The space around the Milky Way is not empty. It is teeming with dwarf galaxies – small, faint and low-mass, only about 1000 stars in each.
This is not unusual. We know from our observations of other large galaxies that dwarf galaxies often congregate nearby and can be captured by the gravitational field of a larger object.
So far, astronomers have identified about 60 smaller galaxies within 1.4 million light-years of the Milky Way, although there is likely much more hiding in the dark. Most look like they hang around like fruit flies around a banana.
However, according to a new analysis of data from the Gaia satellite, most of these galaxies are actually relatively new to the region – too new to orbit the Milky Way, at least not yet, the researchers said.
“We have concluded that because of their unrivaled high energies and angular momenta, most dwarfs cannot be long-lived satellites, and if they could be associated with the Milky Way, they first pass, that is, fall less than 2 billion years ago,” the researchers are writing in a new paper led by astrophysicist François Hammer of the Paris Observatory in France.
The Gaia Mission is an ongoing project to map the Milky Way with the highest precision, including the 3D positions, movements and speeds of stars and objects in it (and beyond).
Using measurements of these properties, Hammer and colleagues used data from the first and third publication of data from Gaia to calculate the motions of 40 dwarf galaxies outside the Milky Way. They then used parameters such as the three-dimensional speed of each galaxy to calculate its orbital energy and angular momentum.
The results were really intriguing – because they showed that most of the dwarf galaxies thought to be satellites of the Milky Way move much faster than objects known to orbit around the Milky Way, such as the stars from Gaia Enceladus and the Sagittarius dwarf … spheroidal galaxy.
Over the course of its long history, the Milky Way has swallowed up other galaxies on several occasions. Gaia Enceladus, also known as Gaia Sausage, was devoured about 9 billion years ago. Its traces remain in the population of stars orbiting at relatively low energies.
The Sagittarius dwarf spheroidal galaxy is currently being destroyed by tidal (gravitational) forces and is being incorporated into the Milky Way, a process that began about 4-5 billion years ago. These stars rotate slightly faster than the stars of Gaia-Enceladus.
Dwarf galaxies move even more vigorously. The team concludes that this means that these dwarf galaxies could not have been close to the Milky Way long enough for the massive galaxy’s gravitational field to slow them down.
The discovery could change our understanding of the interactions between normal galaxies and dwarf galaxies, as well as the properties of dwarf galaxies, the researchers said.
It is possible that some of the dwarf galaxies will be captured in the orbit of the Milky Way (although it is impossible to say which ones), but how long they will remain is an open question.
“The Milky Way is a large galaxy, so its tidal force is gigantic, and it’s very easy to destroy a dwarf galaxy after one or two passes,” explains Hammer.
If a dwarf galaxy can last longer than this – as has been suggested for the Milky Way’s dwarf galaxies – there must be something holding them together, such as higher concentrations of dark matter, the invisible glue that binds the universe together.
The possibility that dwarf galaxies have an amazing amount of dark matter is already strongly hinted at by the motions of their stars, which cannot be explained by the presence of normal matter alone.
The new results show that dark matter need not be included in our models of these galaxies; future research may explore the possibility of whether they are currently being tidal, with a broader set of parameters to play with.
It is also worth noting that the researchers’ results do not differ from the 2006 paper, which found that the velocities of the Large and Small Magellanic Clouds were much higher than suggested by the Hubble telescope data, suggesting that they are not satellites of the Milky Way. It seems that since then this concept has been rejected by at least some researchers.
However, there is a lot we don’t know about objects in and around the Milky Way, and there is no doubt that Gaia is changing our understanding of our little corner of the universe.
“Thanks in large part to Gaia, it has become apparent that the history of the Milky Way is far more legendary than astronomers previously assumed,” says astrophysicist Timo Prusti of the European Space Agency.
“As we explore these seductive clues, we hope to uncover even more fascinating chapters from our galaxy’s past.”
Research published in Astrophysical Journal…