The space around the Milky Way is not empty. It is teeming with dwarf galaxies – small, dim, low-mass galaxies of up to 1,000 stars each.
There’s nothing unusual about that. We know from observations of other large galaxies that dwarf galaxies often cluster nearby and can be captured by the gravitational field of a larger object.
To date, astronomers have found about 60 small galaxies within 1.4 million light-years of the Milky Way, although it’s likely that many more are hiding in the dark. Most of them look like they are hanging 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 — too new to orbit around the Milky Way, at least for now, the researchers believe.
“We conclude that because of their unequally high energies and angular momenta, most dwarf galaxies cannot be long-lived companions, and if they can be associated with the Milky Way, it is at first pass, that is, when they entered less than 2 billion years ago,” the researchers write 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 as accurately as possible, including the three-dimensional positions, movements and velocities of stars and objects in it (and a little beyond).
Measuring these properties, Hammer and his colleagues used data from an early third Gaia data release to calculate the motion of 40 dwarf galaxies outside the Milky Way. They then used parameters such as the three-dimensional velocity of each galaxy to calculate its orbital energy and angular momentum.
The results were really intriguing – they showed that most of the dwarf galaxies thought to be satellites of the Milky Way are moving much faster than known objects in orbit around the Milky Way, such as the stars Gaia Enceladus and the dwarf spheroidal galaxy Sagittarius.
In its long history, the Milky Way has repeatedly swallowed up other galaxies. Gaia Enceladus, also known as the Sausage of Gaia, was consumed about 9 billion years ago. Its traces are preserved in the population of stars orbiting it at relatively low energies.
The dwarf spheroidal Sagittarius galaxy is now destroyed by tidal (gravitational) forces and 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.
The dwarf galaxies are moving even more vigorously. This, according to the team, means that these dwarf galaxies could not have been near the Milky Way long enough for the gravitational field of a massive galaxy to slow them down.
This discovery could change our understanding of the interactions between normal galaxies and dwarf galaxies, as well as the properties of dwarf galaxies, the researchers say.
It is possible that some of the dwarf galaxies will be captured by the orbit of the Milky Way (although it is impossible to say which ones), but how long they will stay there is an open question.
“The Milky Way is a large galaxy, so its tidal force is simply gigantic, and a dwarf galaxy is very easy to destroy after one or two passages,” Hammer explains.
If a dwarf galaxy can last longer than that – as previously thought for the Milky Way dwarf galaxies – something must hold them together, such as a higher concentration of dark matter, the invisible glue that binds the universe together.”
The possibility that dwarf galaxies contain a surprising amount of dark matter is already hinted at by the motions of their stars, which cannot be explained by the presence of ordinary matter alone.
The new results suggest that dark matter need not necessarily be included in our models of these galaxies; future research could explore the possibility of whether they are currently undergoing tidal disruption with a broader set of parameters at play.
It is also worth noting that the researchers’ results are not at odds with the 2006 work, which found that the velocities of the Large and Small Magellanic Clouds are much higher than thought based on Hubble data, suggesting that they are not satellites of the Milky Way. This assumption seems to have been abandoned, at least by some researchers.
Nevertheless, there is much we don’t know about the objects in and around the Milky Way, and there is no doubt that Gaia is changing our view of our little corner of the universe.
“In many ways, Gaia has made it clear that the history of the Milky Way is much richer in events than astronomers had previously assumed,” says astrophysicist Timo Prusti of the European Space Agency.
“By exploring these tantalizing clues, we hope to further uncover exciting chapters of our galaxy’s past.”
The study was published in The Astrophysical Journal.