Katherine M. Blundell
Department of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK ([email protected])
The influence of jet-ejected plasma has been an important theme of this meeting; I draw attention to the prevalence of jet-ejected plasma, in particular that which has not been properly accounted for in the past. There are three strands to this paper: important emission which is prominent only at the lowest radio frequencies; relic radio plasma which must exist if even the most basic aspects of radio source evolutionary models are correct; and evidence that some ‘radio-quiet’ quasars could be FR-I radio sources.
Keywords: AGN; galaxies; quasars; jet output
Source: Phil. Trans. R. Soc. A (2005) 363, 645–654
Jet-ejecting active galaxies are a key constituent of, and influence on, the distant Universe. It is essential to measure and understand their presence for at least three reasons.
(i) The low-energy plasma in high-z radio galaxies is an important contaminant to measurements of anisotropies in the cosmic microwave background (CMB) (Enßlin & Kaiser 2000; Yamada et al. 1999).
(ii) It is incontrovertible that radio galaxies contribute considerable energy and entropy to their surroundings (Binney 2001; Omma et al. 2004; Rawlings & Jarvis 2004; Reynolds et al. 2001); determination of the high-z demography of over-pressured radio lobes is necessary to determine their role in structure formation and the magnetization of proto-galactic material (Gopal-Krishna & Wiita 2001).
(iii) The triggering of powerful radio galaxies is rumoured to result from galaxy– galaxy mergers (because of the need for appreciable quantities of gas). For these reasons, it is imperative to investigate the locality and extent of all the plasma output from active galactic nuclei (AGN) jets in the Universe, complete for all types of radio source, regardless of age or jet power.
We need to consider not just powerful thrusting classical double radio galaxies (Fanaroff–Riley (FR) II objects) (Fanaroff & Riley 1974), which are very easily detected out to high redshift, but also very young and very old sources and those of lower jet power. Such classes include: lower-jet-powered FR-I radio galaxies; ‘flatspectrum’ sources; ‘radio-quiet’ quasars and GHz peak-spectrum (GPS) sources. Members of the last class are thought to be the progenitors of FR-Is and classical doubles (see, for example, Tschager et al. 1999), depending on their intrinsic jet powers, and to have ages between 102 and 104 yr. All these classes of object have rather different spectral and spatial characteristics, and different types of observations are required to detect a true representation of their populations. The radio sky is sampled at different discrete frequencies because of the availability of observing bands. The relative contributions of different parts of a source vary as a function of frequency (because of different energy distributions and absorption) and angle to the line of sight (Doppler beaming). The frequency domains least afflicted by these effects are the low frequencies, corresponding to metre wavelengths. Observations at these wavelengths played a remarkable role in the early history of radio astronomy, but over the subsequent three or four decades their usefulness has been dominated by observations at higher frequencies in which higher angular resolution can be achieved; low-frequency observations have been used as the finding frequency of surveys because they yield samples of radio sources with isotropically distributed jet axes. In recent years, spatially well-resolved imaging at low frequencies has become possible. Powerful classical double radio sources have identical outer boundaries at 74 MHz compared with (fully sampled) images at GHz frequencies (Blundell et al. 1999b, 2005a), which has important implications for the energy-loss mechanisms in these sources (Blundell & Rawlings 2000). However, the situation with much lessluminous radio sources, emerging through very rich cluster environments, does not appear to be so simple: images of Perseus A (3C 84) show curious new features at 74 MHz not seen at frequencies of 330 MHz and above (figure 1) (Blundell et al. 2002). The spectrum of these spurs is steep (α 1.4, where Sν ∝ ν−α and Sν is the flux density at frequency ν). Remarkably, these spurs point to the outer pair of cavities in the X-ray emission surrounding this cluster (figure 1) (Fabian et al. 2002). Future telescopes which are significantly more sensitive in this frequency regime, such as the Square Kilometre Array (SKA) and LoFar, hold much promise for detection of emission that is unique to the low-frequency sky.
