Radio relics
Encyclopedia
Radio Relics are diffuse synchrotron
radio
emission found in the peripheral regions of galaxy cluster
s. Similar to the case of radio halo
s, they do not have any obvious galaxy
counterpart, but their shapes are much more elongated and irregular compared to those of radio halos. Their energy distribution is steep (much more energy at low radio
frequency
that at high radio frequency), with hints of a distribution of different ages for the emitting electron
s across the whole dimension of the emitting region. Their cause is still debated, but they may come to exist due to the direct acceleration of relativistic
electrons in case of a large scale shock wave
, and/or to the adiabatic compression of a pre-existing mildly relativistic plasma
, which is compressed and re-energised by the passage of a compressive wave.
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
emission found in the peripheral regions of galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
s. Similar to the case of radio halo
Radio halo
Radio halos are large-scale areas of radio emission found in clusters of galaxies. They do not have an obvious galaxy counterpart, as opposed, for example, to radio galaxies which have AGN counterparts...
s, they do not have any obvious galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
counterpart, but their shapes are much more elongated and irregular compared to those of radio halos. Their energy distribution is steep (much more energy at low radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
that at high radio frequency), with hints of a distribution of different ages for the emitting electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s across the whole dimension of the emitting region. Their cause is still debated, but they may come to exist due to the direct acceleration of relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
electrons in case of a large scale shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
, and/or to the adiabatic compression of a pre-existing mildly relativistic plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, which is compressed and re-energised by the passage of a compressive wave.