New research suggests galaxies undergo a dramatic change when they enter a cluster as part of a group instead of by themselves. The study's sophisticated simulation indicates galaxies can lose more than one-third of their mass due to a gravitational phenomenon called tidal stripping.

The study, led by Gandhali Joshi from McMaster University, is being presented at the annual general meeting of the Canadian Astronomical Society. It focuses on the different paths galaxies take to become members of a cluster. Clusters of galaxies have hundreds and sometimes thousands of members, and how they get together tells us about their properties.

Joshi investigated two possible ways for galaxies to join clusters: They can either enter them individually or they can join groups first, where up to 30 galaxies become bound together. However, according to the computer model, joining a group is not always ideal as galaxies can lose up to 40 percent of their mass, which is then taken in by the rest of the cluster.

“If we can definitively say how much mass loss is happening, that helps us ultimately understand the physical processes that affect galaxies,” Joshi said in a statement. “It all adds up to a picture of how galaxies evolve, and what happens to them in these dense environments.”

The research suggests that as galaxies enter a group, they start to interact and the mutual gravitational forces pull masses from the individual galaxies. This tidal stripping is common when galaxies collide in mergers, although not every galaxy in a group or cluster ends up colliding with another.

But mergers due happen. The central galaxy in a cluster is usually massive and elliptical, and has fed off several other members. Even in groups, galaxies tend to be more evolved than the quiet spiral galaxies you find in more isolated regions.

The mass loss in the simulated scenario is just dark matter, the mysterious and invisible substance that is believed to surround and make up galaxies. Dark matter is around five-sixths of the mass of the average galaxy, which is why the simulation focused on that.

Joshi used the largest supercomputer center in Canada, called SciNet, to run her model. Yet even with the power of the supercomputer, producing a model with gas, stars, planets, and dark matter becomes very complex.

To follow-up, Joshi currently plans to study what happens to the gas, dust, and stars when a galaxy enters a group.