Although relic radio plasma may be below current telescope sensitivity limits, that it not to say it is not there. Ignoring simple, yet inevitable, evolution of the light curves of radio sources based on well-understood physics means we underestimate the amount of activity in the past. This section illustrates a way to obtain an inventory of what might exist, based on a model whose starting point is the sources born throughout cosmic time, not merely the sources which make it above flux limits, assuming sources are bright when they are very young and thereafter fade.
The ‘birth function’ of a population of extragalactic sources (i.e. the number of sources of a given jet power born per unit cosmic time per unit co-moving volume between z and z+dz) is a distinct function from the more traditionally studied ‘luminosity function’. The former gives the triggering rate of these phenomena, while the latter includes only objects above survey flux limits at redshifted finding frequencies. The luminosity function contains no source physics and does not decouple the individual luminosity evolution of sources from the birth rate and hence is not a direct measure of the triggering rate. Accurate determination of the birth function is of key cosmological importance since it constrains triggering and merging rates at early cosmic epochs, and hence perhaps the formation epoch of massive galaxies. In generating birth functions and simulated luminosity functions, proper account of the sampling by our light cone of extragalactic objects as a function of their age is made, following the algorithm developed in Blundell et al. (1999a). This can be easily adapted to allow for any particular underlying birth function and any prescriptive light curve or other properties, such as spectral shape. The simulations follow the outline indicated in figure 2.
Figure 3 illustrates how two populations of toy objects having the same birth function give rise to two rather different observed distributions of sources with redshift. In one population the sources have constant luminosity, and in the other source gradually fade, although at constant spectral index. It is clear that, if sources have declining light curves, it is not sensible to naively extrapolate back from the luminosity function to the birth function.
Accurate determination of the birth function is possible only with a robust physical model for how objects change in luminosity and spectral shape throughout their life cycles. Classical double radio galaxies are unique among extragalactic objects in having a robust physical model for how they evolve (for example, there is no such model for the optical evolution of quasars). Our work on complete samples of classical double radio sources led us to develop a robust physical model for the evolution of the luminosities, sizes and spectral shapes with age (Blundell et al. 1999a) which, importantly, includes the role played by the hotspots. This led us to the realization that, as these powerful objects age, their luminosities must decline dramatically, the details of this decline depending on redshift and jet power. A population of objects whose luminosities decline as they age, in combination with a survey flux limit applied to sources at different redshifts, inevitably gives rise to an effect called the ‘youth–redshift (YZ) degeneracy’ (Blundell & Rawlings 1999). The YZ degeneracy means that, in a flux-limited sample, the objects at higher redshift must be younger than their low-redshift counterparts. This has important implications for studies of underlying populations. While a high luminosity source requires high jet power, the converse is not true: a high jet-power source will only have high luminosity if it is observed sufficiently early in its life. There is a vast unobserved population of high-jet-power, low-luminosity sources out there.
Monte Carlo simulations run along the lines indicated in figure 2, and indeed consideration of the toy model shown in figure 3 shows that:
(i) only contrived forms of the birth function would cause a redshift cut-off in the luminosity function to fall at the same place as a redshift cut-off in the birth function;
(ii) the parent population is only sparsely sampled, i.e. the space density of sources, on our light cone, is considerably larger than the space density of sources which make it into our surveys.
The view that there are two distinct types of quasar, differing only in their radio characteristics, has prevailed for a number of years (Falcke et al. 1996; Kellermann et al. 1989, 1994; Miller et al. 1990, 1993)). It now seems hard to avoid the conclusion that the foundation of these studies, the Bright Quasar Survey (Schmidt & Green 1983), has serious and systematic incompletenesses (Goldschmidt et al. 1992; Miller et al. 1993; Wisotzki et al. 2000). Moreover, four recent studies have suggested that the basis for believing in a bimodality, rather than a continuity, of the radio properties of quasars may have evaporated.
(i) Quasars selected from the FIRST survey show no bimodality in 1.4 GHz radio luminosity (White et al. 2000).
(ii) Quasars from the optically selected Edinburgh survey show no bimodality in 5 GHz radio luminosity (Goldschmidt et al. 1999).
