Triangulum II: A 'Dead' Galaxy Located At The Edge Of Milky Way May Provide Clues To Dark Matter's Composition

dark matter stuff — Dwarf galaxies have few stars but lots of dark matter. This Caltech FIRE (Feedback in Realistic Environments) simulation from shows the predicted distribution of stars (left) and dark matter (right) around a galaxy like the Milky Way. The red circle shows a dwarf galaxy like Triangulum II. Although it has a lot of dark matter, it has very few stars. Dark matter-dominated galaxies like Triangulum II are excellent prospects for detecting the gamma-ray signal from dark matter self-annihilation. A. Wetzel and P. Hopkins, Caltech

In astronomy, seeing isn’t always believing. Take Triangulum II, for instance. It is a faint dwarf galaxy, made up of only about 1,000 stars, located at the edge of the Milky Way, and seems unremarkable in all aspects until you measure its mass.

When scientists at the California Institute of Technology (Caltech) in Pasadena did that, they unearthed a surprising finding. They found that Triangulum II was an amazingly dense galaxy apparently filled to the brim with the elusive and much sought after dark matter.

“The total mass I measured was much, much greater than the mass of the total number of stars --implying that there's a ton of densely packed dark matter contributing to the total mass,” astronomer Evan Kirby from Caltech, said, in a statement released Wednesday. “The ratio of dark matter to luminous matter is the highest of any galaxy we know. After I had made my measurements, I was just thinking -- wow.”

Dark matter, as the name suggests, is dark, except when it emits high-frequency gamma rays during collisions with other dark matter particles. As of now, very little is known about this exotic matter that makes up 85 percent of all matter in the universe.

While scientists have long been hunting for clues to its composition, the fact that it does not emit or absorb any radiation and interacts with visible matter only through gravity makes it next to impossible to detect and study.

This is where Triangulum II comes in. While current theories suggest that dark matter is constantly emitting gamma rays almost everywhere in the universe, isolating these signals from other galactic noises -- such as gamma rays emitted from pulsars -- is an insurmountable challenge. Triangulum II, on the other hand, is a “dead” galaxy, lacking any star forming regions. So, in theory, any gamma ray bursts coming from colliding dark matter particles -- the Weakly Interacting Massive particles (WIMPs) -- should be clearly visible.

However, other measurements taken by a team of researchers from the University of Strasbourg in France appear to show that stars just outside of Triangulum II are moving faster than the ones close to the galactic center -- which were used by Kirby to determine the galaxy’s mass. If true, this finding would contradict the galactic mass estimate.

“My next steps are to make measurements to confirm that other group's findings,” Kirby said, in the statement. “If it turns out that those outer stars aren’t actually moving faster than the inner ones, then the galaxy could be in what's called dynamic equilibrium. That would make it the most excellent candidate for detecting dark matter with gamma rays.”