framework for such a discussion.
In the widely accepted `unification' scheme (Scheuer & Readhead 1979, Orr & Browne 1982, Scheuer 1987, Barthel 1989) the appearance of sources, including this steep-spectrum/flat-spectrum dichotomy, depends primarily on their their axis orientation relative to the observer. This paradigm stems from the discovery of relativistic jets (Cohen et al. 1971, Moffet et al. 1972) giving rise to strongly anisotropic emission. In the radio regime (Fig. 1), a line-of-sight close to the source jet-axis offers a view of the compact, Doppler-boosted, flat-spectrum base of the approaching jet. Doppler-boosted low-radio-power (Fanaroff & Riley 1974, type I (FRI; edge-dimmed)) sources are associated with BL Lac objects, characterized by optically-featureless continua, while the powerful type II (FRII; edge-brightened) sources are seen as flat-spectrum radio quasars (FSRQs). The view down the axis offers unobstructed sight of the black-hole - accretion disk nucleus at wavelengths from soft X-rays to UV to IR, and this accretion-disk radiation may outshine the starlight of the galaxy by 5 magnitudes. The source appears stellar, either as a FSRQ or a BL Lac object. FSRQs and BL Lacs are collectively referred to as blazars. In the case of a side-on view, the observed low-frequency emission is dominated by the extended, optically-thin, steep-spectrum components, the radio lobes; and the optical counterpart generally appears as an elliptical galaxy. A dusty torus (Antonucci & Miller 1985) hides the black-hole - accretion-disk system from our sight (Fig. 1). At intermediate angles between the line-of-sight and the jet axis, angles at which we can see into the torus but the alignment is not good enough to see the Doppler-boosted jet bases, the object appears as a `steep-spectrum quasar'.
 |
Figure 1. Unified scheme for high radio-power Fanaroff-Riley (1974; FRII) sources (following Jackson & Wall 1999). |
In general, then, each source has both a compact, flat-spectrum core and extended steep-spectrum lobes (Fig. 2). This already implies that a simple power-law representation of the integrated radio spectrum can only apply to a limited frequency range. The reality is even more complex (Wall 1994). External absorption or, more frequently, self-absorption (synchrotron and free-free) can produce spectra rising with frequency at the low-frequency optically-thick regime, while at high frequencies the synchrotron emission becomes optically thin, power law, and energy losses of relativistic electrons ("electron ageing", Kellermann 1966) translate into a spectral steepening.
 |
Figure 2. Spectral behaviour in the millimeter band of the radio galaxy NGC6251 (left panel) and (right panel) 11-GHz isophotes overlaid on the 0.3 GHz map (Mack et al. 1997). The low-frequency spectrum is due to the steep-spectrum outer lobes while at higher frequencies the flatter-spectrum core-jet system dominates. |
Two classes of ultra-steep-spectrum
( > 1.3) sources have
been discovered. One is associated with galaxy clusters; the objects are of
relatively low luminosity and generally are not associated with any host
galaxy. They are diffuse and of several types, including cluster `radio
halos', `radio relics' and `mini-halos', and each type appears to
involve reactivation of the hot intra-cluster medium by shocks or
cooling flows, the observed form depending on the cluster evolutionary
state
(Feretti
2008).
These `radio ghosts' will not be discussed further here. The second class of
ultra-steep-spectrum source is very radio-luminous and these are mostly
identified with very-high-redshift radio galaxies. The high redshifts
tempt the suggestion that the steep spectral index is due to the effect
of redshift moving the steepest part of the spectrum (where electron
ageing effects are strong) into the observed frequency range. However,
Klamer
et al. (2006)
demonstrated that this is not the dominant mechanism,
and that high-redshift radio galaxies, discovered by the steep-spectrum
technique, have intrinsically power-law spectra. The selection of
ultra-steep-spectrum sources is a very effective, but not the only
(Jarvis
et al. 2009),
way to find high redshift radio galaxies (see
Miley
& De Breuck 2008
for a comprehensive review), including the one
holding the current record, TN J0924-2201 at z =
5.19 (0.3651.4
1.6;
van
Breugel et al. 1999).
The highest-redshift radio-loud quasar known
to date, the z = 6.12 QSO J1427+3312
(McGreer et
al. 2006),
also has a steep radio spectrum (1.48.4 = 1.1) although it was not
discovered through this characteristic.
In very compact regions, synchrotron self-absorption can occur up to very high radio frequencies, giving rise to sources with spectral peaks in the GHz range. At high radio luminosities this category comprises the GHz Peaked Spectrum (GPS) sources (O'Dea 1998) some of which peak at tens of GHz (High Frequency Peakers; Edge et al. 1998, Dallacasa et al. 2000, Dallacasa et al. 2002, Tinti et al. 2005). At low luminosities, high-frequency spectral peaks, again due to strong synchrotron self-absorption, may be indicative of radiatively inefficient accretion, thought to correspond to late phases of the AGN evolution, with luminosities below a few percent of the Eddington limit (advection-dominated accretion flows (ADAF, (Quataert & Narayan 1999) or adiabatic inflow-outflow scenarios (ADIOS, Blandford & Begelman 1999, Blandford & Begelman 2004).
