New Observations Reveal How Giant Structures In Space Connect the Universe and Form Galaxies

Our universe is connected by the cosmic web, a vast network of filaments that spans billions of light years and is made of gas and dark matter, a mysterious substance that has so far eluded explanation.

Now, scientists have spotted galaxies that are aligned in never-before-seen patterns along these filaments, a discovery that sheds light on the evolution of galaxies within the large-scale architecture of the cosmos, reports a new study. The research adds to a growing body of evidence that is exposing the influence of the cosmic web over the evolution of galaxies across space and time.

Researchers led by Stefania Barsanti, an astronomer from the Australian National University, studied hundreds of galaxies captured by the SAMI Galaxy Survey based at Siding Spring Observatory in Australia. The team discovered that the mass of a galaxy’s central bulge correlated with its orientation within the cosmic web, revealing “a memory of the galaxy’s formation” that includes the “halo” structures from which galaxies arise, according to a recent study published in the Monthly Notices of the Royal Astronomical Society.

Past studies have shown that a galaxy’s location in the cosmic web has implications for its chemical content, and that galaxies can be used to track the spinning of cosmic filaments, among many other mind-boggling discoveries. Barsanti and her colleagues aimed to follow up on a 2020 study led by Charlotte Welker, a postdoctoral fellow at Johns Hopkins University who also co-authored the new study, that reported the first observational detection a relationship between the stellar mass of these galaxies, and a specific way they are oriented in the cosmic web.

“We decided to dig more into this story in a consistent way in order to understand what was really going on,” Barsanti told Motherboard in an email. “We investigated a complete set of different galaxy properties, including bulge and disc properties, and we applied a 3D approach to measure galaxy spin-filament alignments.”

Simulations suggest that galaxies with more massive bulges tend to spin on axes that are perpendicular to the filament they are embedded in, whereas galaxies with smaller bulges spin parallel to the web, but nobody had ever spotted the trend out there in actual outer space.

Barsanti and her colleagues were surprised to find that their observations lined up with the simulations by showing that big-bulged galaxies tended to spin on a perpendicular axis relative to the cosmic filament, while galaxies with less massive bulges spin parallel to the web. 

“We actually thought that stellar mass would be the stronger galaxy parameter to unravel spin-filament alignments, confirming the previous study led by Welker,” Barsanti said. “Simulations find a close link to stellar mass, but some studies still show that stellar mass alone was not able to completely explain spin-filament alignments. So, we thought there must be something else going on here.”

This pattern can be attributed to the distinct ways that galaxies can form. The low-mass galaxies mainly coalesce from gas that flows from the filament, so they take on the same alignment as the larger cosmic structure. The high-mass galaxies, in contrast, are likely the product of collisions between galaxies that become flipped into a perpendicular orientation during the process of merging together. 

“How galaxies acquire their angular momentum in the cosmic web is a crucial element in understanding galaxy formation and evolution,” the team noted in the study. “Since galaxies are not randomly distributed in the Universe but found along ordered filaments and walls, their properties are expected to be influenced by their host halos, and by the current location and past history of these halos in the evolving cosmic web.”

“We find an observational link between galaxy spin–filament alignments and the growth of the bulge,” the researchers added. “This link can be explained by mergers, which can cause the flipping and the bulge assembling, as seen in galaxy formation simulations.”

The study presents yet another tantalizing glimpse of the connection between the cosmic web and the galaxies that are entangled with it. These kinds of details can be very difficult to observationally detect, but integral field spectroscopy (IFS) projects, such as the SAMI Galaxy Survey, are increasingly bringing them into view. 

For this reason, Barsanti’s team looks forward to the next generation of IFS efforts, such as the Hector Galaxy Survey, which will be able to observe tens of thousands of galaxies. These advances will help to answer unresolved questions about the relationship between galaxies and the cosmic web, including the role that black holes play in these dynamics.

“I would like to transmit that galaxies acquire their angular momentum from the cosmic web, and that they are able to maintain memory of their formation, such as these alignments with respect to the filaments,” Barsanti said. “These alignments can then show insights on the physical processes that not only form the galaxy, but also its structural components such as bulges and discs.”

“Hector will be crucial to understand if the alignments are affected by the different galaxy environments, investigating filaments in galaxy clusters, galaxy groups and regions with low galaxy density,” she concluded.

Update: This article has been updated to include comments from lead author Stefania Barsanti.