At the simplest level, almost all DRAGNs consist of two lobes which straddle the central Active Galactic Nucleus. In a few percent of DRAGNs, the AGN is surrounded by a halo which cannot sensibly be divided into a pair of lobes, but at least some of these must be ordinary twin-lobed objects seen end-on, so the lobes are superposed. Even more rarely, one finds objects with a single lobe clearly separated from the AGN. One never finds more than two lobes associated with a single AGN (but see the discussion of wings).
The lobes often contain bright compact substructure in the form of jets or hotspots.
We need to have unambiguous, and therefore quantitative, definitions for lobes, jets, and hotspots, because these features are used to define both DRAGNs in general and also the various different classes of DRAGN. Our current definitions are a fairly ad hoc attempt to reflect the features which seem obvious to the eye; they can doubtless be refined by a careful statistical analysis of the structures in the Atlas and other samples. Even so, we believe that they reflect important aspects of the underlying physics. One sign of this is that few structures would have to be re-named if we changed the numbers in our definitions by a factor of two or so. Since we are working only from images of the projected radio emission, rather than a full description of the 3-D structure, there will always be cases where physically similar structures are given different names, and vice-versa. With rigid definitions we at least ensure that, in borderline cases, classifications cannot be biassed to support some particular hypothesis that we are trying to test (e.g. DRAGNs with hotspots are brighter than those without, etc.).
A Jet is a feature that is (a) at least four times as long as it is wide, (b) distinguishable from other extended structure (if any) either spatially or by brightness contrast, and (c) is aligned with the centre of activity where it is closest to it.
The centre of activity can generally be identified with the location of the compact core. If no core is detected, the centre is identified with the brightest Active Galactic Nucleus that lies within the radio structure, or the centre of the brightest galaxy if there is no obvious AGN.
There is little doubt that radio jets trace the path of collimated outflows from a central AGN, although the evidence for this is largely indirect. The rest of the radio structure of DRAGNs is believed to be a by-product of such outflows. In this sense jets are the crucial components of DRAGNs, but they are usually the faintest component and in many cases are not detected at all.
The structure of jets can take two distinctlu different forms, christened "strong-flavour" and "weak-flavour' by Bridle (1992); individual jets may change from strong to weak flavour with distance from the centre.
We define a jet to be strong flavour if it has both an average FWHM opening angle of <5° and also a projected magnetic field on the jet axis which is generally parallel to the jet, except in short regions with length at most twice the jet width.
Archetypical strong-flavour jets have small or zero opening angles (<4°) and projected magnetic fields parallel to the jet. Their brightness structure is rather irregular, with bright compact peaks known as "knots" separated by fainter emission. At high resolution, the magnetic field in the knots is parallel to the local intensity contours, rather than to the overall jet axis. The transverse profiles of strong jets are complex, and can be edge-brightened between the knots.
Strong-flavour jets occur in DRAGNs of all luminosities; probably all jets are initially strong-flavour. They are generally very faint relative to the rest of the structure, although less so in quasars than in radio galaxies. In powerful DRAGNs they appear to connect the core with the hotspots, although in many cases only a segment of this presumed path produces detectable radio emission. In weak radio galaxies they flare close to the core, forming the base knots at the start of weak-flavour jets, or else disrupting to form a tail.
In quasars and broad-line radio galaxies strong-flavour jets are usually only detected in one lobe; in some well studied cases any jet in the other lobe would have to be more than a hundred times fainter. On the other hand, in narrow-line radio galaxies twin strong jets are found almost as often as one-sided ones.
The best example of a strong-flavour jet in the Atlas is in 3C 200. Better examples are found in Alan Bridle's Quasar Gallery.
It is widely believed that the asymmetries in strong-flavour jets are caused by relativistic beaming in jets travelling at close to the speed of light, although it is difficult to completely rule out the possibility that they are intrinsically asymmetric. The larger asymmetries in quasars and BLRG is consistent with the predictions of unified schemes that these AGN should be oriented closer to the line of sight than narrow-line objects. These models also imply that strong-flavour jets are highly supersonic with respect to their internal sound speed.
