Cosmic web determines the fate of galaxies

All the galaxies in the universe are trapped in a gigantic spider web, with threads of gas, among other things, and with nodes where those threads intersect. The closer a galaxy is to a node of that spider web, the more variation in chemical elements a galaxy has. That’s what a team of British and French researchers concluded using the Sloan Digital Sky Survey, a map of all the bright galaxies over about a quarter of the sky. The researchers published the results this week in the magazine Nature Astronomy

Like hundreds of billions of other planets, Earth orbits a parent star. The sun (Earth’s parent star) again, along with hundreds of billions of other stars, orbits the center of the Milky Way, an egg-shaped galaxy about 100,000 light-years in diameter. The Milky Way, like most other galaxies, is part of a galaxy cluster. Some of those clusters are back together in a supercluster.


And now it gets really dizzying: multiple superclusters and separate galaxies together form a gigantic, spider web-like structure. That so-called cosmic web is as big as the universe itself and almost all matter in the universe is contracted into it. The threads of the web, called “filaments,” are made up of rarefied hydrogen gas and dark matter. Around those filaments are galaxies. The clusters are located where the wires intersect. The spider web itself is invisible, by the way. Luminous galaxies nestled in the web reveal its structure.

The cosmic web affects galaxies. At the beginning of this century there were several indications that the direction of rotation of a galaxy is influenced by the filament where the galaxy is located. Astronomers saw that small galaxies rotate roughly around the axis of a filament. Massive systems rotate perpendicular to it.

Now British and French researchers believe they have seen an effect of the cosmic web. They studied the composition of 6,984 galaxies from the Sloan Digital Sky Survey. What was striking was that the closer a galaxy is to a node, the more variation in chemical elements the galaxy contains. Galaxies far from a node have mostly light elements helium and hydrogen. Closer galaxies also have heavier elements such as carbon.

To explain how this is possible, the astronomers mimicked the observations with one of the most detailed cosmos models, the IllustrisTNG. The simulation showed that the difference is partly because galaxies closer to nodes make stars faster. Due to the high density around nodes, galaxies have a lot of matter around them from which they can form new stars. And that in turn leads to the formation of new types of elements.

nuclear fusion

Within a few million years, the first generation of stars in a galaxy convert lighter elements into heavier elements in their interior by means of nuclear fusion reactions. Some stars explode as soon as they die, spreading heavy elements throughout the galaxy. Then new stars can form from it. They then make even heavier, new elements from the heavier elements. As a result, a galaxy that has been home to many different generations of stars will generally have more distinct and heavier chemical elements.

The researchers saw the same effect in the simulations for the distance between galaxies and filaments, but to a lesser extent.

“Fascinating work,” says Rien van de Weijgaert. He is a cosmologist at the University of Groningen. “Although I am not yet convinced of the model that the researchers use to explain the observations. IllustrisTNG does not take into account the distance between a galaxy and the edge of a filament, but only calculates the distance of a galaxy from its axis. Anyway, the observations themselves show that the influence of the cosmic web on galaxies goes further than we thought. That is.”

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