Free Astronomy Magazine September-October 2019

21 SEPTEMBER-OCTOBER 2019 SPACE CHRONICLES A rtist impression of the Milky Way. [Gabriel Pérez Díaz, SMM (IAC)] ten billion years ago there was a vio- lent collision between the more mas- sive system and Gaia-Enceladus. As a result, some of its stars and those of Gaia-Enceladus were set into chaotic motion, and eventually formed the halo of the present Milky Way. After that, there were violent bursts of star formation until six billion years ago, when the gas settled into the disc of the Galaxy and produced what we know as the “thin disc”. “All the cosmological predictions and observations of distant spiral galaxies similar to the Milky Way in- dicate that this violent phase of merging between smaller structures was very frequent” explains Matteo Monelli, a researcher at the IAC and a co-author of the article. Now we have been able to identify the speci- ficity of the process in our own Galaxy, revealing the first stages of our cosmic history with unprece- dented detail. same time, what differentiates one from the other? “The final piece of the puzzle was given by the quantity of ‘metals’ (elements which are not hydrogen or helium) in the stars of one component or the other” ex- plains Tomás Ruiz Lara, an IAC re- searcher and another of the authors of the article. “The stars in the blue component have a smaller quantity of metals than those of the red com- ponent” . These findings, with the addition of the predictions of simu- lations which are also analyzed in the article, have allowed the re- searchers to complete the history of the formation of the Milky Way. Thirteen billion years ago, stars be- gan to form in two different stellar systems which then merged: one was a dwarf galaxy which we call Gaia- Enceladus, and the other was the main progenitor of our Galaxy, some four times more massive and with a larger proportion of metals. Some first author of the article published in the journal Nature Astronomy . Previous studies had discovered that the Galactic halo showed clear signs of being made up of two distinct stellar components, one dominated by bluer stars than the other. The movement of the stars in the blue component quickly allowed us to identify it as the remains of a dwarf galaxy (Gaia-Enceladus) which impacted the early Milky Way. How- ever, the nature of the red popula- tion and the epoch of the merger between Gaia-Enceladus and our Galaxy were unknown until now. “Analyzing the data from Gaia has allowed us to obtain the distribution of the ages of the stars in both com- ponents and has shown that the two are formed by equally old stars, which are older than those of the thick disc,” says IAC researcher and co-author Chris Brook. But if both components were formed at the !