They discover how the stars of the center of our galaxy were formed

The dense central regions of our galaxy, the Milky Way, are a natural laboratory for the study of rapid star formation, a common phenomenon in galaxies, especially in the first billion years of the history of the universe. However, the high density of stars in this region has prevented its study in detail.

Now, for the first time, it has been possible to reconstruct the history of star formation in the galactic center. Thanks to the data provided by the Galacticnucleus project, led by the Institute of Astrophysics of Andalusia (IAA), dependent on the Higher Council for Scientific Research (CSIC) in Spain, it has been observed that stars did not form in massive and tightly bound clusters but in isolated stellar associations that spread out over the last few million years.

“Our Milky Way is not a very productive galaxy. As a whole, the stars that form in a year amount to no more than a few solar masses,” says Francisco Nogueras-Lara, a researcher at the Max Planck Institute for Astronomy (MPIA) in Germany who heads the work. “In contrast, the much more productive so-called starburst galaxies give rise to tens or even hundreds of solar masses of stars per year over episodes lasting a few million years. More generally, about ten billion years ago, that kind of high productivity seems to have been the norm among galaxies,” he adds.

Although it might seem that our unproductive galaxy is not good for studying starbursts, in the 1300 light-year regions around the central black hole, star formation rates over the last hundred million years have been ten times higher than the average. That means that the core of our galaxy is as productive as a starburst galaxy, or the hyperproductive galaxies of ten billion years ago.

The center of our galaxy, however, imposes two observational challenges: on the one hand, it is hidden behind copious amounts of dust that impede its view, which was resolved by resorting to observations in the infrared, in millimeter waves and in radio. On the other hand, there is the observational challenge of the very crowding of the stars, which prevents them from being distinguished and which only made it possible to discern very massive and bright individual stars. Thus, until recently only 10% of the stellar mass of the galactic center had been detected, so it was unknown what the stars in the center of the Milky Way were like.

To resolve this issue, the Galacticnucleus project was developed, led by researcher Rainer Schödel from the Institute of Astrophysics of Andalusia. This is a survey that made use of the HAWK-I infrared camera on the VLT (Very Large Telescope) of the European Southern Observatory and that covered a total area of ​​64,000 square light-years around the galactic center. The project has made it possible to obtain individual data for three million where previously only a handful of stars had been mapped.

These new data made it possible to confirm that the region of the galactic center known as Sagittarius B1 is different, as well as to study its stars in detail. “Although even their high-resolution study only allowed us to distinguish giant stars, we were able to reconstruct the luminosity of each star and the intrinsic brightness, corresponding to the amount of light that a star emits per unit of time,” says Nogueras-Lara.

Sagittarius B1 region in the central sector of the Milky Way. (Photo: Galacticnucleus)

Particularly interesting was the statistical distribution of the luminosity of these stars. For stars that form at the same time, that luminosity distribution changes over time in a regular and predictable way. In turn, this allows the approximate history of star formation to be deduced. The research team found that, indeed, there were several phases of star formation in Sagittarius B1: an older population that formed between two billion and seven billion years ago, and a large population of much younger stars, with just ten million years or even younger.

Although the newly discovered young stars are found in a specific region, their example suggests a more general mode of star formation in the galactic center: stars that are not born in massive clusters, but rather in more dispersed stellar associations, which then dissolve as they they orbit the galactic center on scales of several million years. That would explain why young stars in the galactic center can only be found by high-resolution surveys, such as the present work.

The presence of the oldest population of stars in Sagittarius B1 is also interesting. In the innermost regions of the galactic center, stars older than seven billion years exist, but virtually no stars in the middle age range of two billion to seven billion years. This could indicate that star formation in the center of the Milky Way began in the innermost region and then spread to the outer regions. This inside-out mechanism for building the so-called nuclear disk, a disk made up of stars surrounding the galactic center, had already been observed in other galaxies. The new results indicate that the same thing could be happening in the central region of our galaxy.

The study is titled “Detection of an excess of young stars in the Galactic center Sagittarius B1 region”. And it has been published in the academic journal Nature Astronomy. (Source: Silbia López de la Calle / CSIC)