The remnants of four different galaxy clusters comprise the large cluster MACSJ0717. The galaxy cluster labelled “C” is thought to be the “original” cluster, while the motions (arrows) of clusters B, D, and A are all thought to have been funnelled into the cluster from an attached filament (lower left). (The cluster labelled “A” is thought to have moved through the larger cluster once and is now falling back towards its centre.) (Image: X-ray (NASA/CXC/IfA/C Ma et al.); Optical (NASA/STScI/IfA/C Ma et al.))

The first tantalising signs of gas within a filament of dark matter have been glimpsed at the site of a cataclysmic collision between galaxy clusters. If future observations confirm the preliminary detection, it would provide an important test of computer simulations that show how large-scale cosmic structures form.

The simulations suggest that matter is distributed in a cosmic web, with material flowing along filamentary structures and pooling where the filaments intersect. Dark matter is thought to act as the scaffolding for this web, and researchers say as much as 40% of all dark matter in the universe may lie in the filaments.

But although observations have found that galaxies and galaxy clusters do indeed lie in filaments, so far no gas or dark matter have been confirmed in the highway-like structures.


“It’s only logical that the gas and the dark matter trace the same structure, but quantitatively, we do not know whether the simulations and the observations match,” says Harald Ebeling of the University of Hawaii.

Observing the gas in a filament could help gauge how much dark matter lies in the filament and thereby test theoretical models, he says, but “the gas in a filament has never been unambiguously detected”.

Now, he and a team led by Cheng-Jiun Ma say they see hints of gas in a filament that appears to be funnelling galaxies into a galaxy cluster already crammed with them.

‘800-pound gorilla’

In 2004, Ebeling and colleagues discovered that a filamentary structure of galaxies seemed to be connected to a gigantic cluster of galaxies discovered several years earlier. The cluster, called MACSJ0717.5+3745, lies about 5.4 billion light years from Earth and has quite an odd structure – it’s far from a simple, round ball of galaxies.

“This is the proverbial 800-pound gorilla in space,” Ebeling told New Scientist. “It’s one of the most massive known clusters anywhere, and certainly one of the most messed up I know.”

That kind of disarray suggested the cluster had been involved in a merger of some kind, but its history was difficult to tease apart.

3D motions

Now, the researchers believe they have decoded the cluster’s history, by combining data at a variety of wavelengths to reveal the 3D motions of regions within the cluster.

Spectral observations can reveal the motions of objects towards and away from Earth, since that kind of motion compresses or stretches the wavelengths of the objects’ light in a detectable way.

Measuring side-to-side motion can be more difficult. To do this, the team looked for offsets between bright X-ray regions – which represent superheated gas – and regions bright in optical light, which represent stars and galaxies.

Such offsets can occur when galaxy clusters collide – their stars tend to pass through the smashups unimpeded, but the hot gas in the clusters lags behind (see Dark matter and normal matter divorce in cosmic clash).

Four clusters

The observations revealed what appear to be the remnants of four galaxy clusters within MACSJ0717.5+3745 – one large one that may have been the ‘original’ member of the cluster and three others whose motions suggests they were funnelled into the system from the connected filament.

Intriguingly, the researchers identified a very wide region of extremely hot, dense gas where the filament hits the cluster. “We speculate that continuous heating from the filament – with gas flowing in all the time – causes this,” Ebeling says.

‘Holy grail’

The team has submitted a proposal to observe the cluster again – for a longer period – with the Chandra X-ray Observatory.

They hope to measure the temperature of X-rays at a higher spatial resolution to see “whether there is indeed a step [in temperature] at the location where we expect there to be” if gas from the filament is flowing into the cluster, Ebeling says.

They also hope to see gas within the filament that lies farther away from where the filament connects with the cluster. “Right now, we’re only just barely seeing [filament gas] where it hits the cluster,” he says.

“But as you move away from the cluster, the gas is still unperturbed,” Ebeling says. “This is the pristine, original state of gas in filaments – the holy grail for observational X-ray searches.”

Difficult analysis

The filament gas is less dense than gas within the cluster, and much cooler – gas within the cluster can exceed 200 million °C, while gas within the filament may be 10% as hot.

Still, the observation may be challenging to interpret because groups of galaxies are likely embedded within this cooler, low-density filament gas, says Ebeling. “But if we can ever do this, this is the system to do it with – it’s the biggest and best understood,” he says.

Ralph Kraft of the Harvard-Smithsonian Center for Astrophysics agrees with the team that their evidence that gas is falling from the filament into the cluster is only “suggestive”.

But he says it is “potentially the most interesting aspect” of the work: “There aren’t a whole lot of observational constraints” on theoretical models that show how filaments feed the formation of large-scale cosmic structures, he told New Scientist.

Journal reference: Astrophysical Journal Letters (vol 693, L56)