Astronomers track dwarf galaxies to better understand the Milky Way’s make-up and evolution
By Nola Taylor Redd
At the turn of this century, astronomers were confident they understood the Milky Way’s relationship with its galactic neighbors. Our home galaxy is the second-largest member of the Local Group, an assembly of more than 50 galaxies that spans roughly 10 million light-years. Many of the group’s smaller galaxies are satellites of the Milky Way — two of the closest companions are the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Streams of gas and dust ripped from these dwarf galaxies seemed to show that they had made several trips around the Milky Way, bearing witness to the destruction wrought by our galaxy.
But this neat picture was upended almost a decade ago when researchers used NASA’s Hubble Space Telescope to precisely measure the clouds’ motion. Suddenly, it became clear that the LMC and SMC are actually making their first orbit around the Milky Way. Since then, researchers have been trying to pin down the masses and movements of the clouds, details that could rewrite the story of our own galaxy’s evolution.
This year, the European Space Agency (ESA) space telescope Gaia has provided some vital clues in that quest. Gaia found that the LMC itself boasts several ultra-faint dwarf galaxies as satellites, observations that are helping constrain the masses of the cloud and the Milky Way. They also provide strong support for a leading theory suggesting that the universe’s vast collection of galaxies emerged from a primordial fog of gas and dust.
Clarifying the Cosmos
According to the Lambda Cold Dark Matter theory (LCDM), the reigning cosmological explanation of how the universe evolved, small clumps of dark matter seeded the galaxies we see today. Although dark matter eludes visible detection, its gravitational pull on regular matter betrays its presence, and cosmologists estimate that it makes up approximately 85% of the universe’s matter.
“Having two completely independent measurements using different instruments, and seeing that they agree within uncertainty limits — that’s kind of the dream for science.”
— Tony Sohn
LCDM suggests that these small clumps of dark matter merged and eventually grew into small galaxies, which merged, in turn, to become giants like the Milky Way. LCDM also predicts that large galaxies should have satellites such as the LMC, the leftover scraps of this formation process. Even the satellite galaxies should have satellites; some researchers estimate that the Magellanic Clouds ought to keep 20 to 30 smaller collections of stars orbiting around them. Although the Milky Way’s satellites are easy to spot, their diminutive secondary satellites have been more elusive.
Recent surveys of the night sky have finally uncovered a crop of these ultra-faint galaxies. Since 2015, researchers have discovered 32 candidate low-mass dwarf galaxies near the Magellanic Clouds. And in April 2018, data from Gaia conclusively showed that four of these dwarfs orbited the LMC, substantiating the LCDM’s prediction that the cloud should have its own secondary satellites. “Being able to actually confirm that kind of structure formation picture, that the Milky Way accretes smaller galaxies but smaller galaxies are also coming in with these satellite galaxies, I think that’s the context here,” says Nitya Kallivayalil of the University of Virginia in Charlottesville, VA, who coauthored a recent article linking the movement of the ultra-faint galaxies with the LMC.
The Faintest Galaxies
To understand how a galaxy is moving, researchers must first know its direction and speed. The motion of a galaxy toward the Milky Way, its radial velocity, can be tracked by studying how the wavelength of its light shifts because of its speed, a phenomenon known as the Doppler effect. By 2000, researchers had used multiple methods to calculate the radial velocity of the LMC, and the accuracy of those numbers has continued to improve. But the LMC’s proper motion — how it moves tangentially across the sky — remained tantalizingly imprecise. In 2002 and again in 2004, Kallivayalil and her colleagues turned Hubble toward the LMC to solve the problem.
The measurements made by Kallivayalil and her team relied on bright and extremely active galaxies known as quasars, far beyond the Local Group but sitting directly behind the LMC in the sky. “They’re essentially fixed reference points, fixed beacons of light,” Kallivayalil says. Hubble’s precision allowed the researchers to measure how far the LMC moved relative to the quasars, and the results were surprising. The findings suggested that that the Magellanic Clouds were moving faster than originally thought, and that prompted researchers to take another look at the orbital history of the pair. “Those were the observations that really changed the picture,” Kallivayalil says.
