Milky Way: its thick disk formed two billion years earlier than previously thought

Coming out of big Bangspecifically after theepisode from cosmic radiationthe observable cosmos contains atoms ofhydrogen and D’heliummany of their isotopes and a bit of lithium but no trace of heavier elements such as nitrogen, carbon and theoxygen. The first stars which will form and ultimately bring out theUniverse from dark ages can therefore to an excellent approximation be considered as containing no metals and with a metallicity zero as they say astrophysicists in their language, that is to say devoid of elements heavier than helium.

According to standard cosmological modelit must first form small galaxies which will merge to give large galaxies which will quickly grow by accreting gas channeled by filaments of black matter cold for the most part according to the new paradigm of the evolution of galaxies which makes these filaments play a fundamental role and only secondary to all the mergers of galaxies, contrary to what we still thought more than a decade ago.

It is still believed, however, that essentially a spiral galaxy initially forms much like a protoplanetary disc with a cloud of rotating matter that gravitationally collapses. The centrifugal force opposing this contraction perpendicular to the axis of rotation, the initial cloud flattens.

A galaxy whose chemistry is changing

The theory of stellar structure and evolution demonstrates that stars evolve all the more quickly as they are massive. Beyond 8 masses solar cells, they will explode in supernovae after synthesizing heavy nuclei up to iron, enriching an interstellar medium in galaxies where new stars will be born. There is therefore a chemical evolution of stars and galaxies that allows dating. Thus, a red dwarfpoor in metallic elements beyond lithium, will indicate a star born more than 10 billion years ago while a yellow dwarf like our Sunricher in heavy elements, will be a few billion years younger.

The evolutionary theory stellar also predicts that the size and brightness of a star will vary with its age for a given mass. Thus, some stars will see the thermonuclear fusion into heavier elements of the nuclei in their cores stop while it will continue in an envelope around this core while the star is in a short phase, called subgiant. red, before finally becoming a red giant.

In short, by analyzing the chemical composition and the brilliance of a star, we can date its age and this is what astrophysicists have undertaken to do with precision for approximately 250,000 stars in the Milky Way with the help of Large Sky Area Multi-Object Fiber Spectroscopic Telescope (that is to say Telescope multi-object spectroscopy at optical fiber large field, abbreviated Lamost) – a Chinese optical telescope four meters in diameter.

Maosheng Xiang and Hans-Walter Rix of the Max-Planck Institute for Astronomy in Heidelberg, Germany, together with their colleagues combined the data from Lamost with data on the brightness and position of these same stars in the set of I’Early Data Release 3 (EDR3) from satellite Gaia of the’ESA dedicated to astrometry. The results of this combination have just been published in the famous journal Nature.

A galactic archeology readable in strata of stars

Gaia’s astrometric data concerns fine measurements of positions and gears of about 1.5 billion stars in the Milky Way. The latest were published in December 2020 and Lamost provided those of 9 million stars in 2021. Only some of these stars were in the red subgiant phase, the 250,000 studied with Lamost, but in the end the structure and dating of the halo and the disk of the Milky Way have been clarified.

Its structure is twofold. First there is a thin, dense disk, the thickness of which is about 2,000 light years and where are the arms of our Galaxy, where the stars are mainly born today. He himself is immersed in a disc more diffuse whose thickness is about 6,000 light-years.

The thin disk contains most of the stars we see as the hazy band of light in the sky nocturnal what we call the Milky Way. The thick disc is more than triple the height of the thin disc but smaller in radius, containing only a few percent of the Milky Way stars in the solar neighborhood.

the stellar halo is itself made up of an almost spherical population of stars andglobular clusters surrounding the Milky Way. The stars there are old with low metallicity, just like in the case of the central bulge of the Milky Way. There is no dust there unlike the disc. This halo itself is immersed in a hot plasma halo, which in turn should be enveloped by a dark matter particle halo according to the standard cosmological model.

A different galactic timeline

It now appears, and this is the surprise according to an ESA statement, that there would have been two distinct important phases in the history of the Milky Way.

In the first phase, beginning just 0.8 billion years after the big Bang and so about 13 billion years ago, the thick disc is already there contrary to what we thought and it begins to form stars there. But about 2 billion years later, after an acceleration in the rate of star formation, a large peak occurs and is explained by the merger between the young Milky Way and a dwarf galaxy, a galaxy called Gaia-Enceladus. This name comes from Greek mythology: Enceladusone of the Giants, son of Gaia (Earth) and Ouranos (Heaven), during the Gigantomachy, was put out of action by Athena and buried under Mount Etnacausing since earthquakes and eruptions.

This fusion was certainly comparable to a mass thrown and hitting a pool, in the self-gravitating star fluid of our Galaxy, and it was thought that it was this which had, in some way, “heated” the star gas of the thin disk, causing it to evaporate and expand to form the thick disk.

It was also at this time that the stellar halo would have formed at the same time and for the same reason.

But, in the scenario proposed now, it was only after this merger that the thin disk would have been born and star formation in the thick disk would have continued until its gas content was largely depleted around 6 billion years after the Big Bang. During this time, the metallicity of the thick disc would have increased by more than a factor of 10.

As a bonus, the astrophysicists determined that the metallicity in this disc was relatively uniform, implying that turbulent mixing processes provided efficient transport and mixing of newly formed elements released into the interstellar medium by supernova explosions.

This scenario is perhaps that of many large spiral galaxies. We will perhaps find out with the observations that will be made in the decade to come by the James Webb Space Telescope.