A Bridge Quite Far

Specifically, a bridge between two galaxy clusters that spans a whopping 2 Mpc. They'd seen some earlier hints of this in previous data, but now have higher sensitivity from multiple telescopes to try and confirm it. And it's rock solid at 144 MHz and (in my opinion - they're more cautious) 53 MHz, but tentative at best at 383 MHz and not visible at all at 1.5 GHz.

Such bridges are rare in the extreme - this is one of only two such features known, making them hard to understand. You can't, for example, say what types of system most commonly host them, so it's difficult to comment on their origins. Even the emission mechanism is uncertain, since it's only clearly detected at one frequency. They say :

According to Brunetti & Vazza (2020), radio bridges may originate from second-order Fermi acceleration of electrons interacting with turbulent motions triggered by the complex dynamics in the overdense region between premerging clusters.

Basically it's from very low energy, low density, ionised gas. Interestingly there's a correlation with the much hotter X-ray emission, pointing to a connection between thermal and non-thermal sources, but that's about all they can say from this.

The larger question of whether this gas is primordial, i.e. we're seeing the cosmic web itself, or non-primordial gas from interactions, isn't addressed directly. And fair enough really - when you've discovered a bridge of gas 2 Mpc in length, you're absolutely entitled to write a short letter than says little more besides, "we found a bridge of gas 2 Mpc in length". Interestingly though, both cases of giant radio bridges (or in one slightly amusing typo, radio brides) are found in massive, pre-merging clusters that themselves consist of interacting sub-structures. So I guess both options are open, though presumably the sheer length of the thing would more naturally suggest a primordial structure. But until more of these are found, there's little else to be said apart from, "hey, that's neat !".

A giant radio bridge connecting two clusters in Abell 1758

Collisions between galaxy clusters dissipate enormous amounts of energy in the intra-cluster medium (ICM) through turbulence and shocks. In the process, Mpc-scale diffuse synchrotron emission in form of radio halos and relics can form. However, little is known about the very early phase of the collision.