Two galaxies found to be linked by an 'astonishingly' massive gas bridge, the longest such structure ever seen

Two small, misshapen galaxies are wandering together on the outskirts of the Virgo Cluster, a huge city of galaxies. Astronomers have been watching this galaxy pair, NGC 4532 and DDO 137, for years because their gas looks messy and stretched.

Fresh radio maps now show why. Using a survey that traces invisible, cold hydrogen gas, researchers mapped a massive bridge between the two galaxies and a far longer tail streaming away into space.

The simple question – what built that structure? – turns out to have a layered answer.

Hidden galaxy bridge, huge tail

The new observations reveal a bridge of gas linking the two galaxies, plus extra arms and clouds feeding into a tail that extends 185,000 light-years long. That makes it the longest such structure ever seen.

Even at 53 million light-years away from Earth, it’s obvious that the tail is the main feature, stretching across intergalactic space and pointing the pair toward their future inside the cluster.

The study appears in the Monthly Notices of the Royal Astronomical Society and was led by Professor Lister Staveley-Smith of ICRAR at the University of Western Australia.

“Our modelling showed that the tidal forces acting between these galaxies, alongside their proximity to the massive Virgo cluster of galaxies, played a crucial role in the gas dynamics we observed,” Professor Staveley-Smith said.

“As the galaxies rotated around each other and moved toward the hot gas cloud surrounding the Virgo cluster, which was 200 times hotter than the Sun’s surface, they experienced what is known as ram pressure, which stripped and heated the gas from the galaxies.”

How the gas bridge is made

Two engines are at work. Tidal forces from the galaxies pulling on each other can tear gas from their edges and stretch it into a shared bridge and streamers.

At the same time, as the pair plunges toward the Virgo Cluster, the thin but hot cluster atmosphere pushes back.

That push – ram pressure – does not need to be fierce to matter over long timescales. Over about a billion years, it can peel away gas that tides have already loosened.

The team estimates the pair’s inward speed at roughly 880 km/s. Combine that with the local density of the cluster’s hot gas and you get a steady headwind strong enough to carry off material that no longer sits snug inside the galaxies’ gravity wells.

Reading the radio maps

The radio data show the gas disks still rotating, but not cleanly. NGC 4532 spins faster, with gas reaching about 93 km/s. DDO 137 turns more slowly, around 38 km/s.

Their overall recession speeds differ by only tens of km/s, so they move together as a bound pair rather than unrelated galaxies.

The bridge has its own distinct speed, slightly offset from either galaxy and leaning toward NGC 4532’s value.

That smooth change across the bridge is exactly what you expect when gravity has pulled gas out from both sides and stitched it into one structure.

“The process is akin to atmospheric burn-up when a satellite re-enters the Earth’s upper atmosphere, but has extended over a period of a billion years,” Professor Staveley-Smith explained.

“The density of electrons and the speed at which galaxies are falling into the hot gas cloud are enough to explain why so much gas has been pulled away from the galaxies and into the bridge and surrounding areas.”

Magellanic Clouds have a galaxy bridge

There is a local analog of this system. The Large and Small Magellanic Clouds near the Milky Way show a gas bridge and a long trailing stream.

Astronomers point to the same recipe: tides between the two dwarfs plus ram pressure from the Milky Way’s hot halo.

NGC 4532 and DDO 137 look like a larger, faster version of that playbook, now set against the broader backdrop of a galaxy cluster.

Another detail stands out: the gas arms and bridge have little to no starlight. That means most of the stripped material has not formed many stars, at least not yet.

Near the end of the tail sits a small blue galaxy (Tololo 1232+052). It may be forming stars from gas in the tail – an example of how stripped material can seed new systems.

Why this galaxy bridge matters

Environment shapes galaxies. Pairs and small groups supply fuel and neighbors; clusters set the final look by moving gas around or taking it away. Gas is the raw material for making stars. Remove it, and galaxies fade with time.

This work shows that galaxies do not have to sit in a cluster’s crowded core to feel its influence. Even out on the rim, the one-two punch of tides plus a persistent cluster wind can rearrange where gas lives and where future stars will form.

Computer simulations back that up: tides can throw out arms over many billions of years, but without the cluster’s gravity and wind, you do not get the enormous tail seen here.

In that type of circumstance, the two dwarf galaxies would likely merge. With the cluster environment included, the picture comes together over long timescales.

To sum it all up, NGC 4532 and DDO 137 are not just traveling companions. Their mutual pull stretches gas into a bridge and arms, and the Virgo Cluster’s hot atmosphere sweeps that loosened gas into a million-plus-light-year tail.

It is a clear test case of how galaxies change as they fall into bigger structures, captured in radio maps with enough detail to watch the transformation unfold.

The full study was published in the journal Monthly Notices of the Royal Astronomical Society.

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