On December 3, the first part of the third batch of Gaia data, called Gaia Early Data Release 3 , was made available to scientists. It revealed new position and velocity data for many stars already in its database, a small part of which were in these satellite galaxies.
Using this new data, which includes more precise measurements of hundreds of millions of these stars, scientists plan to probe our galaxy and its surroundings in exquisite detail. And in doing so, we are about to learn more than ever about our satellite galaxies.
The Milky Way has at least 50 to 60 satellite galaxies, although the exact number is unknown; some are simply too faint to see. The most populated contain billions of stars, compared to hundreds of billions in our own galaxy, while the least populated have only hundreds. They vary over distances from about 26,000 to a million light years away.
Airplane
While they come in different shapes and sizes, most share a strange trait. “Many of the satellites move in an airplane, similar to how planets move around the sun in our solar system,” said Dr. Marius Cautun of the University of Leiden in the Netherlands. “This is quite puzzling, because (according to current theoretical models) we would expect a more random movement.”
Dr. Cautun and his team made this discovery as part of a project called DancingGalaxies. Using the vast and unprecedented data from Gaia, they have been able to monitor the motion of the stars within these satellite galaxies, and thus track their overall motions.
“Most of the data from Gaia is about stars in the (Milky Way),” Dr. Cautun said. But you can measure the motion of bright stars up to 100,000 light years, and maybe even further. And we can average the motion of those stars and get the motion of satellite galaxies. “
This revealed the strange motion of these satellite galaxies, and could provide insight into the evolution of the Milky Way. “Normally, galaxies grow by accumulating (gathering) matter from the outside,” said Dr. Cautun. “ What we think happened in the case of our own Milky Way is that matter, instead of being spherical accreted, accreted in a plane, including satellite galaxies. If this is the case, the Milky Way is an extreme example of accretion on an airplane. ” This could make the Milky Way a bit unusual, as other galaxies are believed to have grown spherical.
Dark matter
Studying the motion of these galaxies has also allowed astronomers like Dr. Cautun to probe dark matter, specifically a halo of dark matter known to surround our galaxy. The atypical motion of our satellites has suggested that rather than being spherical, the dark matter halo is more rugby-ball shaped and twisted.
“The puzzling thing is that at some distance, maybe 40,000 to 100,000 light-years from the center of our own galaxy, the halo undergoes a sudden change,” said Dr. Cautun, a feature that could again mean that the Milky Way is unique, as less than one percent of galaxies are believed to have such a shift. “It’s like rugby balls on top of each other, but at some point the rugby ball is flipped 90 degrees, a twist in the halo of dark matter. This is a strange feature that only occurs in very, very few galaxies. “
In the future, it should be possible to use the Gaia data to probe some of our fainter satellite galaxies in greater detail than before. “We are going to have more precise measurements for the motion of the weaker satellites,” said Dr. Cautun, with at least a factor ten improvement expected in the known motion of such galaxies. ‘It will make a big difference.’
Two satellite galaxies in particular are of great interest to astronomers, because they are in the process of interacting with each other and with our Milky Way. These are known as the Small and Large Magellanic Clouds, or SMC and LMC, located about 200,000 and 163,000 light-years away, respectively.
Professor Maria-Rosa Cioni from the Leibniz Institute for Astrophysics in Potsdam in Germany is the leader of a project called Interclouds that uses LMC and SMC, the former being the largest of our satellite galaxies, to understand more about how galaxies behave. “(Our) idea is to use these two galaxies to learn about the interactions of the galaxies,” he said. Their proximity to Earth makes them easy candidates to study, as even individual stars can be seen.
To study them, Professor Cioni and her team have been analyzing the populations and movements of the stars in each cloud. Currently, both clouds are moving away from the Milky Way at about 320 kilometers per second , this being the end of a close pass that began recently in astronomical terms in the last two billion years. However, an important unanswered question is whether this was his first or second pass.
“They seem to be moving too fast to be in a limited orbit, so we think they just made the first step into the Milky Way,” said Professor Cioni. “But there are other people who think they are in their second passage, which (would) mean that they are already linked to the Milky Way.”
If the former is true, it is possible that the Magellanic Clouds are continuing to recede, and therefore they may not actually be satellites. To find out, we will need to know exactly how their mass compares to the Milky Way to discern if there is a gravitational pull strong enough to bring them back or not, and the Gaia data is vital to getting to that precise measurement of their masses.
“If the Milky Way is not very massive, then the gravitational force that the Magellanic Clouds would feel is not very strong,” said Professor Cioni. “(But) if the mass of the Milky Way is significantly greater than the mass of the Magellanic Clouds, they will slow down tremendously so that they will stay closer.”
Professor Cioni also hopes to probe the ages of the Magellanic Clouds, their chemical compositions, and even their structures, something that is simply not possible without large-scale data sets like Gaia. “Gaia allows us to do that on a level that was not possible before,” says the expert.
Original article
This article was originally published in Horizon, the EU Research and Innovation Magazine
