Study seeks explains process of formation of the Milky Way disk

Uthe international team discovered with great precision how the disk of the Milky Way became more flat, a process that, according to the investigation, took place in two distinct stages.

The study was attended by researchers from the Spanish universities of Laguna and Granada, the University of Victoria (Canada), the Carlos I Institute of Theoretical and Computer Physics, the National Institute of Astrophysics (INAF, Abruzzo Astronomical Observatory) and the National Institute of Nuclear Physics (both of Italy).

In a statement released on Monday, the Canary Astrophysics Institute (IAC) announced that, based on data from the Gaia satellite and the precise ages determined by the ChronoGal project, the researchers identified two phases, a very fast “finishing” process, which ended about 10.8 billion years ago, and another, much slower, which lasts until today.

The IAC pointed out that most of the stars in the Milky Way are distributed by two disks, one thin and young, and another thicker and older.

The question of how these records formed has been one of the great challenges of galactic astrophysics.

So far, the observations were compatible with two explanations. In the first, the stars are always born on a thin disc and, over time, encounters with clouds of gas, spiral arms or the galactic bar end up “swinging them” away from the central plane.

In the second explanation, stars are born with increasingly thin distributions, as the gas from which they form is settling.

Distinguishing between them required knowing precisely the age of the stars, something very difficult to obtain, especially in the case of the older ones.

With this study, published in The Astrophysical Journal Letters, researchers showed that the younger the stars, the closer they are to the plane, but not at a constant pace.

There is a turning point about 10.8 billion years ago, detailed the astrophysical center.

Before, the disc flattened very quickly, in just 2 billion years, its thickness went from over 4,100 light-years (1.26 kpc) to about 1,600 light-years (0.5 kpc).

Then thinning became very gradual, reaching a thickness of about 400 light-years (0.1 kpc) for newly formed stars.

If stars were always born on a thin disc and moved away from the plane bit by bit, the disc would have become thicker slowly and continuously. However, the data shows something different.

Thus, such a rapid change cannot be explained by a slow and gradual process, especially in a young galaxy that often collides with other galaxies.

On the other hand, the scenario is consistent with the model known as ‘upside-down’: the disc is not born thin, but thick, because the gas from which the stars form is very agitated and becomes thinner as this gas calms and stabilizes.

According to the authors of the study, the thick disc was formed in this first agitated stage, while the thin disc appeared later, in a much quieter phase.

“ This result solves a classic problem of astrophysics”, highlighted David Mirabal, a researcher at the IAC and the University of La Laguna (ULL) and main author of the study.

“ In the simulations, the two models of disc formation were possible, but there was no observational evidence to determine which one occurred in the Milky Way. This is the first clear observational evidence that points to one of the two models regarding the initial phase of the Galaxy. And this was possible thanks to the great precision we obtained for the more remote ages ", highlighted.