A stream of hydrogen gas has been found connecting two galaxies 2.6 million light years apart. The previous record for this sort of bridge was 1.6 million, in the Virgo Cluster, the closest galactic cluster to ourselves.

In this case the galaxies are 500 million light years away, making faint elements much harder to see than at the 65Mly distance of Virgo. Detection was conducted with the giant William E Gordon Telescope at Arecibo, Puerto Rico.

"This was totally unexpected. We frequently see gas streams in galaxy clusters, where there are lots of galaxies close together, but to find something this long and not in a cluster is unprecedented,” said Dr Rhys Taylor of the Czech Academy of Sciences and lead author of a paper in Monthly Notices of the Royal Astronomical Society.

Given the mind-warping scale of such an object, it is perhaps best to note that our own galaxy is usually considered 100,000 light years in length, although component stars have been found 900,000ly from Earth.

Roberto Rodriguez of the University of Puerto Rico said, "We normally find gas inside galaxies, but here half of the gas -- 15 billion times the mass of the Sun -- is in the bridge. That's far more than in the Milky Way and Andromeda galaxies combined!" Rodriguez was one of three undergraduate students who are authors of the paper.

Although the authors note "H I... is less gravitationally bound to the galaxy and therefore easier to remove" the formation of such a huge bridge poses a puzzle. Taylor’s team are exploring the possibility that either the large galaxy anchoring one end of the stream passed by the group of galaxies at the other drawing gas with it, or that the large galaxy passed through the middle of the group and pushed gas in front of it. The latter might be expected to leave a more disturbed galactic group behind, but would explain the gas extending from the large galaxy away from the group.

Either way the coincidence of the almost equally impressive stream of gas coming from another nearby galaxy towards the group also needs explaining.

The bridge is formed from atomic hydrogen (H I), rather than molecular hydrogen (H2). The paper notes, “The galaxy density is extremely high (15 deg-2) and many (~24%) show signs of H I emission.” Galaxies in densely populated areas are often depleted in H I, presumably because it has been draw out into streams such as this. It is thought that this is also the reason such galaxies show signs of reduced star formation, having lost much of the gas that would otherwise provide fuel for stars, and Taylor’s team hope to use this extreme case as a study example.