Dance of small galaxies surrounding the Milky Way

Credit: Instituto de Astrofísica de Canarias

An international team led by IAC researchers used data from the Gaia satellite system to measure the motion of 39 bars of galaxies. These data provide 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 (rodlike galaxies) that can be ten times or even millions of times shiny from the Milky Way. Compared to normal or giant galaxies, dwarf galaxies contain fewer stars and have lower luminosity.

These small galaxies were the subject of a study by an international team of astronomers led by Tobias K. Fritz and Giuseppina Battaglia, both researchers at the Astrophysics de Canarias Institute (IAC). Thanks to the data obtained by the space mission ESA Gaia, which became available in the second edition of April 2018, researchers could measure the movement of 39 mountain galaxies in the sky, which determines direction and speed.

Before the second publication of Gaia satellite data, these measurements could not be performed for the 29 galaxies analyzed by the team. Researchers have found that many are moving in a plane known as a large polar structure. "It was already known that many more massive galaxies were found in this plane, but now we know that this smaller galaxy can belong to this structure," says Fritz, the lead author of the scientific article.

Battaglia points out that the origin of a large polar structure is still not fully understood, but its characteristics appear to contradict the cosmological models of the formation of the galaxy. In this planar structure, there is also a large magellanic cloud, which could mean that they are connected.

By analyzing the data on the movements, the team found that it has several galaxies from the orbit of dwarfs that lead them closer to the interior of the Milky Way. The gravitational attraction that the Milky Way performs on these galaxies can be compared with the tidal action. It is likely that some of the plasma galaxies that have been studied are concerned about these tides, which extend them as a stream.

"This could explain the observed properties of some of these items, such as Hercules and Crater II," says Fritz.

On the other hand, new questions arise. "Over the years, it has been observed that some galaxies have characteristic properties that could be caused by tides on the Milky Way (eg Customs I)," says Battaglia. "But their orbit does not believe that this hypothesis would be confirmed. Perhaps they should have supposed that these encounters with other galaxies from dwarves could be culprits."

The researchers found that most of the galaxies examined are near the pericent of their orbit (the point closest to the center of the Milky Way). Nevertheless, basic physics explains that you should spend most of your time near the apocenter of your orbit (the furthest distance from the Milky Way center). "This suggests that there should be many more galaxies that do not yet have to be discovered and hidden long distances from the center of the Milky Way," says Fritz.

Red galaxies, besides being interesting in themselves, are one of the rare tracers of dark matter that can be used on the outermost parts of the Milky Way. It is believed that this type of matter represents about 80 percent of the total mass of the universe. However, it can not be observed directly, but detection is difficult. For measuring the total mass of a substance in volume, we can use movements of celestial bodies, such as dwarf galaxies. This is determined by subtracting the mass of the light objects that are detected in order to obtain an estimate of the amount of dark matter. From these data, researchers could conclude that the amount of dark matter on the Milky Way is high, about 1.6 trillion solar masses.

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More information:
T. K. Fritz et al. Gaia DR2 corresponding movements from dwarf galaxies within 420 kPc, Astronomy and astrophysics (2018). DOI: 10.1051 / 0004-6361 / 201833343

Reference number:
Astronomy and astrophysics

Provided by:
Instituto de Astrofísica de Canarias

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