Galaxies are locked in place by their surroundings

Galaxies can be used as tracers for numerous characteristics of the universe, including the “cosmic web” of hydrogen gas that connects galaxies and follows the distribution of dark matter filaments throughout the cosmos. Looking at where galaxies sit sheds light on these otherwise invisible structures, while also providing clues about the amount of mass in galaxies and how they influence their neighbors over time. Now, astronomers have traced the alignment of massive galaxies back 10 billion years, showing these objects have been in tune with their environment since the universe was just one-third its current age.

The work was led by Lowell Observatory astronomer Michael West, who together with his collaborators used Hubble Space Telescope images of 65 galaxy clusters located billions of light-years away to study the orientation of the massive elliptical galaxies in the centers of these clusters. What they found suggests that the biggest, brightest galaxies in galaxy clusters have been heavily influenced by their unique environment since very early times. The study appears in Nature Astronomy online today.

Galaxy clusters present a very different environment from “the field,” which is an astronomer’s term for the majority of the sky, which shows no preferential structure or clustering of galaxies. Inside galaxy clusters, individual galaxies are subjected to intense gravity, a hot intracluster medium of gas, and many more “flybys” between neighboring galaxies than could ever occur in the less dense field. And while galaxies in the field tend to be oriented any which way, galaxies in clusters are different. The massive galaxies at the centers of clusters show preferential alignment with their neighbors, and astronomers are still looking to find out why.

One reason for this alignment could be that over time, gravity simply tends to orient large galaxies in the same direction as their neighbors. Alternatively, because large galaxies grow by absorbing smaller galaxies, these smaller galaxies could impart orientation on the galaxies that eat them due to the progenitors’ preferred orientation along the cosmic web.

The results of West’s study don’t rule out either scenario, but they do help constrain the alignment by showing that it occurs very early on in galaxy evolution. “It’s an important new piece of the puzzle,” said West in a press release, “because it says that whatever caused these alignments happened early.”

What’s next? West’s group plans to push the envelope further by trying to observe even more distant galaxies. Despite the precision achievable with Hubble, however, this will be challenging, as even massive galaxies appear fainter and smaller as the distance between Earth and these clusters grows. But such observations at the earliest epochs may help astronomers finally determine the reason for this preferred orientation, helping to complete our picture of early galaxy evolution.