As the `flat' spectra are actually the superposition of emitting regions peaking over a broad frequency range (Kellermann & Pauliny-Toth 1969, Cotton et al. 1980), whose emission is strongly amplified and blue-shifted by relativistic beaming effects, a power-law description is a particularly bad approximation. The spectral shapes are found to be complicated, and generally show single or multiple humps. Many of these show flux-density variations, attributed to the birth and expansion of new components and shocks forming in relativistic flows in parsec-scale regions. The variations may be on times scales from hours to months or even years, and substantial resources have been devoted to monitoring these variable sources, led by groups at Michigan (USA) and Metsahövi (Finland) (e.g. Aller et al. 2003, Valtonen et al. 2008). The latter reference shows how global (multi-wavelength and multi-telescope) these monitoring programmes have become; moreover the quasi-periodicity for the object in question, OJ 287, indicates that it is probably a binary black-hole system. With regard to flux variations, we also note the `Intra-Day Variables' (IDVs), blazars whose flux densities vary wildly on time scales from minutes to days: these are flat-spectrum objects with extremely small components that show inter-stellar scintillation (ISS) via the turbulent, ionized inter-stellar medium (ISM) of our Galaxy (e.g. Lovell et al. 2007). Detailed discussion of all these variable objects is beyond the scope of this review.
The discovery of Compact Steep Spectrum sources (CSS;
Kapahi
1981,
Peacock
& Wall 1982,
O'Dea
1998)
originally appeared to be an exception to the conventional wisdom that
steep and flat spectra are associated with extended and compact sources
respectively. CSS sources are unresolved or barely resolved by standard
interferometric observations (arcsec resolution), and the integrated
spectra show peaks at < 0.5 GHz, above which the spectral
indices (on average,
~ 0.75) are typical of
extended radio
sources. There is compelling evidence that these objects, as well as GPS
and associated types of object (HFPs and CSOs - Compact Symmetric
Objects) are young radio galaxies, as summarized concisely by
Snellen
(2008).
It follows from the above that the conventional two-population approach (flat- and steep-spectrum) assuming power-law spectra is particularly defective at high radio frequencies, where several different factors (emergence of compact cores of powerful extended sources, steepening by electron energy losses, transition from the optically-thick to the optically-thin synchrotron regime of very compact emitting regions, etc.) combine to produce complex spectra (see Fig. 3). Nevertheless, for many practical applications the conventional approach remains useful in describing the bulk population properties of AGN-powered radio sources.
 |
Figure 3. Examples of radio-source spectra
at mm wavelengths: a
flat-spectrum source (top left panel); a steep-spectrum source
(bottom left panel); a source whose spectrum flattens at
|
The radio emission of star-forming galaxies is mostly optically-thin synchrotron from relativistic electrons interacting with the galactic magnetic field, but with significant free-free contributions from the ionized interstellar medium (Condon 1992, Bressan et al. 2002, Clemens et al. 2008). At mm wavelengths, however, the radio emission is swamped by (thermal) dust emission, whose spectrum rises steeply with increasing frequency. The well-known tight correlation between radio and far-IR emission of star-forming galaxies (Helou et al. 1985, Gavazzi et al. 1986, Condon et al. 1991) vastly increases the body of data relevant to characterize, or at least constrain the evolutionary properties of this population. However, to date few attempts have been made to build comprehensive models encompassing both radio and far-IR/sub-mm data (but see Gruppioni et al. 2003).
In this paper we first review the observed radio to mm-wave source
counts (Section 2), the data on the local
luminosity function of different radio source populations
(Section 3), and the source spectral properties
(Section 4). Next
(Section 5) we look at evolutionary models
for the classical
radio sources as well as for individual populations, such as GPS
sources, ADAF/ADIOS sources, and (Section 6)
star-forming galaxies and
-ray
afterglows at radio wavelengths. We deal briefly with the Radio Background
(Section 7) and the Sunyaev-Zeldovich effect
on cluster and galaxy scales (Section 8).
Section 9 contains a summary of the
information on large
scale structure stemming from large-area radio surveys. Finally, in
Section 10 we summarize perspectives for the
future, and Section 11 contains some
conclusions.
Unless otherwise noted, we adopt a flat
CDM cosmology with
=
0.7 and H0 = 70 km s-1 Mpc-1.
 |
Figure 4. Differential source counts at
150, 325, 408, 610 MHz normalized to c
S |
1 We note that this
negative sign convention for
is not universal;
however the convention has been adopted for the K-corrections of
optical quasars and for the extrapolation from optical to X-rays
(`ox').
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