The Milky Way is not a uniform and isolated set of stars, it was formed as a result of mergers with several smaller galaxies which eventually dissolved in it. Researchers have identified the remains of one of these objects. Their method could provide information on the history of our galaxy.
In the collective imagination, and it is true in most cases, stars are born in the galaxy they occupy, they only rotate within the galactic disc as do the planets around their sun. However, everything is not as simple and the history of galaxies, including the Milky Way, is much more turbulent. In a study published in Nature Astronomy on July 6, 2020, a team of American astronomers has just proved the existence of a dwarf galaxy which struck our own, and whose vestiges are still found today in the region of the Sun. .
More concretely, we are talking here of some 200 stars, all that remains at first sight of the passage of this galaxy. But they are slightly different from the « native » stars as explained to Numerama Lina Necib, the main author of the study: » These stars rotate with those of the disc of the galaxy, but they do not have the same speed by compared to their neighbors, that’s how we spotted them. It is about a current of star that the researcher of the Walter Burke Institute in California chose to name Nyx, of the name of the Greek goddess of the night. » There are already a lot of gods in astronomy , » she explains , » so I was looking for names of goddesses instead! » «
The stars come from other galaxies
A collision between two galaxies is not an extremely rare phenomenon. It is even this type of event that has largely shaped our Milky Way. Sagittarius, for example, is considered one of the capital architects , whose passage on several occasions would have allowed the birth of millions of stars around us, including the Sun perhaps. But where Nyx differs from other galaxies is that the collision seems to have been parallel to the galactic disc, that is to say that its stars rotate with the disc, which makes it extremely difficult to locate. “ If confirmed , enthuses Lina Necib, it would be the first evidence of a parallel collision . «
But to arrive at this discovery, it was necessary to pass the stars under the magnifying glass. Fortunately, this is possible with the data collected by Gaia. The ESA satellite is considered to be the great cartographer of the Milky Way and has scanned millions of stars to draw the most accurate picture possible of what surrounds us.The Gaia satellite. // Source: Wikimedia / CC / Gaia (cropped photo)
Scientists used this data to separate stars that had been added to the Milky Way from those that were born there. To achieve this, they used the Ananke program , a galaxy simulation system, and trained the program to differentiate the stars according to their origin according to their trajectory or their speed. This is where the authors managed to isolate a little more than 200 stars going in the same direction as the others, but slightly more slowly: about 90 kilometers per second less. Next step, go back in time. Still using the simulator, they went back a little billion years to see how the stars of Nyx moved, and it became obvious that its stars came from elsewhere.
Next steps: spectroscopy and dark matter
At this point, everything suggests that Nyx is the remnant of a dwarf galaxy’s fusion, but there is another possibility. It could be a disturbance of the disc, due for example to the passage of Sagittarius which can modify the direction of the stars. There are indications that this is not the case, particularly the general shape of the current, but to be sure, the authors plan to use the Keck and Magellan telescopes, which have the advantage of being able to perform spectroscopic analyzes. An essential step for Lina Necib: » Thus, we will be able to know the chemical composition of the stars of Nyx, and establish once and for all if they have indeed come from another galaxy or if they have just been shaken in their orbit. Other analyzes could therefore take place. Especially since the authors think they have spotted a second stream of stars which could be linked to Nyx. Called Nyx-2, it could be the remains of a second passage from the same dwarf galaxy in the Milky Way. The properties are roughly the same as those of Nyx stars.
Other research brought by this discovery concerns dark matter. Indeed, Lina Necib is above all a specialist in this strange matter omnipresent in the Universe but still impossible to formally detect with current tools. She has published numerous papers on its distribution and on techniques to detect it. Currently, the theoretical effects of dark matter are visible by looking at the movements of the stars and galaxies, since it is its weight that influences their movements. There are also projects underway on Earth to detect it by looking for particle interactions, but without clear success for the moment, even if some recent results (with the XENON1T dark matter detector) suggest that hope is allowed. .nullIllustris Collaboration
Anyway, if Nyx is the rest of a galaxy, this set of stars has reached the Milky Way with its dark matter, which changes a few things about the distribution of these particles across our galaxy. In another study published last year, the author was already reconstructing the properties of dark matter from satellite galaxies. With Nyx, she has a real laboratory available to test her theory other than by simulations. With further studies, it should be possible to better understand how dark matter is distributed in our galaxy, and therefore to be able to detect it more effectively with terrestrial experiments such as XENON1T.