What faint dwarf galaxies can tell astronomers about galaxy formation
Here we sit, situated on Earth amid the Milky Way galaxy. And here the Milky Way galaxy sits, having grown inside a large dark matter halo
Dark matter is a curious thing: It doesn’t interact with light, so it should really be called transparent matter, says Eric Bell, professor of astronomy at the University of Michigan.
But it makes up about 85% of the matter in our universe and drives galaxy formation. In order to study it further, astronomers find and examine really faint dwarf galaxies that should be embedded in smaller, dark matter “subhalos.”
“One of the big reasons for studying these little dwarf galaxies is that you get to learn about what small dark matter clumps, or halos, would look like,” Bell said. “You’d expect this to impact the properties of the faint galaxies.”
Bell and a team of researchers were looking for dwarf galaxies alongside a nearby, Milky Way-sized galaxy called M81. M81 is slowly shredding two smaller, satellite galaxies, meaning that its gravity is pulling those two smaller galaxies, M82 and NGC 3077, into 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 largest mass, then fewer around M82, and possibly one around NGC 3077. Instead, his group found that nearly all the faintest galaxies clumped around NGC 3077.
The group found one definite galaxy—one of the faintest ever discovered outside of the Milky Way galaxy and Andromeda galaxy (M31)—and a further six likely candidate galaxies. Bell will present the findings at the June meeting of the American Astronomical Society.
“Six of the seven new galaxies and galaxy candidates are all clumped around this pretty moderately sized galaxy. It’s the third-biggest one in the group, but for some reason, it has almost all of the satellites, and we have no idea why,” Bell said. “To go back to our dark matter models and our galaxy formation models, they basically say, ‘Bigger is more.’ If I have a bigger galaxy with a bigger dark halo, it should have more little friends, and this system appears to be violating that.”
Galaxy formation models simulate how galaxies would grow from tiny fluctuations in dark matter early in the history of the universe into galaxies like we might observe today. Dark matter plays a critical role—without it, no galaxies could form at all, and even small changes in the dark matter behavior changes the predicted properties of galaxies, especially the faintest dwarf galaxies.
To look for these faint dwarf galaxies, the astronomers used Subaru’s Hyper Suprime-Cam, a digital camera the size of a small car perched atop the Subaru Telescope in Hawaii. With this camera, the astronomers were able to take incredibly sensitive images that can detect light more than 100 million times fainter than the human eye can see.
These galaxies are very diffuse, but Subaru is sensitive enough to detect the brightest few stars in that galaxy. Bell and his colleagues find these galaxies by looking for clumps of these faint stars. For each 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 this seems like a lot, but is 1 million times smaller than the number of stars in the Milky Way. “These are really bad at being galaxies. They do their job—making stars—terribly. You would not hire them to be a galaxy.”
Bell says finding these faint galaxies near the smaller satellite galaxy has been a puzzle.
“Our models predict that bigger galaxies should have more faint galaxies, but they don’t. Or maybe on the whole they do, but there are some exceptions and we just happened to take a picture of one weird one,” he said. “But the models don’t particularly expect that kind of variation. What it means is that there’s something interesting about how galaxies form and survive in little dark matter halos that we don’t understand.”
Astronomers have a few theories as to why these faint dwarf galaxies might be found near smaller galaxies rather than large ones, though Bell says the theories aren’t entirely satisfying. One possibility may be that the gravity of a large galaxy might tear apart little galaxies more effectively 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 get stripped apart,” Bell said.
This effect has been taken into account by mathematical models of how these faint galaxies should act near larger companions. But, Bell explains, the models might not have incorporated enough of this effect—such as a satellite being closer to the larger galaxy than previously estimated—from the gravitational pull, called tides, from the larger galaxy.
Another possibility could be that big galaxies like the Milky Way do something to their surroundings that stops little galaxies from even forming to begin with. Galaxies heat the surrounding gas, and it is possible that this process is stronger than current models predict.
“It’s possible. I don’t really like either idea, but we’re going to try them out in our galaxy formation models,” Bell said.
He says his findings bring up more questions than answers, but that they are intriguing questions to consider.
“It could be that these faint galaxies don’t form around big guys, but do around smaller, medium-sized guys. Or it could be that they don’t survive around big guys, but do around little guys,” he said. “And so that means we’ve got something about the formation and survival of these things a little bit wrong—or a misunderstanding about how galaxies form or survive.”
Bell and his team have applied for telescope time on the Hubble Space Telescope, which should give them even clearer pictures of these faint galaxies—and a chance to solve some of these problems.