What is this weird long radio structure in between two clusters of galaxies?
It's a....bridge! 🌉
In particular, it is the radio bridge connecting two of the most massive clusters in the nearby Universe: Abell 399 and Abell 401.
Huge discovery in 2019, by F.Govoni and collaborators, using LOFAR: https://arxiv.org/abs/1906.07584 #Astrodon#Astronomy
Let me use this new #AstrophysicsFactlet to tell you a bit about this discovery, and why we care so much!
The emission was the first discovered of this kind: it is the largest unique radio structure in the sky so far discovered (nearly 3.5 Megaparsecs from side to side!) and it is a surprise, because it appears between two clusters of galaxies which still have to undergo a merger.
It was discovered using the LOFAR telescope, High Band Antenna (HBA), at about 200 MHz, in a deep observation by F. Govoni (INAF) and collaborator (including your truly) in 2019.
This target was very interesting: it is the cluster association producing the highest signal-to-noise in the PLANCK maps of the Sunyaev Zeldovich effect from clusters of galaxies ( https://en.wikipedia.org/wiki/Sunyaev%E2%80%93Zeldovich_effect ) which is a strong evidence for the presence of hot and "dense" plasma in between the two systems, who were also previously known for their X-ray emission.
This is the Sunyaev-Zeldovich map, with the Moon to show their wide angular scale in the sky.
This is the overlay of the newly discovered emission by LOFAR-HBA, on top of the optical and SZ maps of the cluster.
See that huge bluish area between the cluster, 3,5 Megaparsec long? that's the radio bridge.
While radio emission was already observed in nearly fifty clusters which underwent collisions between them, in this case the big surprise was the strong detection of emission between the two cluster - who have not merged yet.
Until then, it was widely accepted that the kinetic energy developed and dissipated during merger events between clusters (yielding a ~1e44 erg/s kinetic power on Mpc scales) was used to power Fermi II re-acceleration and produce radio emission.
But explaining the acceleration of radio emitting electrons to ~GeV energies, in the absence of a merger, was/is a big theoretical challenge!
What can power them, if it is not the energy taken from cluster-cluster collision?
For theorists will to explain radio emission on such large scales, it became a very nice testbed of theories and models.
First, I tested if the shocks developed along the merger axis would be enough to explain the emission.
Barely, i.e. only an unexpected multitude of weak (Mach ~2-3) shocks might have formed there, but they could have produce the emission only in case they were running over a pool of already energetic electrons.
The geometry made this quite implausible, even if not crazy.
A few months later, with my collaborator and former PhD advisor G. Brunetti, we came up with hopefully a viable solution: Gianfranco devised a Fermi II scenario which can become very efficient if the velocity field of the gas is mostly solenoidal, and magnetic field lines are likely to undergo "reconnection".
A cosmological simulation of A399-401showed that both conditions should be met in the case of the real A399-A401 pair.
The paper is complex, and it mostly describes this electron re-acceleration model (originally developed by Brunettti & Lazarian 2016 https://arxiv.org/abs/1603.00458 and also applied to pulsars wind nebulae!) in which magnetic turbulent re-connection regions channel a fraction of their energy into the acceleration of already mildly relativistic electrons.
the model was so appealing because to reproduce the observed radio spectra, it only needs a 0.001% of the kinetic energy involved in the merger.
Having found a nice binary system in a a large set of my cosmological simulations, which well reproduced the thermal and geometrical properties of the real pair, we could also test if this mechanism could produce the desired time and energy properties which we could desume for the real population of bridges.
This a movie showing how we imagine (based on the Fermi II model!) the formation of the radio bridge to have happened - while at the same time keeping the Xray emission between the two clusters basically invisible, as in reality.
So, this idea of turbulent re-acceleration seems to be able to do the proper job in a number of ways: it requires little energy, it naturally produces wide and intermittent radio structures, with the right spectrum and power.
..not really!
Only a pairs of such bridges have been discovered since 2019. This suggests that they are rare- about 1 every 500Mpc^3 in the local Universe.
They should also have a "steep" spectrum, meaning that only low frequency observations (LOFAR or MWA, or SKA-LOW in the future) should be able to detect them.
So the real testing of our (or other) models need to await for a larger collection of observations!
But with some patience, they will surely keep appearing in the radio sky.
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