An international team led by IAC researchers used data from the Gaia ESA to measure 39 dwarf galaxies. This data provides information on the dynamics of these galaxies, their history and their interactions with the Milky Way.
Around the Milky Way, there are many small galaxies (dwarf galaxies), which can be tens of thousands of times or even millions of times less bright than the Milky Way. Compared to normal or giant galaxies, dwarf galaxies contain fewer stars and have lower brightness.
These small galaxies have been the subject of an international team of astronomers headed by Tobias K. Fritz and Giuseppina Battaglia, both of the Instituto de Astrofisica de Canarias (IAC) researchers. Thanks to the data obtained from the Gaia ESA space mission, which was available in second release in April 2018, the researchers were able to measure the sky motion of 39 dwarf galaxies, defining direction and speed.
Prior to the second release of data from the Gaia satellite, it was not possible to perform such measurements for 29 of the galaxies analyzed by the group. The researchers found that many of them are moving to a level known as the huge polar structure. "It was already known that many of the most massive dwarf galaxies were found at this level, but now we know that many of the less massive dwarf galaxies may belong to this structure," says Fritz, principal author of the scientific article.
Battaglia underlines that the origin of the vast polar structure is not yet fully understood, but its features seem to be causing cosmological galaxy-forming patterns. Also, the large Magellanic Cloud is in this flat structure, which can mean that the two are connected.
Analyzing motion data, the team found that several of the dwarf galaxies have orbits that bring them near the inner parts of the Milky Way. The gravitational pull exerted by the Milky Way on these galaxies can be compared to tidal action. It is possible that some of the dwarf galaxies studied are disturbed by these tides, which tend to be like a current.
"This could explain the observed properties of some of these objects, such as Hercules and Crater II," says Fritz.
On the other hand, new questions arise. "Over the years, there have been some galaxies that might be due to tidal galaxies (eg Carina I)," says Battaglia. "However, their orbits do not seem to confirm that. We may assume that encounters with other dwarf galaxies may have been the culprit."
The researchers found that most of the galaxies studied were approaching their dangerous orbit (the point closest to the center of the Milky Way). However, basic physics explains that they have to spend most of their time near the excerpt of their orbit (the point that is more distant from the center of the Milky Way). "This suggests that there should be more dwarf galaxies that have not yet been discovered and that lie long distances from the center of our Milky Way," says Fritz.
Dwarf galaxies, in addition to being interesting on their own, are one of the few dark matter tracers that can be used in the outermost parts of the Milky Way. It is believed that this type of matter accounts for about 80 percent of the total mass of the universe. However, it can not be directly observed and detection is difficult. The movements of celestial bodies, such as dwarf galaxies, can be used to measure the total mass of matter within a volume. This is determined by subtracting the mass of light objects detected to obtain an estimate of the amount of dark matter. From these data, researchers could conclude that the amount of dark matter in the Milky Way is high, about 1.6 trillion solar masses.
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T. K. Fritz et al. Gaia DR2 suitable movements of dwarf galaxies within 420 kpc, Astronomy & Astrophysics (2018). DOI: 10.1051 / 0004-6361 / 201833343