The grand “cosmic web” is a picture that has fascinated us since the beautiful Millennium Simulation was run in 2005, showing us a Universe full of matter often clumped together but intricately bound by thin filaments. For the first time, these thin filaments of gas have been directly seen using one of the largest telescopes on Earth.
Clouds of gas, by their nature, are diffuse and difficult to detect. Often, we can see gas that lies along a line of sight to distant bright object, such as a quasar. However, that only gives insight into one small piece of that cloud.
A quasar is the bright central region of a galaxy that is being powered by a supermassive black hole that is gobbling up surrounding material. A team of astronomers, with lead author Sebastiano Cantalupo, looked towards such a quasar 10 billion light years away and found it illuminating a gas cloud far too large to be part of a galaxy cluster.
This gas resides in the intergalactic medium, the spaces between galaxies. It is two million light years across, a distance equal to roughly the distance between the Milky Way Galaxy and the Andromeda Galaxy. It was detected in the faint glow given off by hydrogen atoms that were excited by the radiation of the nearby quasar using the 10-meter Keck I telescope in Hawaii.
This cosmic web is not only poetic but important to the formation of galaxies and galaxy clusters. According to the Millennium Simulation, these filaments help to funnel gas into the “nodes” of the web, the sites of large galaxies and galaxy clusters. Our Milky Way Galaxy is thought to have formed in a small such node in this web.
Much of the material, over 80 percent, is dark matter, or matter that does not directly interact with electromagnetic radiation. Although we’ve detected its presence indirectly for decades due to its gravitational effects, we are no at the point where dark matter in the cosmos has been thoroughly mapped out. This gas, the “normal” matter that makes up people, stars, and planets, however, can be seen.
Since it is so diffuse, this gas proved to be a challenging target. It was found near a quasar in part because the quasar made it glow. It is also an ideal place for searching for such gas because quasars tend to inhabit the more densely populated nodes of the web, thus are likely to be surrounded by more of these filaments.
But, as always, there was a surprise in store. The mass of the gas cloud is ten times what has been predicted by simulations. A more massive cloud may have been an outlier, just the easiest to spot, but it is likely that the simulations don’t quite model reality exactly. Some factors may be missing that would create gas clouds of this larger size as is seen in these recent observations. This just goes to show that even when you think you know what you are looking for, the Universe is going to surprise you.
This research was published in Nature; and a preprint is available at arXiv.org.