The archetypical weak-flavour jet has a relatively large opening angle (> 8° on average, but varying along the jet), and a projected magnetic field perpendicular to the jet (at least near the jet axis). Their transverse profiles are centre-brightened, and the brightness declines smoothly along the jet from an initial bright "base knot" situated of order a kiloparsec from the compact core.
In some weak-flavour jets, high-resolution imaging of the region before the base knot has revealed a faint strong-flavour jet. Much more commonly, the magnetic field near the base of weak-flavour jets is parallel to the jet, as in strong-flavour jets. This has given rise to the suspicion that all jets start strong-flavour, and sometimes undergo a transition to weak-flavour at the base knot.
Weak-flavour jets occur exclusively in DRAGNs with luminosities below the Fanaroff-Riley break. They have a fairly constant brightness per unit length, and are almost always seen in both lobes if they are detected at all, usually differing by a factor of less than four in brightness. When detected, they tend to contribute a significant fraction of the total radio power (> 10%).
For good examples, see the supplementary images of 3C 66B and 3C 272.1, and Alan Bridle's image of the jets of 3C 31.
The fact that weak-flavour jets have similar brightnesses in the two lobes shows that they cannot consist of material moving relativistically away from the core (in fact the speed v < 0.2 c) since otherwise even intrinsically identical jets would appear very asymmetrical because of relativistic beaming. It seems likely that the transition from strong- to weak-flavour in jets is associated with deceleration and reduction in Mach number of the jets. Weak-flavour jets are often interpreted as turbulent flows travelling at about their internal sound speed (e.g. Bicknell 1990), although the details remain controversial.
A hotspot component is a brightness peak which: (a) is neither a core nor part of a jet; (b) has a largest dimension (at the half-maximum brightness contour) <10% of the main axis size of the DRAGN; (c) is separated from any nearby brighter peak by a minimum falling to two-thirds or less of the fainter peak.
To avoid peaks due to noise, we also impose the condition: (d) has a peak brightness more than ten times the rms noise level in the image.
By hotspot complex we mean one or more hotspot components, grouped together in a small region (peaks separated by <10% of the largest angular size of the DRAGN), together with surrounding high-brightness emission. If the complex contains just one component, it can be called a hotspot without ambiguity.
Notice that by defining the size at the half-maximum contour we require hotspot components to be more than twice as bright as any surrounding large-scale emission.
This definition does not specify a "boundary" for the hotspot, which would be necessary if we wanted to measure hotspot sizes or flux densities. No satisfactory way of defining such a boundary has been invented, as hotspots often merge smoothly into surrounding fainter emission. In this Atlas, as a crude measure of hotspot flux density, we use the peak flux density of the lobe as measured in the C20 maps. There are a number of obvious drawbacks. Firstly, we get a finite flux density even for DRAGNs lacking hotspots, given by the peak of the jet or diffuse lobes. Secondly, in a few cases with very weak hotspots the C20 peak refers to the lobes rather than the hotspots (3C 123W, 223S, 285E, 303E, 436N, 465W). These two problems would be reduced if we had used images of higher resolution. On the other hand, some of the larger hotspot complexes are well resolved even at C20 resolution, e.g. the double hotspots in 3C 20S and 390.3N; in these the C20 peak is definitely an underestimate of the hotspot complex flux density, and a lower resolution would give a better estimate. Clearly, perfection is not possible, but the C20 resolution seems to be a reasonable compromise.
DRAGNs which appear to have more than one hotspot complex in a lobe are quite rare.
Hotspot complexes appear to be sites in which the energy carried by strong-flavour jets is dissipated. The multi-peaked structure of many hotspot complexes suggests this is a complicated, multi-stage process. Fluid-dynamical modelling of DRAGNs suggests that hotspots are localised high-pressure regions confined by strong shocks which arise when the jet flow is disrupted.
A lobe is an extended region of emission which is not a jet, showing billowy or filamentary substructure, whose perimeter is mostly well-defined in the sense that the projected magnetic field is parallel to the edge, the intrinsic polarization is >40%, and the intensity tends to zero as the perimeter is approached.
This definition is rather specific, in order to separate DRAGN lobes from other diffuse radio nebulae, for instance the disk emission in spiral galaxies. On the other hand it does cover haloes.
