By Manuela Zoccali, Millennium Institute of Astrophysics
Our Galaxy, the Milky Way, is a giant spiral containing, in its center, a region very dense of stars, called the bulge. This region includes approximately 1/4 of the stellar mass budget of the Milky Way, while the other 3/4 are in the disk, where spiral arms are located. Being older than disk stars, bulge stars were present during the early phases of formation of the Galaxy.
For this reason they keep, both in their surface and in their orbits, some traces of this process, and can help us understanding how did occur it.
At MAS we study this kind of stars within the project called VISTA Variables in the Vía Láctea (VVV) https://vvvsurvey.org/ that is constantly monitoring an area of 20×15 square degrees of the bulge. We also follow up the study of some of these stars using spectroscopy. Stellar spectra allow us to measure chemical abundances on the surface of stars, and their velocities along the line of sight.
In this study we obtained chemical abundances and velocities for a sample of 5000 stars, spread in 26 fields across the galactic bulge.
The results demonstrate that the stars containing little iron on their surface (those that were born earlier) have a spheroidal spatial distribution, with a high concentration towards the center. Conversely, the stars whose surface is rich in iron (belonging to a second generation) show a rectangular projected density on the sky, typical of the bars of several external galaxies, when seen edge on. The large difference in the spatial distribution of the two components suggests two different formation scenarios. Moreover, the stars in each of the two components have different velocity distributions, confirming the above conclusion.
It is the first time that a study of this kind reaches so close to the galactic center, with such a large sample of stars in different fields. This allowed us to construct, for the first time, the detailed density maps shown in the figure.