Identified the ‘old’ heart of the Milky Way

A map of metal-poor red giant stars in the galactic center. (H.-W. Rix/MPIA)

A group of astronomers has managed to identify the “poor old heart of the Milky Way”, a population of stars left over from the earliest history of our home galaxy. For this feat of “galactic archaeology”, researchers at the Max Planck Institute for Astronomy (MPIA) analyzed data from the latest version of ESA’s Gaia mission, using a neural network to extract the metallicities of two million bright giant stars. of the inner region of our galaxy. The detection of these stars as well as their observed properties corroborate cosmological simulations of the earliest history of our galaxy, the MPIA reports in a statement.

Knowing that their analysis had to reach the central regions of the Milky Way, the team looked specifically at the red giant stars in the Gaia sample. Typical red giants are about a hundred times brighter than subgiants and easily observable even in the distant central regions of our galaxy. These stars also have the added advantage that the spectral features that encode their metallicity are comparatively conspicuous, making them especially well-suited for the kind of analysis astronomers were planning. For the analysis itself, the astronomers turned to machine learning methods. In this particular case, the neural network was trained using selected Gaia spectra as input: Gaia spectra for which the correct answer, metallicity, was already known from another study (APOGEE, high-resolution spectral observations as part of the Sloan Digital Sky Survey [SDSS]). The internal structure of the network was adapted so that, at least for the training set, it could reproduce the correct metallicities. The scientists trained their neural network with only half of the APOGEE data. In a second step, the algorithm was put to the test with the rest of the APOGEE data, with spectacular results: the neural network was able to deduce precise and exact metallicities even of stars it had never encountered before.

Finally, they applied the algorithm to their complete data set of Gaia’s red giant spectra. With the results, the researchers had access to an unprecedentedly large sample of precise metallicities, consisting of 2 million bright giants from the inner galaxy. With that sample, it was relatively easy to identify the ancient heart of the Milky Way, a population of stars that lead author Hans-Walter Rix has dubbed the “poor old heart,” given its low metallicity, inferred age, and central location. . On a map of the sky, these stars appear to be concentrated around the galactic center. The distances conveniently provided by Gaia (via the parallax method) allow a 3D reconstruction showing those stars confined within a comparatively small region around the center, approximately 30,000 light-years across. The stars in question complement co-author Maosheng Xiang and Rix’s earlier study of the Milky Way’s adolescence: They have just the right metallicity to have given birth to the most metal-poor stars that later formed the thick disk of the Milky Way. Milky Way. Since that earlier study provided a timeline for the formation of the thick disk, this makes the ancient heart of the Milky Way older than about 12.5 billion years.

For the small subset of objects for which APOGEE spectra are available, it is possible to go one step further: These spectra provide additional properties of poor old-hearted stars in this subset, namely the abundance of elements such as oxygen, silicon and neon. These elements can be obtained by successively adding alpha particles (helium-4 nuclei) to existing nuclei in a process called “alpha augmentation”. Their presence in such quantities indicates that the first stars obtained their metals from an environment in which the heavier elements were produced on comparatively short time scales through the supernova explosions of massive stars.