The waltz of the small galaxies that surround the Milky Way

The waltz of the small galaxies that surround the Milky Way https://i1.wp.com/www.eresviral.com/wp-content/uploads/2018/11/El-vals-de-las-pequeñas-galaxias-que-rodean-la-Vía-Láctea.jpg?fit=260%2C136&ssl=1

The waltz of the small galaxies that surround the Milky Way







Around the Milky Way there are many dwarf galaxies, which can be thousands or even millions of times less luminous than the Milky Way. Compared to normal or giant galaxies, dwarfs have much fewer stars and, therefore, their brightness is lower.



These small galaxies have been the object of study of an international team, led by Tobias K. Fritz and Giuseppina Battaglia, both researchers from the Institute of Astrophysics of the Canary Islands (IAC) (Spain). Thanks to the data compiled by the space probe Gaia, of the ESA, published in April 2018, they have been able to measure the movement of 39 dwarf galaxies, determining the direction of their displacements and their speeds.



Before the publication of these data from the Gaia space probe, it had not been possible to measure the displacement of 29 of the galaxies that the team analyzed. The researchers found that many of them are moving in a plane known as the vast polar structure ("the great polar structure"). "Previously it was known that in this plane there were many massive dwarf galaxies," says Fritz, the main author of the scientific article, "but now we know that there are also many less massive galaxies here."



Battaglia points out that 'the vast polar structure' is still not fully understood. The concentration of galaxies in their interior is greater than expected by our current understanding of the assembly of dark matter. "In it is also the Large Magellanic Cloud, which may mean that the two are connected," he explains.







Movements of the 39 dwarf galaxies. As a background, the image constructed from point sources of Gaia is shown. In it you can only see the brightest dwarf galaxies, and even then they are barely visible. Galaxies are labeled by their name and the arrows show how they will move in the future in relation to the Galactic Center. The color indicates the radial movement. The blue galaxies approach the galactic center, retreat in red. (Photo: IAC)



When analyzing the data relative to the movement, the team found that several of the dwarf galaxies in the sample had orbits that carry them to the inner regions of the Milky Way. The gravitational attraction exerted by the Milky Way on these galaxies can be compared with the action of the tides. "It is likely that several of the dwarf galaxies studied are disturbed by this tide, stretching like a stream. This could explain the characteristics observed in some of these objects, for example, in Hercules and Crater II, "says Fritz. On the other hand, new questions arise. "Certain galaxies show peculiar characteristics - Battaglia indicates - that could be due to the disturbance of the tides (ex Carina I), but that have orbits that do not seem to confirm this hypothesis. Perhaps we should postulate that these disturbances may have been caused by encounters with other dwarf galaxies. "



The orbits determined allowed us to detect that most of the galaxies in the study are near the pericenter of its orbit (closest to the center of the Milky Way). But basic physics explains that they should be closer to the orbital apocenter (points farther to the center of our galaxy). "This suggests that there must be many dwarf galaxies that have not yet been discovered and that hide at great distances from the center of the Milky Way," says Fritz.



Dwarf galaxies, in addition to being interesting by themselves, are one of the few traces of dark matter that can be used in the outer parts of the Milky Way. It is believed that this type of matter accounts for 80% of all the existing in the Universe. But it can not be observed directly, so its detection is not simple. The movements of objects such as dwarf galaxies can be used to measure the total mass of dark matter in a volume. For this, the mass of those detected luminous objects is subtracted and an estimate of the dark matter present is obtained. From all these data, they could deduce that the presence of dark matter in the halo of the Milky Way is very massive, since it has around 1.6 billion solar masses. (Source: IAC)

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