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‘Tailed’ radio galaxies tell stories about the Universe

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Posted January 2, 2015

Radio galaxies constitute an interesting class of objects in the Universe. According to astrophysicists, the most of relatively isolated radio galaxies have a fairly simple and symmetrical structure: one could expect to find a symmetrical structure of these objects when observing both in the visible optical and in the wider radio frequency ranges. “And how could these images be different?”, you may ask. Well, even our Moon has different faces when looking at it in a visible, ultraviolet, X-ray, gamma or infrared frequencies. When making a galaxy-scale observations you could expect to see things which can be even stranger.

Very close to the “head” of this radio galaxy a faint feature extending perpendicular to the radio trails can be seen, which has been interpreted as evidence for a bow shock of the host galaxy, caused by its supersonic speed due south-east (lower left). The 3C 129 tail length is 460 kpc or ~1.5 million light years long. Only very few tailed radio sources are known to be larger than this.

Very close to the “head” of this radio galaxy a faint feature extending perpendicular to the radio trails can be seen, which has been interpreted as evidence for a bow shock of the host galaxy, caused by its supersonic speed due south-east (lower left). The 3C 129 tail length is 460 kpc or ~1.5 million light years long. Only very few tailed radio sources are known to be larger than this. Image source: Galaxy Zoo

The first examples of the radio galaxies whose radio emissions extend compared to the optical galaxy in one direction were found back in 1968. When observing such objects in the radio frequency range, they appear to have a usually bright distorted “tail” which is not visible to the naked human eyes.

Astronomers say there exist at least several potential large-scale reasons for such tails to form and most of them are related to the environment the galaxy resides in. For example, when galaxy moves through a relatively dense medium, the movement generates a strong ram pressure, strong enough to distort its structure. The structure can be further distorted by precession, gravitational effects of nearby objects and perturbations in the density of galactic medium. In some cases you have to combine all these effects in a single model to explain the shape of a particular ‘tailed’ radio source.

Pseudo-colour image of the large-scale radio structure of the FRI radio galaxy 3C31. Jets and plumes are labelled. Image credit: Mhardcastle via Wikimedia Commons

Pseudo-colour image of the large-scale radio structure of the FRI radio galaxy 3C31. Jets and plumes are labelled. Image credit: Mhardcastle via Wikimedia Commons

“Tailed sources are unique probes of a number of mechanisms, over a range of physical scales from cluster-wide winds to binary galaxy pair gravitational interactions”, say the scientists from Victoria University of Wellington, New Zealand. In their recent article published on arXiv.org the team notes that the structure (or morphology, if speaking in more general scientific terms) of the so-called bent-tail radio galaxies directly depends on their surroundings. This statement is also supported by the fact that most of these objects are typically found in regions of the Universe with high galaxy concentrations like in galaxy clusters and groups.

Based on this observation, a number of earlier studies suggested that tailed radio sources could be used to measure density, dynamics, magnetic field strength and, possibly, other physical properties of host medium. Meanwhile, the authors of the current study say this concept could be advanced further: parameters of tailed radio galaxies could be used as probes of even larger scale structure and pertaining intra-cluster environmental conditions.

According to their study, the association between bent-tail radio galaxies, galaxy clusters and groups could be used to trace large scale structures both in the local and distant Universe. The researchers say there is a growing evidence that such an association is likely to hold up to redshift Z=2, which in turn means that it could be possible to investigate locations containing the first formed clusters of galaxies. In this case the detection of a bent-tail radio galaxy would be the first critical indication of a galaxy cluster located in the observed region.

Furthermore, large scale structures and their configurations could be detected based on the morphology of radio galaxy tail. Application of detailed modeling could be used to reconstruct the dynamical history of intra-cluster medium, including reconstruction of trajectories or even detection of mergers of neighboring galaxies.

The `corkscrew' galaxy, ESO 137-G 007, in the Norma cluster at 843 MHz from the Sydney University Molonglo Sky Survey (SUMSS). Note the tightly wound helical path of the radio jet. Image courtesy of researchers.

The `corkscrew’ galaxy, ESO 137-G 007, in the Norma cluster at 843 MHz from the Sydney University Molonglo Sky Survey (SUMSS). Note the tightly wound helical path of the radio jet. Image courtesy of researchers.

The authors of the study predict that the next generation radio telescopes could detect ‘tailed’ radio galaxies in great numbers. For example, it is estimated that the all-sky continuum survey Evolutionary Map of the Universe performed by ASKAP radio telescope should detect approximately half a million of such objects out to a redshift of Z=2, with sizes down to tens of kpc. The expected number of bent-tail radio galaxies is expected t oreach 1 million when the SKA telescope enters its Phase 1.

“Even if these instruments do not achieve the currently planned sensitivities, it is clear that vast numbers of BTs [bent-tail radio galaxies] will be detected, signalling the location of new clusters, groups and dense regions in the Universe”, conclude the researchers.

Written by Alius Noreika

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