What faint dwarf galaxies can tell astronomers about galaxy formation

Here we sit, located on Earth in the middle of the Milky Way galaxy. And here lies the Milky Way galaxy, having grown inside a large halo of dark matter

Dark matter is a curious thing: it doesn’t interact with light, so it really should be called transparent matter, says Eric Bellprofessor of astronomy at the University of Michigan.

But it represents about 85% of the matter of our universe and is at the origin of the formation of galaxies. In order to study it further, astronomers are finding and examining really faint dwarf galaxies that should be embedded in smaller dark matter “subhalos.”

“One of the main reasons to study these little dwarf galaxies is that you learn what little clumps of dark matter, or halos, would look like,” Bell said. “We would expect this to impact the properties of faint galaxies.”

Bell and a team of researchers were looking for dwarf galaxies next to a nearby Milky Way-sized galaxy called M81. M81 is slowly tearing two smaller satellite galaxies apart, meaning its gravity is pulling these two smaller galaxies, M82 and NGC 3077, towards itself.

Bell expected to find the faintest galaxies yet identified outside of the Milky Way or the Andromeda galaxies to be clustered around M81, the galaxy with the greatest mass, then least around M82, and possibly one around NGC 3077. Instead, his group found that nearly all of the fainter galaxies clustered around NGC 3077.

The group found a definite galaxy – one of the faintest ever discovered outside of the Milky Way Galaxy and the Andromeda Galaxy (M31) – and six other likely candidate galaxies. Bell will present the results at the June meeting of the American Astronomical Society.

“Six of the seven new galaxies and candidate galaxies are all clustered around this fairly mid-sized galaxy. It’s the third largest of the bunch, but for some reason it owns almost all the satellites, and we don’t know why,” Bell said. “Going back to our models of dark matter and our models of galaxy formation, they basically say, ‘Bigger is bigger. If I have a bigger galaxy with a bigger dark halo, it should have more little friends, and this system seems to violate that.

Galaxy formation models simulate the growth of galaxies from tiny dark matter fluctuations early in the history of the universe to galaxies like those we might observe today. Dark matter plays a critical role – without it, no galaxy could form, and even small changes in the behavior of dark matter alter the predicted properties of galaxies, especially fainter dwarf galaxies.

To search for these faint dwarf galaxies, astronomers used Subaru’s Hyper Suprime-Cam, a small car-sized digital camera perched atop the Subaru Telescope in Hawaii. With this camera, astronomers were able to take incredibly sensitive images that can detect light more than 100 million times fainter than what the human eye can see.

These galaxies are very diffuse, but Subaru is sensitive enough to detect the few brightest stars in this galaxy. Bell and his colleagues find these galaxies by searching for clusters of these faint stars. For every star they see, they expect there to be a few thousand fainter stars.

“So these galaxies have between 50,000 and 200,000 stars,” Bell said, explaining that that sounds like a lot, but it’s 1 million times smaller than the number of stars in the Milky Way. “These are really bad for being galaxies. They do their job of making stars terribly. You wouldn’t hire them to be a galaxy.

Bell says finding these faint galaxies near the smaller satellite galaxy has been a headache.

“Our models predict that large galaxies should have more faint galaxies, but this is not the case. Or maybe on the whole they do, but there are a few exceptions and we just took a picture of a weird one,” he said. “But the models don’t particularly expect that kind of variation. That means there’s something interesting about the way galaxies form and survive in little halos of dark matter that we don’t understand.

Astronomers have a few theories as to why these faint dwarf galaxies might be near smaller galaxies rather than large ones, though Bell says the theories aren’t entirely satisfactory. One possibility could be that the gravity of a large galaxy is tearing apart smaller galaxies more efficiently than previously thought.

“If I’m next to a big guy, that big guy will pull harder on one side of me than the other, and so I’ll be undressed,” Bell said.

This effect has been accounted for by mathematical models of how these faint galaxies should act near larger companions. But, Bell says, the models may not have sufficiently factored in this effect — such as a satellite being closer to the larger galaxy than expected — from the gravitational pull, called tides, of the larger galaxy.

Another possibility could be that large galaxies like the Milky Way do something to their surroundings that prevents even small galaxies from forming to begin with. Galaxies heat the surrounding gas, and it’s possible that this process is stronger than current models predict.

“It’s possible. I don’t really like either idea, but we’ll try them in our models of galaxy formation,” Bell said.

He says his findings raise more questions than answers, but they are intriguing questions to consider.

“It could be that these faint galaxies do not form around large types, but around smaller and medium-sized types. Or it could be that they don’t survive around the big guys, but they survive around the little ones,” he said. “And so that means we have something a little off about how these things form and survive – or a misunderstanding about how galaxies form or survive.”

Bell and his team have requested telescope time on the Hubble Space Telescope, which should give them even clearer images of these faint galaxies and a chance to fix some of these problems.