A river of hydrogen gas flows from the Large and Small Magellanic Clouds, two dwarf galaxies located on the outskirts of the Milky Way. The contents of this river, called the Magellanic Stream, have puzzled astronomers for decades. But now, for the first time, a team of researchers has been able to locate stars within the stream's gaseous clouds. The discovery will not only help them better understand the evolution of the galaxies that host the stream, but also the distribution of matter in the Milky Way itself. This was done using the 6.5-meter Baade Telescope at the Las Campanas Observatory in Chile.
With the help of a detailed map of the Milky Way, created by the European Space Agency with the Gaia space telescope, the researchers observed 200 stars located at the farthest reaches of our galaxy, in the direction of the Magellanic Stream. They analyzed the spectrum of light emanating from these stars and found that the chemical composition of 13 subjects matched that of the Magellanic Clouds.
The Milky Way's nearest neighboring galaxies—the Small and Large Magellanic Clouds—are shown on the right side of this artist's illustration. As these galaxies move to the right, the gaseous Magellanic Stream billows behind them, intertwining and stretching across the southern sky. The illustration also shows the 13 red giant stars discovered in the Magellanic Stellar Stream by Chandra et al.
The stream, originally discovered in the 1970s, extends over a region of the southern sky equivalent to the size of 300 full moons as seen from Earth. But despite its enormity, some sensitive equipment is needed to see it.
Astronomers believe that the gas forming the stream was pulled from dwarf galaxies by the gravitational pull of the Milky Way. The new observations may reveal more about the nature of the stream and help scientists seeking to understand how it interacts with our galaxy. For now, scientists say, the stream appears to be falling into the Milky Way.
"With these and similar results, we hope to gain a much better understanding of the formation of the Magellanic Stream and Magellanic Clouds, as well as their past and future interactions with our galaxy," said Charlie Conroy, Professor of Astronomy at the Harvard-Smithsonian Center for Astrophysics (CfA), and co-author of the research.
Unlike the Magellanic Stream, the Magellanic Clouds have been known to mankind since ancient times because they are clearly visible to the naked eye. However, astronomers still have many questions about the origin and history of these two galaxies.
"The beauty of having a vast stellar stream like the Magellanic Stream is that we can now do a lot of astrophysical research with it," said Vedant Chandra, a Ph.D. student in astronomy and astrophysics at CfA and lead author of the study. "As our spectroscopic study continues and we find more stars, we are excited to see what other surprises the galactic outskirts hold."
The scientists believe that when hydrogen gas from the stream falls into the Milky Way, the right conditions are created for star formation. In analyzing the data, the researchers also discovered that the stream is about twice as massive as they thought, meaning that the Milky Way must be devouring much more gas than previous calculations had suggested.
"The Magellanic Stream is the dominant source of stellar calories for the Milky Way: it's our breakfast, lunch and dinner," said Ana Bonaca, co-author of the study and former postdoctoral fellow at the CfA, now a scientist at Carnegie Observatories. "Based on the new higher mass estimates for the Magellanic Stream, the Milky Way may end up weighing more pounds than initially thought."
By better constraining the mass of the current and gaining a better understanding of the Magellanic Clouds, astronomers will be able to better estimate the mass distribution of the entire Milky Way, the scientists said in the release.
"Much of that mass is in the form of dark matter, a poorly understood substance that exerts gravity," the scientists said. "Better measuring the mass of our galaxy in its distant inner regions will help account for ordinary matter versus ordinary matter. contained dark matter, which constrains the possible properties of the latter."
The study was published in October in the journal Astrophysics.