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Dances of small galaxies that surround the Milky Way

Credit: Instituto de Astrofísica de Canarias

An international team led by researchers from IAC used data from the ESA Gaia satellite to measure the movements of 39 dwarf galaxies. This data provides information about 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 dimmer than the Milky Way. Compared to normal or giant galaxies, dwarf galaxies contain fewer stars and have lower luminosity.

These small galaxies have been the subject of research by a team of international astronomers led by Tobias K. Fritz and Giuseppina Battaglia, both researchers from the Instituto de Astrofísica de Canarias (IAC). Thanks to data obtained by the ESA Gaia space mission, which became available in the second release in April 2018, researchers have been able to measure the movement of the sky from 39 dwarf galaxies, determining direction and speed.

Before the release of the two data from the Gaia satellite, it was not possible to make these measurements for 29 of the galaxies analyzed by the team. The researchers found that many of them moved on planes known as extensive polar structures. "It is well known that many more massive dwarf galaxies are found on this plane, but now we know that some less large dwarf galaxies may belong to this structure," said Fritz, lead author of the scientific article.

Battaglia highlights that the origins of a vast polar structure are still not fully understood, but its characteristics seem to challenge the cosmological model of galaxy formation. Also, the Great Magellanic Cloud is found in this planar structure, which might imply both are connected.

By analyzing data on movement, the team found that some dwarf galaxies had orbits that brought them close to the inner regions of the Milky Way. The gravitational attraction given by the Milky Way Galaxy in these galaxies can be compared to tidal action. It is possible that some of the galaxies studied were disturbed by these tides, which stretched them like streams.

"This can explain the properties observed in some of these objects, such as Hercules and Crater II," said Fritz.

On the other hand, new questions arise. "Over the years, several galaxies have been observed to have strange characteristics that could be potentially due to tidal disturbances by the Milky Way (eg Carina I)," Battaglia said. "However, their orbit does not appear to confirm this hypothesis. Maybe we should postulate that encountering other dwarf galaxies might be the culprit."

The researchers found that most galaxies were studied close to the center of their orbit (the closest point to the center of the Milky Way). Nevertheless, basic physics explains that they must spend most of their time close to their orbital apocenter (the farthest point from the center of the Milky Way). "This shows that there must be more undiscovered dwarf galaxies and hiding at a great distance from the center of the Milky Way," said Fritz.

Dwarf galaxies, besides being interesting in themselves, are one of the few tracers of dark matter that can be used in the most external part of the Milky Way. It is estimated that this kind of material contributes about 80 percent of the total mass of the universe. However, it cannot be observed directly, and detection is difficult. Movement of celestial bodies such as dwarf galaxies can be used to measure the total mass of matter in volume. This is determined by reducing the mass of luminous objects detected to get an estimate of the amount of dark matter. From this data, researchers can conclude that the amount of dark matter in the Milky Way is high, around 1.6 trillion solar masses.

Explore more:
Four newly discovered neighbors of the Milky Way

Further information:
T. K. Fritz et al. Gaia DR2 is the exact movement of a dwarf galaxy in 420 kpc, Astronomy & Astrophysics (2018). DOI: 10.1051 / 0004-6361 / 201833343

Journal reference:
Astronomy & Astrophysics

Provided by:
Instituto de Astrofísica de Canarias

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