Radio galaxy
A radio galaxy is a
Alcyoneus is a low-excitation radio galaxy, identified as having the largest radio lobes found, with lobed structures spanning 5 megaparsecs (16×106 ly). For comparison, another similarly sized giant radio galaxy is 3C 236, with lobes 15 million light-years across. A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated by relativistic jets powered by the central galaxy's supermassive black hole. Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than the diameters of their host galaxies.
Emission processes
The
A sister process to the synchrotron radiation is the inverse-Compton process, in which the relativistic electrons interact with ambient photons and Thomson scatter them to high energies. Inverse-Compton emission from radio-loud sources turns out to be particularly important in X-rays,[4] and, because it depends only on the density of electrons, a detection of inverse-Compton scattering allows a somewhat model-dependent estimate of the energy densities in the particles and magnetic fields. This has been used to argue that many powerful sources are actually quite near the minimum-energy condition.
Synchrotron radiation is not confined to radio wavelengths: if the radio source can accelerate particles to high enough energies, features that are detected in the radio wavelengths may also be seen in the
Processes, collectively known as particle acceleration, produce populations of relativistic and non-thermal particles that give rise to synchrotron and inverse-Compton radiation. Fermi acceleration is one plausible particle acceleration process in radio-loud active galaxies.
Radio structures
Radio galaxies, and to a lesser extent, radio-loud quasars display a wide range of structures in radio maps. The most common large-scale structures are called lobes: these are double, often fairly symmetrical, roughly ellipsoidal structures placed on either side of the active nucleus. A significant minority of low-luminosity sources exhibit structures usually known as plumes which are much more elongated. Some radio galaxies show one or two long narrow features known as jets (the most famous example being the giant galaxy
the most widely accepted model has been that the lobes or plumes are powered by beams of high-energy particles and magnetic field coming from close to the active nucleus. The jets are believed to be the visible manifestations of the beams, and often the term jet is used to refer both to the observable feature and to the underlying flow.In 1974, radio sources were divided by
In more detail, the FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosities in more massive galaxies.
Names are given to several particular types of radio source based on their radio structure:
- Classical double refers to an FRII source with clear hotspots.
- Wide-angle tail normally refers to a source intermediate between standard FRI and FRII structure, with efficient jets and sometimes hotspots, but with plumes rather than lobes, found at or near the centres of clusters.
- Narrow-angle tail or Head-tail source describes an FRI that appears to be bent by ram pressure as it moves through a cluster.
- Fat doubles are sources with diffuse lobes but neither jets nor hotspots. Some such sources may be relics whose energy supply has been permanently or temporarily turned off.
Life cycles and dynamics
The largest radio galaxies have lobes or plumes extending to
Host galaxies and environments
These radio sources are almost universally found
There are several possible reasons for this very strong preference for ellipticals. One is that ellipticals generally contain the most massive
Unified models
The different types of radio-loud active galaxies are linked by unified models. The key observation that led to the adoption of unified models for powerful radio galaxies and radio-loud quasars was that all quasars appear to be beamed towards us, showing
Uses of radio galaxies
Distant sources
Radio galaxies and radio-loud quasars have been widely used, particularly in the 80s and 90s, to find distant galaxies: by selecting based on radio spectrum and then observing the host galaxy it was possible to find objects at high redshift at modest cost in telescope time. The problem with this method is that hosts of active galaxies may not be typical of galaxies at their redshift. Similarly, radio galaxies have in the past been used to find distant X-ray emitting clusters, but unbiased selection methods are now preferred. The most distant radio galaxy currently known is TGSS J1530+1049, at a redshift of 5.72.[18]
Standard rulers
Some work has been done attempting to use radio galaxies as standard rulers to determine cosmological parameters. This method is fraught with difficulty because a radio galaxy's size depends on both its age and its environment. When a model of the radio source is used, though, methods based on radio galaxies can give good agreement with other cosmological observations.[19]
Effects on environment
Whether or not a radio source is expanding supersonically, it must do work against the external medium in expanding, and so it puts energy into heating and lifting the external plasma. The minimum energy stored in the lobes of a powerful radio source might be 1053 J. The lower limit on the work done on the external medium by such a source is several times this. A good deal of the current interest in radio sources focuses on the effect they must have at the centres of clusters at the present day.[20] Equally interesting is their likely effect on structure formation over cosmological time: it is thought that they may provide a feedback mechanism to slow the formation of the most massive objects.
Terminology
Widely used terminology is awkward now that it is generally accepted that quasars and radio galaxies are the same objects (see above). The acronym DRAGN (for 'Double Radiosource Associated with Galactic Nucleus') was coined by Patrick Leahy in 1993 and is in use.[21][22] Extragalactic radio source is common but can lead to confusion, since many other extragalactic objects are detected in radio surveys, notably starburst galaxies. Radio-loud active galaxy is unambiguous, and so is often used in this article.
See also
- Relativistic jet
- X-shaped radio galaxy
- List of spiral DRAGNs
- M–sigma relation
- Death Star Galaxy
- ESO 0313-192
References
- ISBN 978-0-521-54623-2.
- ^ "9.3 Fanaroff-Riley Classification". NASA/IPAC Extragalactic Database (NED). California Institute of Technology. Retrieved 24 March 2022.
- doi:10.1086/146237.
- S2CID 10241874.
- ^ .
- .
- ^ .
- ISBN 978-0-937707-73-9.
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- Bibcode:1989A&A...219...63M.
- ^ Pratik Dabhade- https://astronomycommunity.nature.com/posts/giant-radio-galaxies-the-cosmic-behemoths
- S2CID 15207553.
- S2CID 16971626.
- S2CID 18712724.
- doi:10.1086/167038.
- S2CID 45906162.
- S2CID 4347023.
- .
- S2CID 5423628.
- ^ "Perseus Cluster: Chandra "Hears" a Supermassive Black Hole in Perseus". Retrieved 2008-08-24.
- ^ Leahy JP (1993). "DRAGNs". In Röser, H-J; Meisenheimer, K (eds.). Jets in Extragalactic Radio Sources. Springer-Verlag.
- ISSN 0035-8711.
External links
- Atlas of DRAGNs A collection of radio images of the 3CRR catalogue of radio-loud active galaxies.
- Radio and optical images of radio galaxies and quasars
- The on-line 3CRR catalogue of radio sources