However, reservations about the lack of a bimodality found by (i) and (ii) include concerns about the high frequency at which the radio data were obtained: 1.4 GHz and 5 GHz, respectively. Selection and measurement in a waveband where Doppler boosting is prevalent could contaminate the measured radio luminosities, and in principle might blur out any underlying bimodality.
(iii) Consideration of a simple diagram from Blundell & Rawlings (2001), reproduced in figure 4, shows the potential for the bimodality perceived in the past to be a consequence of selection in different radio wavebands. For example, a standard classification of radio-quiet quasars (e.g. Miller et al. 1990) is that a radio-quiet quasar’s luminosity at 5 GHz is less than 1024 WHz−1sr−1. It is instructive to see how this translates to classifications concerning FR-I and FRII types at the lower frequency of 178 MHz. Although there have been assertions in the literature that FR-I radio structures are never associated with quasars (Baum et al. 1995; Falcke et al. 1995b), figure 4 suggests that low-frequency extended emission resembling that well known around nearby low-power FR-I radio galaxies might sensibly be searched for around ‘radio-quiet’ quasars.
The generality of extended radio emission (presumably from FR-I-type, i.e. lowpower, highly dissipative, jets) in the population of quasars hitherto deemed ‘radio quiet’ has not been adequately investigated. Clarification of this issue would considerably benefit studies of the mechanisms by which quasar central engines work and the relationship between accretion and jet power. To achieve such aims requires avoiding selecting the quasars in a way that depends on an unknown and ill-understood mixture of different physical effects (e.g. Doppler boosting, for the FIRST quasars selected at (1 + z) × 1.4 GHz). It is important to select on low-radio-frequency flux density which should be dominated by lobe or plume (i.e. non-Doppler-boosted) jet output.
It is essential to establish the prevalence of FR-I quasars, and to deduce their jet powers, if we are ever to make definitive deductions concerning the so-called ‘radio– optical correlation’ (Rawlings & Saunders 1991) or ‘jet–disc symbiosis’ (Falcke et al. 1995a,b) for quasars and their central engines. Only with low-frequency data (corresponding to non-Doppler-boosted jet output) can one hope to do a refined ‘radio– optical correlation’ analysis and move closer to an understanding of the relationship between accretion and jet output in the quasar phenomenon.
(a) A different picture with long baselines? In addition to significantly shorter integration times, existing observations of radioquiet quasars are inferior to those of radio-loud quasars in another respect: most of the observations listed in the left column of figure 5 have been made with a single configuration of the Very Large Array (VLA). Typically this configuration has been one of the most extended VLA configurations (either A or B). Observations in extended configurations are less sensitive to large-scale structure (such as that from FR-I jets, for example) than the more compact configurations, as figure 6 demonstrates. On the assumption that the FR-I quasar discovered by Blundell & Rawlings (2001) is not some exception, there is the interesting possibility that many more of the ‘radio-quiet quasar’ population have FR-I radio structures. Their recognition is contingent on deep, sensitive observations at low frequency, and/or with sufficiently short baselines that emission on all size scales is sampled.