Note that hotspot complexes are simply the brightest regions of lobes, whereas jets are not parts of lobes. We call the lobe emission excluding the hotspot complex the diffuse lobe, bearing in mind that there is actually no clear boundary between the two.
Diffuse lobes can usually be subclassified according to the position of the end of the jet, into Bridges and Plumes (the latter are known as Tails in some cases). We classify a lobe as relaxed if the brightest peak at high resolution is not part of a hotspot or jet. Thus relaxed lobes may contain very weak hotspots or jets, although in most cases neither is visible. Because of this, the bridge/plume distinction cannot be reliably made for relaxed lobes. Haloes are almost inevitably relaxed because if they contained compact structure one could use that as a basis to divide the halo into a pair of lobes.
A few lobes show faint extensions known as wings.
The diffuse lobes appear to contain the majority of the energy of the DRAGN, in the form of magnetic fields and relativistic particles. They are believed to be "cavities" where the normal interstellar or intergalactic medium has been displaced by very hot, low-density material from the jets.
The archetypical bridge trails back towards the centre from a hotspot at or close to the outer edge of the lobe. Thus the emission forms a "bridge" between the core and the hotspot.
Bridges are expected to form when the ends of the jets move outwards from the centre supersonically through the surrounding medium. Numerical simulations show that a significant fraction of the bridge material is likely to be flowing towards the host galaxy; so-called "backflow".
Diffuse lobes which fail the criteria for bridges are defined to be plumes.
The archetypical plume begins at the end of a narrow jet by flaring in width and fading in surface brightness (perhaps after an initial hotspot), and extends outwards from the centre. Plume-type lobes which are "bent back" behind the host galaxy (hence showing C-shaped distortion) are generally known as tails.
Plumes are believed to be flowing subsonically through the surrounding medium, or to be roughly static. Their dynamics may resemble smoke or steam plumes from factory chimneys, in that they will be strongly affected by both buoyancy and any "winds" in the surrounding medium. Some plumes may be relics of an earlier phase of bridge-type expansion.
Since lobes often show distortions near the host galaxy, we define true wings to be cases where the distortions are at least 80% of the length of the main lobes (Leahy & Parma 1992). The distribution of angles between wings and main axes seems random (although only a dozen or so cases are known). There are only two winged DRAGNs in the Atlas: 4C 12.03 and 3C 315.
Wings are usually taken as relics of previous outbursts of AGN activity, along a different axis from the current one. It is not known whether the change of direction of the ejection axis is associated with the re-birth of the DRAGN; note that if the axis did not change, the older lobes would be taken over by the new outburst and the evidence for a previous "event" would be much less clear.
At present we can only speculate about the reason for the change of ejection axis (if that is really what has happened).
SSCs are rather rare. The outer structure is often relaxed (e.g. in 3C 315), but they have been found in objects with well-defined hotspots in their large-scale structure, (e.g. 3C 236).
There are two main ideas for why some DRAGNs have SSCs. In the first, there is continuous flow of material through the SSC to the outer lobes, and the SSC is simply a region in which, for whatever reason, much more radio emission is produced by the flow. The second idea is that these are cases where the jets have somehow been cut off from the outer structure, so that the SSC is a "reborn" DRAGN inside a fossil of its previous phase.
VLBI images (which are only available for the brighter cores) show that they are typically a few parsecs or less in size. Usually a significant part of the emission is still contained in an unresolved component, with the remainder consisting of the beginning part of a jet. In a very few objects (usually FR Is) the core shows twin jets. The jet in the core is in the same direction as the brighter large-scale jet, if there is one, and in some cases the jet can be traced directly from the core to the kiloparsec scales resolved in the Atlas images. It is usually assumed that even the emission unresolved by VLBI is produced by the base of the jet(s).
The surface brightness of cores is extremely high (as it must be for such a small region to produce a detectable flux density). As a result, the low-frequency radio emission is affected by synchrotron self-absorption, giving cores a characteristically flat radio spectrum. They lack a well-defined spectral peak because they contain a sequence of regions along the expanding jet with progressively larger sizes and turnovers at progressively lower frequency, which combine to give a roughly flat spectrum.
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