University of Groningen astronomers have found relics of merger occasions in the Milky Way halo. Five small teams of stars seem to symbolize mergers with smaller galaxies, whereas an enormous ‘blob’ comprising lots of of stars seems to be the remnant of a giant merger occasion. These outcomes had been printed in the Astrophysical Journal Letters on 12 June.
The examine relies on the latest Gaia Data Release 2. This supplied the astronomical group with correct info on the place and motion of hundreds of thousands of stars, largely in the Milky Way. Ph.D. pupil Helmer Koppelman is a part of the analysis group of Amina Helmi, professor of dynamics, construction and formation of the Milky Way, who has been concerned in the Gaia mission virtually from its inception. He began analyzing the data proper after the discharge and printed a preprint of the article simply eight days later. This has now been formally printed.
“Our aim is to study how the Milky Way has evolved,” says Koppelman. The concept is that smaller galaxies merge to kind bigger ones. “One of the questions is whether a lot of small galaxies merge, or a few large ones.” As most stars in the Milky Way’s halo—the spherical cloud of stars surrounding the principle disk and bulge of our galaxy—are regarded as remnants of merger occasions, Koppelman and his colleagues targeted on halo stars in the Gaia data.
“We collected information from stars within 3,000 light years of the sun, as the accuracy of the position and movement is highest for stars that are near us,” Koppelman explains. The first step was to filter out the celebs from the Milky Way disk. “These stars move around the centre of the disk, so are easily identified.” What remained had been about 6,000 halo stars.
By calculating their trajectory, Koppelman was in a position to determine stars with a shared origin. “We discovered five small clusters which we believe are remnants of five merger events.” However, most of the remaining stars additionally appeared to have a shared historical past. “These stars form a huge “blob” with a retrograde movement compared to the disk. This suggests they are the result of a merger with a large galaxy. In fact, we believe that this merger event must have remodelled the disk in our Milky Way.” A extra detailed examine of the character of this merger is now underway. “At this point in time, we can say that our Milky Way was shaped by a massive merger event and some smaller mergers.”
Koppelman additionally seemed for stars belonging to the “Helmi stream,” which is known as after his Ph.D. supervisor who recognized it again in 1999 because the remnant of a merger occasion. “Up to now, fewer than 20 stars belonging to the Helmi stream had been identified. The Gaia data has added over 100 new stars.” Further evaluation ought to make clear the character of the galaxy that produced this stream. “We will even be taking a look at stars past three,000 light-years to find extra members of the completely different streams we recognized. Together with simulations of galaxy evolution, this could give us thrilling new insights into the evolution of the Milky Way.”
Stars in the halo of the Milky Way often travel in groups
Helmer Koppelman et al. One Large Blob and Many Streams Frosting the close by Stellar Halo in Gaia DR2, The Astrophysical Journal (2018). DOI: 10.3847/2041-8213/aac882