2024-01-20 15:37:44
The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Topics of pulsating research, astronomical adventures and scientific news about the Universe analyzed in depth.
ORCs
In 2019, a team of astronomers, using data from the radio telescope ASH CAP, in Western Australia, discovered surprising circular structures of colossal size, reaching sizes 50 thousand times larger than that of the Milky Way (that is, up to 3 million light years). They were called “odd radio circles” (ORC).
Circular structures are not strange in astronomy, but those previously known were of much smaller dimensions, generally planetary nebulae or supernova remnants, created during explosions that take place at the end of a star’s life. What made the ORCs unique were their dimensions much larger than those of an entire galaxy.
An ORC observado with MeerKAT.MeerKAT/DES
A dozen of these enormous circles or ORCs are known today. They are only observed thanks to their emissions on radio waves, They are not visible at visible, infrared, or X-ray wavelengths.. In several of them a galaxy is observed in their center, which soon led to the conclusion that each ORC had been created by a large explosion that had taken place within its central galaxy.
Explosions
It was initially speculated that the origin of ORCs could be the collision of a pair of black holes or neutron stars. But the later images, also obtained at centimeter wavelengths, with the large radio telescope Marks in South Africa, opened the possibility that ORCs formed during a burst of star formation in galaxies.
In fact, if at a given moment the massive formation of a large number of stars (millions) occurs, their combined effect can give rise to the ejection of a gigantic wind of galactic dimensions that would drag an enormous layer of gas and dust from the galaxy into outer space.
This would create a thin spherical layer of which we only see the edges (the limbus). for the same reason that when we look at a soap bubble we preferably see the edge: because it is towards the edge where there is more matter in the line of sight.
In some of the known ORCs, it has been possible to estimate their age, that is, the time elapsed from the explosion to the moment in which the ORC is observed, in a few hundred million years.
As this gigantic bubble expands through intergalactic space for so long, it can encounter other surrounding galaxies, interacting with them and producing the complex arcs and structures observed inside some of the ORCs.
shock waves
In a new study carried out by a team led by astronomer Alison Coin, from the University of California in San Diego, the ionized gas around the central galaxy of one of these circles, the so-called ORC-4, has been examined. It is observed in these data that the oxygen emission covers a large region around the central galaxy, although it does not fill the ORC. Astronomers conclude that there was indeed a burst of star formation in this galaxy, about a billion years ago. The most massive stars of this batch must have died, giving rise to a large set of supernovae quasi-simultaneous, which generated a large galactic-scale wind that ejected gas and dust out of the galaxy, creating an expansive shock wave.
Model made for an ORC.Coin et al. (2024, Nature)
In its propagation towards outer space, the shock front drags the magnetic field and, thanks to this, we can detect the bubble, since the charged particles in the medium They emit synchrotron-type radiation which is detected in centimeter-length radio waves.
Simultaneously with the expansion, the hot gas left behind the shock front falls back onto the galaxy while it cools, which causes the emission that we see of ionized oxygen.
In the attached image, taken from the work of Coin and collaborators, we can see the evolution of the bubble in three successive moments. The upper diagonal half of the images shows the temperature of the gas, while the lower half illustrates the expansion and return rates of the gas. The shock front is observed in the form of ORC (speeds in blue), while the gas falling back onto the galaxy (speeds in red) is observed by oxygen ions. It is the result of a numerical simulation that satisfactorily explains the observed data and, therefore, corroborates the hypothesis that ORCs are created thanks to the winds that are ejected in a massive burst of star formation.
Waiting for the SKA
The radio wave emission from ORCs is faint and very spatially extensive. This, together with the fact that they are not abundant, meant that there were no radio telescopes capable of detecting them until ASKAP and MeerKAT were put into operation.
These two large arrays of radio telescopes are, in turn, precursors of the giant radio telescope WILL which is already under construction in both Australia and South Africa. When operational, the SKA will be able to both explore large areas of the sky and study the objects it finds in great detail and sensitivity. Detect a greater number of these puzzling circles It is essential to be able to make meaningful statistics and to be able to definitively conclude about their properties and about the physical mechanisms that create and shape them.
The original article on the discovery of ORCs by Norris et al. (2021) was published in Publications of the Astronomical Society of Australia and can be consulted here. The article by Coin et al. It was published a few days ago in Nature and can be consulted here.
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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