New data from the Gaia spacecraft reveal the full extent of what appears to be the galaxy’s original core, the ancient stellar population around which the rest of the Milky Way grew.
The foundation of our stellar home
The Milky Way began to form about 12.5 billion years ago, when several huge star clusters and gas clusters coalesced to form a protogalaxy. This amalgamation went through a series of mergers and collisions with other galaxies until it finally acquired its present form.
Many scientists have long suspected that traces of this original protogalaxy can still be found in the center of the Milky Way and an international team of researchers set out to test this assumption. One of the obstacles to finding the original stars of the Milky Way is that, by definition, they must be as old as the galaxy. Also, these stars should be near the center of the galaxy , in the constellation of Sagittarius from our perspective. To do this, they turned to the data archive of the Gaia telescope.
An exercise in finding a needle in a haystack
Specifically, they relied on GAIA data to examine 2 million stars within 30 degrees of the galactic center . They focused on the oldest stars in the core of our galaxy, characterized by an exceptionally low metallicity (concentration of elements heavier than hydrogen and helium), which does not exceed 3% of that of the sun. The researchers then analyzed the distribution of these stars. Distinguishing between stars that were part of the original Milky Way and those captured from smaller merging galaxies is not easy, as the latter can also be very metal-poor. But extrapolating to all the stars blocked by dust or other obstacles, the authors estimate that 0.2 percent of the Milky Way’s mass belongs to this original population.
Thus, having ruled out those luminaries whose orbits pass through the halo of the Milky Way, they found 18,000 ancient stars. The conclusion is that these were part of a protogalaxy whose diameter reached 18,000 light years, and whose mass was 50 to 200 million times the mass of the Sun.
This is the original core of our Milky Way. The protogalaxy formed when several large groups of stars and gas coalesced long ago, before the Milky Way’s first disk, the so-called thick disk, emerged. It is a compact protogalaxy, which means that few things have disturbed it all these thousands of years since its formation.
It is curious, because the analysis of this protogalaxy provides us with data such as the fact that the oldest stars of the proto-Milky Way barely revolve around the center of the galaxy, but rather submerge in and out of it, while the slightly younger stars they show more and more movement around the galactic center.
The authors baptized this latest scientific article: “The poor old heart of the Milky Way” , a headline that has received all kinds of praise on Twitter, calling it “the most poetic title of a scientific article in a long time”.
Thus, even though our galaxy is rotating rapidly today, this work puts on the table that the Milky Way began as a modest protogalaxy whose stars continue to shine today and that we can study to learn a little more about its birth and evolution. initials of our little home in the universe.
Referencia: H.-W. Rix et al. The poor old heart of the Milky Way. arXiv:2209.02722. Posted September 7, 2022. V. Belokurov and A. Kravtsov. From dawn till disc: Milky Way’s turbulent youth revealed by the APOGEE+Gaia data. Monthly Notices of the Royal Astronomical Society. Vol. 514, July 2022, p. 689. doi:10.1093/mnras/stac1267.