A Huge Radio ‘Bridge’ Has Been Discovered Connecting 2 Galaxy Clusters About to Merge

A uncommon merger between clusters of galaxies has simply revealed a fair rarer sight. Astronomers have discovered an enormous, low-frequency radio ‘bridge’ between the 2, spanning a 6.5-million-light-year distance – proof of a magnetic discipline connecting them within the early stage of the merging course of.

It is solely the second time such a radio bridge has been recognized between merging galaxy clusters. However already it is offering some essential clues as to how these bridges type.

The galaxy clusters are round three billion light-years away, in a bunch known as Abell 1758. All up, 4 clusters are concerned within the impending smash-up – two huge cluster pairs coming collectively.

Final yr, X-ray knowledge revealed that the intently sure pair within the north section, known as Abell 1758N, has already moved collectively and separated, the cluster cores passing one another round 300 to 400 million years in the past. They will swing again round ultimately to return again collectively. The pair of clusters within the south, Abell 1758S, remains to be approaching one another for the primary time.

Each of those pairs have a radio halo, regarded as generated by the acceleration of electrons in a merger occasion. And it is these pairs which might be separated by a distance of 6 million light-years, a spot that’s slowly closing, for an eventual four-way cluster… bonk.

This situation is similar to the galaxy clusters Abell 0399 and Abell 0401, which final yr grew to become the primary merging galaxy clusters revealed to have a low-frequency radio bridge connecting them.

Utilizing the low-frequency radio telescope LOFAR, which consists of 25,000 antennas throughout 51 areas, astronomers detected a definite radio emission at 140 megahertz.

Now a staff of astrophysicists led by Andrea Botteon of Leiden Observatory within the Netherlands has turned LOFAR to Abell 1758. At 144 megahertz, they found radio emission stretching between A1758N and A1758S, similar to the radio bridge between Abell 0399 and Abell 0401.

(Botteon et al., MNRAS, 2020)

“We verify,” they wrote of their paper, “the presence of an enormous bridge of radio emission connecting the 2 techniques that was reported solely tentatively in our earlier work. That is the second large-scale radio bridge noticed up to now in a cluster pair. The bridge is clearly seen within the LOFAR picture at 144 MHz and tentatively detected at 53 MHz.”

This emission is interpreted as proof of an enormous magnetic discipline connecting the 2 clusters. If this magnetic discipline acts as a synchrotron (particle accelerator), electrons ought to be accelerated alongside it to relativistic velocities, producing synchrotron radiation detectable as a low-frequency radio glow.

However there’s one other potential rationalization – Fermi acceleration, by which electrons interacting with turbulence and astrophysical shock waves are accelerated, boosting electromagnetic emission.

Within the dense area between two pre-merging clusters, such turbulence and shock waves could possibly be generated within the early levels of a merger. And the staff’s findings counsel this could possibly be very true if the clusters have been already gravitationally disturbed in a roundabout way – for example, if every cluster was a pair of smaller interacting clusters in its personal proper.

Botteon and his staff carry two supporting arguments to this situation. Firstly, a decrease mass pair of merging clusters, solely certainly one of which had a radio halo, confirmed no proof of a radio bridge in LOFAR observations, in keeping with a paper final yr.

Chandra statement (left) and LOFAR (proper). (Botteon et al., MNRAS, 2020)

Secondly, the staff additionally checked out observations of Abell 1758 taken utilizing the Chandra X-ray Observatory. They usually discovered that the X-ray emission very intently correlated with the radio emission at 144 megahertz – in line with predictions of the Fermi acceleration situation.

Within the merger of Abell 0399 and Abell 0401, the researchers discovered that synchrotron acceleration couldn’t alone account for the huge distances lined by the electrons. They ran simulations, and located that shock waves generated by the merger re-accelerated high-speed electrons, leading to an emission in line with the LOFAR observations.

So it appears possible that there are a number of sorts of acceleration at play – that a magnetic discipline can stretch hundreds of thousands of light-years throughout house between galaxy clusters, however shock waves and turbulence add that particular one thing that completes the bridge.

“Solely two big intra-cluster radio bridges have been detected up to now,” Botteon and his colleagues wrote.

“These are among the many most big buildings noticed within the Universe to this point, and their origin is  possible associated to the turbulence (and shocks) generated within the intra-cluster medium through the preliminary stage of the merger, which increase each the radio and X-ray emission between the clusters.”

There are fairly a couple of merging galaxy clusters which were recognized on the market within the wider Universe. Looking out them for extra of those mysterious radio bridges may assist determine what generates these huge buildings.

The analysis has been printed within the Month-to-month Notices of the Royal Astronomical Society.