The assertions in the literature that optically luminous quasars with FR-I radio structures do not exist (e.g. Baum et al. 1995; Falcke et al. 1995a) were based on the presumption that if optically powerful quasars had extended radio structures, they would be of the FR-II type (with hotspots towards the outermost edges). Such assertions were based purely on radio images of quasars in the literature, and took no account of whether radio observations of optically selected quasars were sufficient and appropriate to detect any extended FR-I structures, were they to exist. A further insufficiency in past radio observations of optically selected quasars is that the luminosities of FR-Is are below the FR-I–FR-II-break luminosity (1025.5 WHz−1 sr−1 at 178 MHz) and this luminosity falls below the flux limit of most of the low-frequency complete samples (Blundell et al. 1999a) at the rather nearby redshift of 0.4. Most quasars are at redshifts higher than 0.4 (cf. those found in the 2dF and Sloan surveys). Ignoring the above considerations, dogma concerning radio properties of quasars took on board the view that quasars either had powerful FR-II structures with hotspots (and were called radio-loud quasars) or did not have significant extended radio emission (and were called radio-quiet quasars). The first deeply imaged ‘radioquiet’ quasar in fact revealed a 300 kpc FR-I jet structure (Blundell & Rawlings 2001). It seems hard to avoid the conclusion that radio-quiet quasars may be anything but quiet and that at least some have FR-I radio structures and hence low-power jets rather than no jets. Thus, the jet energy from the quasar population could be appreciable, and so a programme to test this hypothesis was initiated using the recently commissioned Giant Metre-wave Radio Telescope (GMRT). The GMRT is an interferometer consisting of 30 antennas, each 30 m in diameter, located in the state of Maharashtra in India. The GMRT is the first of a number of purpose-built low-frequency telescopes and is in some sense a prototype of LoFar and the SKA. We have used the 240 MHz receivers to make pointed observations of regions within the 2df–QSO survey area, each pointing giving an image of the sky 1.8◦ in diameter, and with a resolution of 10 arcsec and a root-mean-squared background noise level of 1 mJy per beam, both unprecedented at this low frequency. An example image from this survey is shown in figure 7. A full analysis of the cross-correlation of the 240 MHz sources with the 2df quasars will be presented by Blundell et al. (2005b), but it is already clear that, in this waveband largely uncontaminated by Dopplerboosted core emission, radio sources with luminosities lower than the FR-I–FR-II break are associated with optically powerful quasars at z > 2. This finding suggests that quasar jet-activity has been underestimated in the past.
I am very grateful to The Royal Society for its support.
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Figure 1. The different structures of Perseus A (3C 84) at low frequencies (Blundell et al. 2002). (a) 74 MHz contours overlaid on an X-ray image (Fabian et al. 2002) and (b) 330 MHz contours, which show no evidence for the spurs seen at 74 MHz.
Figure 2. Schematic of how a simulated survey (and hence simulated luminosity function) is derived from a prescriptive underlying birth function. Objects born at a given redshift zbirth, with a given jet power Q, are evolved for a time tobs - tbirth (which is determined by the separation of the object from our light cone at the time of its birth), according to a physical model for the evolution of its luminosity. The luminosity of the object when it is on our light cone, when sampled at (1 + z) times the observing frequency, determines whether or not the object is above the flux limit. In this way, simulated surveys of objects are made whose luminosity function can be compared with observed luminosity functions, to infer which underlying birth function is most appropriate.
Figure 3. Illustration of three key differences in observed redshift distribution if sources have (a) a constant or (b) a declining light curve: normalization, peak redshift and shape at high redshift.
Figure 4. A plot of low-frequency radio luminosity against high-frequency luminosity. The diagonal lines show tracks of final spectral index. The vertical line distinguishes what are conventionally known as the radio-loud and radio-quiet regimes, while the horizontal line distinguishes what Fanaroff & Riley (1974) observed to separate classical double (FR-II) and edge-dimmed (FR-I) radio sources. E 1821 + 643 is the quasar which was discovered by Blundell & Rawlings (2001) to have an FR-I structure.
Figure 5. The left column lists the mean length of time-on-source (min) for observations of radio-quiet quasars in the recent literature. The VLA has been used to make very deep radio observations (integration times of hours) of radio-loud quasars, e.g. the study of some radio-loud quasars from 3C by Bridle et al. (1994) in which the radio structure on all scales is fully sampled; comparable quality observations of quasars which are deemed to be radio quiet, though which are actually not radio silent, have yet to be made.
Figure 6. Images of the same region of sky (a) from the most compact VLA D-configuration (23 s observation from the NVSS survey) and (b) from the extended VLA B-configuration (200 s observation from the FIRST survey), demonstrating a basic principle of interferometry: long baselines are blind to extended structures.
Figure 7. A typical example of an object in a 1.8. diameter low-frequency image from the newly commissioned Giant Metre-wave Radio Telescope 240 MHz receivers. Peak contour flux density is 5.8533×10-1 Jy per beam; other contour levels are 2.500×10-3(-1.41,-1, 1, 1.414, 2, 2.828, 4, 5.657, 8, 11.31, 16, 22.63, 32, 45.25, 64, 90.51, 128, 181.0, 256, 362.0, 512.0, 724.1, 1020, 1450).