Here we are sitting, lying on Earth in the middle of the Milky Way galaxy. And this is the Milky Way galaxy, which has grown inside great dark matter halo.
Image of the Messier 81 team from the Sloan Digital Sky Survey with the positions of known M81 satellites (red) and discoveries and candidates (yellow). The known and newly discovered candidate galaxies, with a million times fewer stars than Messier 81 (or the Milky Way), are distributed around Messier 81 in a misleading way, clustering around the citadel. 3rd brightest member in the NGC 3077 group. This is surprising as the satellites are predicted to cluster around the brightest members, and is a challenge for galaxy formation models to reproduce. create. Image credit: Sloan Digital Sky Survey
Dark matter is a curious thing: It doesn’t interact with light, so it should be called transparent matter. Eric Bellprofessor of astronomy at the University of Michigan.
But it makes up about 85% of the matter in our universe and drives galaxy formation. To study it further, astronomers find and examine faint dwarf galaxies that should be embedded in dark matter, smaller than “subhalos.”
“One of the big reasons to study these small dwarf galaxies is that you can learn what small clumps of dark matter, or halos, look like,” Bell said. “You might expect this to affect the properties of faint galaxies.”
Bell and a team of researchers are looking for dwarf galaxies along with a nearby galaxy; The galaxy the size of the Milky Way is called M81. M81 is slowly splitting two smaller satellite galaxies, which means its gravity is pulling the two smaller galaxies, M82 and NGC 3077, into itself.
Bell expected to find the faintest unidentified galaxies outside the Milky Way or Andromeda galaxies clustered around M81, the most massive galaxy, then less around M82 , and possibly one around NGC 3077. Instead, his team found nearly all of the faintest galaxies clustered around NGC 3077.
The team found one identified galaxy — one of the faintest galaxies ever detected outside the Milky Way and Andromeda galaxies (M31) — and six other possible candidates. Bell will present the findings at the June meeting of the American Astronomical Society.
“Six of the seven new and candidate galaxies are all centered around this moderately sized galaxy. It’s the third largest of the bunch, but for some reason it has almost all of the satellites, and we don’t know why,” Bell said. “To go back to our dark matter and galaxy formation model, they say, ‘The bigger, the more.’ If I had a larger galaxy with a larger dark halo, it must have many smaller friends, and this system seems to be violating that.”
Models of galaxy formation simulate how galaxies would grow from the microscopic fluctuations of dark matter in the history of the universe into the galaxies we can observe today. Dark matter plays an important role – without it, no galaxies can form, and even small changes in dark matter’s behavior alter the predicted properties of galaxies. , especially the faintest dwarf galaxies.
To look for these faint dwarf galaxies, astronomers used Subaru’s Hyper Supreme-Cam, a small car-sized digital camera parked atop the Subaru Telescope in Hawaii. . With this camera, astronomers can capture extremely sensitive images that can detect light 100 million times dimmer than the human eye can see.
A new satellite galaxy in the Messier 81 group. With just 100,000 stars, it is the faintest known member of the Messier 81 group and one of the faintest galaxies detected outside of the immediate environment. of the Milky Way. Image credit: Eric Bell, University of Michigan, with the Subaru Telescope
These galaxies are very diffuse, but Subaru is sensitive enough to detect some of the brightest stars in the galaxy. Bell and his colleagues found these galaxies by looking for clusters of these faint stars. For each star they see, they expect to have several thousand fainter stars.
“So these galaxies have between 50,000 and 200,000 stars, explaining that this number seems like a lot, but is 1 million times smaller than the number of stars in the Milky Way,” says Bell. “These are lousy galaxies. They do their job – make stars – amazingly. You wouldn’t hire them to make a galaxy. “
Bell says finding these faint galaxies closer to the smaller satellite galaxy is a puzzle.
“Our models predict that larger galaxies will have more faint galaxies, but in reality they do not. Or maybe overall, they have, but there are some exceptions, and we just happened to take a weird picture,” he said. “But the models don’t particularly expect that kind of variation. It means that there is something interesting about how galaxies form and exist in small halos of dark matter that we don’t understand. “
Astronomers have several theories as to why these faint dwarf galaxies might be found closer to small galaxies than large ones, although Bell says these theories are far from perfect. fully satisfied. One possibility is that the gravity of a large galaxy could tear apart small galaxies more effectively than previously thought.
“If I’m with a big guy, that big guy will pull harder on one side of me than the other, and so I’ll get ripped off,” Bell said.
This effect has been accounted for by mathematical models of how these faint galaxies would behave near larger companions. But models may not incorporate enough of this effect — such as a satellite closer to a larger galaxy than previously estimated — from the gravitational pull, known as tides, from the massive galaxy, Bell explains. than.
Another possibility could be that large galaxies like the Milky Way do something to their surroundings to prevent even small galaxies from forming, in the first place. Galaxies heat the surrounding gas, and it is possible that this process is more intense than current models predict.
“It can. I don’t really like either of these ideas, but we’re going to try them out in our galaxy formation models,” says Bell.
He says his findings raise more questions than they answer, but they are intriguing questions to consider.
“It is possible that these faint galaxies form not around large guys but around smaller, medium sized ones. Or it could be that they don’t exist around the big people, but live around the small ones,” he said. “And that means we’ve got a little bit of a misconception about the formation and existence of these things – or a misunderstanding of how galaxies form or exist.”
Bell and his team have timed the telescope on the Hubble Space Telescope, which will provide them with even clearer pictures of these faint galaxies — and the chance to solve some of these problems.
Source: University of Michigan
