Astronomers grew suspicious of Triangulum II when they tried to measure its mass. Using 6 stars as tracers, they measured their speed around the galaxy's center. Known only to contain around a 1,000 stars, this particular galaxy is a welterweight by cosmic standards, but looks can be decieving. What they found was an amazingly dense galaxy apparently filled with 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," said astronomer Evan Kirby, of the California Institute of Technology (Caltech) in Pasadena. "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."
Indeed, dark matter is believed to account for the vast majority of matter in the entire universe - approximately 85 percent is thought to be composed of dark matter particles that do not interact with normal matter, except via the gravitational force.
ANALYSIS: Wait a Minute, Dark Matter May Not be Dark After All
After clocking the speeds of stars inside Triangulum II with the Keck Observatory, located on Hawaii's Mauna Kea, Kirby's team found that to account for their high speed, there had to me more mass that can be explained by adding up all the stars' masses. Even more, they realized that the tiny galaxy possibly possesses the highest concentration of dark matter yet discovered in any galaxy.
So what's going on? One theory is that, for some reason, Triangulum II may be home to a dense cloud of Weakly Interacting Massive particles, or WIMPs. WIMPs are hypothetical particles that carry mass, but do not interact with normal matter. They are ghostly particles that exert a gravitational force and yet cannot be seen (i.e. they do not interact via the electromagnetic force). However, WIMPs do annihilate with one another should they collide, so if Triangulum II is stuffed full of dark matter particles, we should be able to observe an excess of gamma-ray radiation being emitted from the galaxy.
To make things easier, Triangulum II is known as a "dead" galaxy - it lacks star forming regions and is very faint (in fact, the reason why Kirby's team only tracked 6 stars is that only 6 stars are bright enough to be tracked by the Keck telescope). Therefore, the dwarf galaxy shouldn't produce much in the way of high energy radiation, such as gamma-rays. So if we detect gamma-rays, perhpas this would be the "smoking gun" of WIMP annihilation.
ANALYSIS: Dark Matter Just Got Darker (and Weirder)
However, other measurements appear to show that stars outside of Triangulum II are moving faster than the ones tracked by Kirby's team, a finding that would, if confirmed, contradict this galactic mass estimate. If this is the case, the stellar speed contradiction may be a sign that our galaxy's gravitational field is ripping Triangulum II apart.
"My next steps are to make measurements to confirm that other group's findings," said Kirby. "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."