Radio galaxy

The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming.

Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology.

This implies that the radio-emitting plasma contains, at least, electrons with relativistic speeds (Lorentz factors of ~104) and magnetic fields.

In the latter case the responsible electrons must have energies in excess of 1 TeV in typical magnetic field strengths.

It is hard to distinguish observationally between the synchrotron and inverse-Compton radiation, making them a subject of ongoing research.

Since the 1970s,[8][9] 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.

In more detail, the FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosity in more massive galaxies.

For a long time, it was assumed that powerful sources would expand supersonically, pushing a shock through the external medium.

[15] The only unambiguously supersonically expanding system known consists of the inner lobes of the low-power radio galaxy Centaurus A which are probably a result of a comparatively recent outburst of the active nucleus.

One is that ellipticals generally contain the most massive black holes, and so are capable of powering the most luminous active galaxies (see Eddington luminosity).

Another is that ellipticals generally inhabit richer environments, providing a large-scale intergalactic medium to confine the radio source.

It may also be that the larger amounts of cold gas in spiral galaxies in some way disrupts or stifles a forming jet.

[19] Possibly due to availability of new sensitive radio and optical all sky surveys the number of such Speca-like objects or Spiral-DRAGNs have increased to up to three dozens.

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 superluminal motion in the cores[20] and bright jets on the side of the source nearest to us (the Laing-Garrington effect:[21][22]).

In a similar way, low-power radio galaxies are a plausible parent population for BL Lac objects.

Similarly, radio galaxies have in the past been used to find distant X-ray emitting clusters, but unbiased selection methods are now preferred.

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.