Filaments that bind galaxies together illuminated by a quasar

Virgo consortium: the web that holds the galaxies together. Rich Murray

According to cosmologists, galaxies are joined together by filaments, quite literally. These filaments form the cosmic web and are made of mostly dark matter, many stars and some gas. Observing these filaments is difficult, which is the case with everything cosmologists attach the word “dark” to.

But researchers from the University of California Santa Cruz and the Max Planck Institute have found a way to study part of this cosmic web. They identified a quasar – a celestial body that spews intense radiation out from a black hole – that is “bright” enough to allow part of the filament to be observed. Their results have left some physicists baffled.

The part of the filament they have identified is a giant gas cloud, around two million light years across. The gas cloud consists of ten times the amount of cold matter (that does not give off light) as is predicted by standard theories about the cosmic web. Nobody knows why.

Entangled web

Scientists got a hint about the cosmic web from studying the cosmic microwave background (CMB), also known as the oldest light. CMB is the remnant light imprinted on the universe from about 380,000 years after the Big Bang. It shows that some parts of the universe are denser than others. This is because dark matter is unevenly distributed, which tends to attract more ordinary matter towards it.

To understand this tangled web that dark matter creates, cosmologists use computer simulations. Invisible in itself, dark matter still exerts gravitational forces on visible light and ordinary matter nearby. Only 5% or so of the universe is made of the ordinary matter that we can see. This forms the stars and giant gas clouds. But even then, only the high-energy, glowing ordinary matter can generally be detected, cold gas clouds remain hard to detect.

Dark matter has been detected, but only indirectly from the way its gravity bends light from distant galaxies, a process called gravitational lensing. Filaments of dark matter have been inferred in reports coming out over the last couple of years using such methods.

The nebula (cyan), 2 million light-years across was discovered around the quasar UM287 (at the centre of the image). The energetic radiation of the quasar makes the surrounding intergalactic gas glow revealing the morphology and physical properties of a “Cosmic Web” filament. Image obtained at the W.M.Keck Observatory.

In the new study, recently published in Nature, researcher have made the first observations of the cold gas decorating cosmic web filaments using the Keck telescope in Hawaii. The glowing hydrogen cloud, also called a nebula, sits ten billion light years away and is illuminated by the neighbouring quasar in these new observations. The gas traces out an underlying filament of dark matter that is attracted to it by gravity, according to the researchers’ analysis.

Sebastiano Cantalupo, lead researcher of the study, and others have used the same methods previously to look for glowing gas around quasars, and had seen dark galaxies.

“The dark galaxies are much denser and smaller parts of the cosmic web. In this new image, we also see dark galaxies, in addition to the much more diffused and extended nebula,” Cantalupo said. “Some of this gas will fall into galaxies, but most of it will remain diffused and never form stars.”

“The light from the quasar is like a flashlight beam, and in this case we were lucky that the flashlight is pointing toward the nebula and making the gas glow. We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission from the quasar,” he added.

While the observations support the cosmological simulations’ general picture of a cosmic web of filamentary structures, the researchers’ results suggest around ten times more gas in the nebula than predicted from typical simulations models. They postulate that this may simply be due to limitations in the spatial resolution of the current models, or, more interestingly perhaps, may be because the current grid-based models are missing some aspect of the underlying physics of how galaxies form, evolve, and interact with quasars.

Whatever the cause, the remarkable illumination of a cold gas cloud by this distant quasar opens up a new method for search for cold ordinary matter, as well as dark matter itself. It may simply be that there is more ordinary matter distributed in cold gas clouds than previously acknowledged, but until a bigger picture emerges it will be difficult to say more from this first observation.