The Milky Way’s satellite galaxies, such as the Large and Small Magellanic Clouds, can provide important information about the relationship between dark matter halos and galaxy formation. These satellite galaxies are thought to be located within the halo of the Milky Way, and their properties can be used to infer the properties of the dark matter halo they are located in.
Our galaxy has satellite galaxies, some of which may have smaller satellite galaxies of their own, just as the sun has planets and the planets have moons. For instance, recent observations from the European Space Agency’s Gaia mission suggest that the Large Magellanic Cloud (LMC), a rather large satellite galaxy observable from the Southern Hemisphere, took at least six of its own satellite galaxies with it when it first approached the Milky Way.
Researchers at the Department of Energy’s SLAC National Accelerator Laboratory and the Dark Energy Survey have used observations of faint galaxies near the Milky Way to tighten the connection between the size and structure of galaxies and the dark matter halos that surround them. Astrophysicists believe that dark matter is responsible for much of that structure.
At the same time, they have discovered more proof of LMC satellite galaxies’ existence and made a new prediction: If the scientists’ models are accurate, the Milky Way should have an extra 150 or more extremely faint satellite galaxies waiting to be found by future initiatives like the Legacy Survey of Space and Time at the Vera C. Rubin Observatory.
“The new study, forthcoming in the Astrophysical Journal and available as a preprint here, is part of a larger effort to understand how dark matter works on scales smaller than our galaxy,” said Ethan Nadler, the study’s first author and a graduate student at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) and Stanford University.
“We know some things about the dark matter very well how much dark matter is there, how does it cluster but all of these statements are qualified by saying, yes, that is how it behaves on scales larger than the size of our local group of galaxies,” Nadler said. “And then the question is, does that work on the smallest scales we can measure?”
Shining galaxies’ light on dark matter
Long known to exist, the Milky Way’s satellite galaxies include the Large Magellanic Cloud, which can be seen with the unaided eye from the Southern Hemisphere, but until about 2000, it was believed there were only a few hundred of them. Since then, there have been a significant increase in the number of satellite galaxies observed.
Thanks to the Sloan Digital Sky Survey and more recent discoveries by projects including the Dark Energy Survey (DES), the number of known satellite galaxies has climbed to about 60.
The new study, forthcoming in the Astrophysical Journal and available as a preprint here, is part of a larger effort to understand how dark matter works on scales smaller than our galaxy. We know some things about the dark matter very well how much dark matter is there, how does it cluster but all of these statements are qualified by saying, yes, that is how it behaves on scales larger than the size of our local group of galaxies.
Ethan Nadler
Such findings are always thrilling, but what the data might reveal about the cosmos may be much more so.
“For the first time, we can look for these satellite galaxies across about three-quarters of the sky, and that’s really important to several different ways of learning about dark matter and galaxy formation,” said Risa Wechsler, director of KIPAC.
Last year, for example, Wechsler, Nadler, and colleagues used data on satellite galaxies in conjunction with computer simulations to place much tighter limits on dark matter’s interactions with ordinary matter.
Now, Wechsler, Nadler, and the DES team are using data from a comprehensive search over most of the sky to ask different questions, including how much dark matter it takes to form a galaxy, how many satellite galaxies we should expect to find around the Milky Way and whether galaxies can bring their own satellites into orbit around our own a key prediction of the most popular model of dark matter.
Hints of galactic hierarchy
The answer to that last question appears to be a resounding “yes.”
When DES discovered more satellite galaxies nearby the Large Magellanic Cloud than they would have anticipated if those satellites were dispersed randomly throughout the sky, the potential of discovering a hierarchy of satellite galaxies first surfaced some years ago. Those observations are particularly interesting, Nadler said, in light of the Gaia measurements, which indicated that six of these satellite galaxies fell into the Milky Way with the LMC.
To study the LMC’s satellites more thoroughly, Nadler and team analyzed computer simulations of millions of possible universes. These simulations, which were first performed by Yao-Yuan Mao, a former graduate student of Wechsler’s who is currently a professor at Rutgers University, simulate the formation of the dark matter structure that permeates the Milky Way and include specifics like smaller dark matter clumps inside the Milky Way that are anticipated to house satellite galaxies.
The relationship between galaxies’ brightness and the mass of dark matter clumps in which they form, as well as other uncertainties in the current understanding of galaxy formation, were taken into account by the researchers using a flexible model to establish the link between dark matter and galaxy formation.
An effort led by the others in the DES team, including former KIPAC students Alex Drlica-Wagner, a Wilson Fellow at Fermilab and an assistant professor of astronomy and astrophysics at the University of Chicago, and Keith Bechtol, an assistant professor of physics at the University of Wisconsin-Madison, and their collaborators produced the crucial final step: a model of which satellite galaxies are most likely to be seen by current surveys, given where they are in the sky as well as their brightness, size, and distance.
With these elements in hand, the team ran their model with a variety of parameters and looked for simulations in which LMC-like objects were pulled by a galaxy similar to the Milky Way. They were able to determine a variety of astrophysical characteristics, including how many satellite galaxies should have traveled with the LMC, by contrasting those cases with galactic data.
The findings, according to Nadler, were in line with Gaia observations: Six satellite galaxies ought to be visible right now in the area of the LMC, traveling at nearly the correct velocities, and positioned roughly where scientists had previously noticed them.
Additionally, based on the simulations, the LMC first approached the Milky Way some 2.2 billion years ago, which is in line with precise Hubble Space Telescope observations of the LMC’s speed.
Galaxies yet unseen
The researchers placed restrictions on the relationship between dark matter halos and galaxy structure in addition to the LMC findings. For instance, the smallest galaxies that astronomers may currently view should have stars with a total mass of around 100 suns and approximately a million times as much dark matter, according to simulations that most closely matched the history of the Milky Way and the LMC. The model’s extension predicts that halos up to a hundred times less massive than that might form around the weakest galaxies that have ever been seen.
“And there could be more discoveries to come: If the simulations are correct,” Nadler said, “there are around 100 more satellite galaxies more than double the number already discovered hovering around the Milky Way.”
According to him, the discovery of those galaxies would support the researchers’ theory of the connections between dark matter and galaxy formation and probably impose more stringent limitations on the characteristics of dark matter.
Sidney Mau, an undergraduate at the University of Chicago, and Mitch McNanna, a graduate student at the University of Wisconsin–Madison, were among the junior members who made significant contributions to the research, which was a collaborative effort within the Milky Way Working Group of the Dark Energy Survey.
The research was supported by a National Science Foundation Graduate Fellowship, by the Department of Energy’s Office of Science through SLAC, and by Stanford University.