In 2007, Gurtina Besla of the University of Arizona in Tucson, AZ, and her colleagues used Hubble’s quasar measurements to demonstrate that the speed of the Magellanic Clouds was much higher than expected for long-term companions of the Milky Way. Rather than making yet another loop of the galaxy, the pair is on its first pass, a discovery that has completely changed how astronomers understand the LMC and SMC. “The real game changer in this picture has been the development of proper motion,” Besla says.
In all, Hubble determined the proper motion of 11 Milky Way satellite galaxies. But when the additional ultra-faint galaxies were discovered several years later, their light was too dim for Hubble to track their proper motion. It would take Gaia to connect those faint galaxies to the LMC.
Gaia provided the most accurate positional data on one billion stars in the Local Group, enabling astronomers to calculate the proper motion of 30 ultra-faint dwarfs. Combined with previous measurements of their speed toward the Milky Way, it revealed the dwarf galaxies’ true trajectories, providing strong evidence that four of them are satellites of the LMC. “It does seem like we found the smallest companions, these relics of LCDM theory,” Besla says.
Another benefit of Gaia’s observations is to confirm Kallivayalil’s Hubble measurements of the LMC’s proper motion. “Having two completely independent measurements using different instruments, and seeing that they agree within uncertainty limits — that’s kind of the dream for science,” says Tony Sohn, who studies the proper motion of nearby stellar systems at the Space Telescope Institute in Baltimore.
Milky Weigh
These observations have helped tackle another astronomical enigma: the mass of the Milky Way. It’s difficult to see the galaxy’s shape and structure from our position inside, and not all of the galaxy is visible. Although the familiar spiral disk and central bulge are predominantly composed of gas and stars, the outer halo that surrounds it is mostly dark matter, making it tough to locate the halo’s boundary. Some researchers argue that it extends all the way to the outskirts of the nearby Andromeda Galaxy, where the two galaxies’ dark matter halos push against one another.
Stars or satellite galaxies moving through the outer regions of the halo can help show how much mass it contains. By studying how fast these tracers travel, researchers can tease out information about the gravity that pulls at them, a characteristic determined by the mass of the Milky Way. That calculation also depends on the mass of the satellite, including its own load of dark matter.
But as the Milky Way strips gas and dust from a smaller companion such as the LMC, it also steals some of its unseen dark matter. So if the LMC is only coming in for its first orbit, it should be carrying much more dark matter — and thus more mass — than previously anticipated. “If it’s on its first approach, then it’s a big thing,” says Denis Erkal, an astronomer at the University of Surrey in the United Kingdom, who studies the Milky Way and its satellites. “It’s kind of a massive beast just in our neighborhood,” he says.
“The LMC is a factor of 10 more massive than we’ve been assuming it has been,” adds Besla. That makes it approximately 100 billion times more massive than the Sun, and it implies that the Milky Way must also be heavier than expected, tipping the scales at somewhere between 1 and 1.5 trillion solar masses. That’s significantly higher than some previous mass estimates, which fell as low as 500 billion solar masses.
Further insights will come from NASA’s James Webb Space Telescope and its Wide Field Infrared Survey Telescope (WFIRST), the ground-based Large Synoptic Survey Telescope, as well as ESA’s Euclid. These instruments will map the outskirts of the Milky Way’s halo, revealing structures such as tidal streams from disrupted dwarf galaxies and globular clusters that are too faint to be seen by today’s telescopes. Tracking the faintest galaxies in the outer halo should help astronomers refine their view of the Milky Way’s interactions with its satellites.
The James Webb Space Telescope and WFIRST should also reveal the faint satellites of Andromeda, allowing astronomers to unpick the evolution of the largest galaxy in the Local Group. This wealth of new instruments will enable astronomers to make significant strides in understanding how galaxies grow and change, says Besla: “It really does open up an entirely new era for study.”
Published under the PNAS license. More information, including full references, is available at PNAS.org.
