Here we sit, located on earth in the middle of the Milky Way galaxy. And here sits the Milky Way galaxy, having grown inside a great halo of dark matter
Dark matter is a strange thing: it does not interact with light, so it should really be called transparent matter, says Erik BellProfessor of Astronomy at the University of Michigan.
But it makes up about 85% of matter in our universe and drives galaxy formation. To study it further, astronomers find and study really faint dwarf galaxies that should be embedded in less dark matter “subhalos”.
“One of the big reasons to study these little dwarf galaxies is to learn what small lumps of dark matter, or halos, would look like,” Bell said. “You can expect this to affect the properties of the faint galaxies.”
Bell and a team of scientists were looking for dwarf galaxies next to a nearby Milky Way galaxy called the M81. M81 slowly tears two smaller satellite galaxies, which means that its gravity pulls the two smaller galaxies, M82 and NGC 3077, into itself.
Bell expected to find the weakest galaxies so far identified outside the Milky Way or the Andromeda galaxies to be gathered around M81, the galaxy with the largest mass, then fewer around M82, and possibly one around NGC 3077. Instead, his group found that almost all of the the weakest galaxies cluster around NGC 3077.
The group found a definite galaxy – one of the weakest ever discovered outside the Milky Way and the Andromeda galaxy (M31) – and six more likely candidate galaxies. Bell will present the results at the June meeting of the American Astronomical Society.
“Six of the seven new galaxies and galaxy candidates are all clumps around this rather medium-sized galaxy. It is the third largest in the group, but for some reason it has almost all satellites, and we have no idea why,” says Bell. go back to our dark matter models and our galaxy formation models, they say in principle, ‘Bigger is more’. If I have a bigger galaxy with a bigger dark halo, it should have more little friends, and this system seems to be breaking that. “
Galaxy formation models simulate how galaxies would grow from small fluctuations in dark matter early in the history of the universe to galaxies that we can observe today. Dark matter plays a crucial role – without it no galaxies could form at all, and even small changes in the behavior of dark matter change the predicted properties of galaxies, especially the weakest dwarf galaxies.
To search for these faint dwarf galaxies, astronomers used the Subaru’s Hyper Suprime-Cam, a digital camera the size of a small car sitting on top of 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 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 every star they see, they expect there to be a few thousand weaker stars.
“So these galaxies have between 50,000 and 200,000 stars,” Bell said, explaining that this seems to be a lot, but is 1 million times smaller than the number of stars in the Milky Way. “These people are really bad at being galaxies. They do their job – make stars – awful. You would not hire them to be a galaxy.”
Bell says it has been a puzzle to find these faint galaxies near the smaller satellite galaxy.
“Our models predict that larger galaxies should have more weak galaxies, but they do not. Or maybe they do at all, but there are some exceptions and we just happened to take a picture of a strange one,” he says. But the models do not particularly expect that kind of variation. What that means is that there is something interesting about how galaxies form and survive in small dark matter halos that we do not understand. ”
Astronomers have some theories as to why these faint dwarf galaxies can be found near smaller galaxies rather than large ones, although Bell says the theories are not entirely satisfactory. One possibility may be that the gravity of a large galaxy can tear apart small galaxies more efficiently than previously thought.
“If I’m next to a big guy, the 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 taken into account in mathematical models of how these weak galaxies should act close to larger companions. But, Bell explains, the models may not have incorporated enough of this effect – as if a satellite is closer to the larger galaxy than previously estimated – from the gravitational force, called tides, from the larger galaxy.
Another possibility could be that large galaxies such as the Milky Way do something with their surroundings that prevent small galaxies from even forming to begin with. Galaxies heat up the surrounding gas, and it is possible that this process is stronger than current models predict.
“It’s possible. I do not really like any of the ideas, but we will test them in our models for galaxy formation,” Bell said.
He says that his results raise more questions than answers, but that these are exciting questions to think about.
“It could be that these weak galaxies are not formed around big guys, but around smaller, medium-sized guys. Or it could be that they do not survive around big guys, but do around little guys,” he said. means we have something wrong with the formation and survival of these things – or a misunderstanding about how galaxies are formed or survive. “
Bell and his team have applied for telescope time on the Hubble Space Telescope, which should give them even clearer images of these faint galaxies – and a chance to solve some of these problems.
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