Our universe is connected by a cosmic web made of giant threads of dark matter and gas that stretch across millions of light years and intersect at “nodes” populated by dense clusters of galaxies. This vast network shapes the distribution and evolution of galaxies in fundamental ways that scientists are trying to unravel with ever-sharper observations and advanced simulations.
Now, a team led by Callum Donnan, a postgraduate student in astronomy at the University of Edinburgh, have identified a key correlation between the chemical makeup of galaxies and their location within the cosmic web. Using both real-life observations and computer simulations, the team found that “galaxies closer to nodes [display] higher chemical enrichment than those farther away,” a discovery that reveals some of the mysterious dynamics that link the universe, according to a study published on Monday in Nature Astronomy.
“It’s been postulated for a while that there is a link between how galaxies evolve and their position in the cosmic web,” said Donnan in an email. “Getting observational evidence however has been difficult due to the need for large, dense spectroscopic surveys covering much of the sky. Results from this have come recently but how the gas properties link to the cosmic web hadn’t been explored in much detail before.”
To home in on this question, Donnan and his colleagues examined galaxies within about a billion light years of the Milky Way observed by the Sloan Digital Sky Survey in New Mexico, which covers a huge area of the sky. The team studied the elemental makeup of gasses in the interstellar spaces within these real-life galaxies, a property that is known as gas-phase metallicity.
The results revealed that galaxies close to the nodes of the cosmic web were richer in “metals,” which in astronomy refers to any element heavier than helium. A weaker correlation was also observed with proximity to the web’s filaments, which are the threads that stretch across the universe and link nodes together. The team ran sophisticated cosmological simulations using the IllustrisTNG platform, which supported the observational findings.
Significantly, the approach revealed that a galaxy’s position in the cosmic web modulates its chemical content even when other factors, such as the density of a particular region in the universe, are taken into account.
“We suspected there would be a relationship as galaxies are not isolated systems and interact with their environment,” Donnan noted. “However, we were not sure exactly what to expect as there are numerous physical processes at play here. There has been some evidence in the past that galaxies in overdense regions of the universe are chemically enriched but nothing looking at the full scale of the cosmic web.”
Naturally, that raises the question of why galaxies located near nodes are enriched with more metals compared to those distributed along filaments or in empty “voids” within the cosmic web. Donnan’s team isolated two major drivers of this relationship: The absorption of gas from outside of galaxies and the evolution of stars and dark matter inside of them.
Galaxies feed on gasses that are strewn across space in the intergalactic medium, but those that are further from nodes consume much more of this outside material than those close to nodes. Since intergalactic gas is metal-poor, it dilutes the enriched gas of far-flung galaxies, lowering their overall gas-phase metallicities. Galaxies near nodes don’t consume as much of this metal-poor material, which helps to keep them chemically enriched with higher concentrations of heavier elements.
In addition, galaxies close to nodes seem to have matured earlier than those located at a distance. These galaxies had a head-start in birthing new stars and collecting dark matter, which is a mysterious substance that makes up most of the matter in the universe.
“We think that galaxies close to nodes had more active star formation in their past and other results show that galaxies close to nodes assembled their dark matter earlier,” Donnan said. “We suggest that this shows a link between the underlying assembly of the large dark matter structure in the universe, and the gas metallicity through increased early star formation.”
Teasing out these nuanced connections between the cosmic web and the evolution of galaxies is a difficult task, given the scale and complexities of these astronomical interactions. Donnan and his colleagues said their findings represent “an important first step towards that goal” in the study, but they also emphasize that new technologies will refine these mysteries in the future. In particular, the Dark Energy Spectroscopic Instrument (DESI), due for completion in the mid-2020s, will help to expose some of the hidden links between this epic cosmic structure and the galaxies within and around it.
“With the Dark Energy Spectroscopic Instrument (DESI) we will have spectra for an order of magnitude more galaxies and this will allow us to push this question forward and start to really disentangle the ways in which the cosmic web influences galaxy evolution,” Donnan said. “DESI will also allow us to see this effect further back in time and therefore we can see how the role of the cosmic web in galaxy evolution changes over time.”
“The big picture here is to try and generate a complete picture of galaxy evolution and we have shown that in order to do this, we need to consider the role of the cosmic web,” he concluded. “There is a lot of uncertainty particularly on the complex gas physics of galaxies and we have shown the cosmic web plays a role in this. Also, trying to connect how the large-scale structure of the universe grows, with how galaxies evolve, is important to understanding the evolution of the universe as a whole as it can help us better understand cosmology. This helps create a bridge between physics on the largest scales and on smaller, galactic scales.”