The evolutionary history of the Milky Way determined in more detail than ever

The Milky Way, the whiteish strip of light which is prominent in both the summer and the winter skies, is the densest part of the disc of the Galaxy which we are inside. However, over very long periods it has not always looked the same, and its evolution is a challenge to current astrophysics.

To study this evolution, ESA’s Gaia mission is measuring the luminosities, positions, motions, and the chemical composition of a large number of individual stars in our Galaxy.

Combining apparent luminosity with measured distance, astronomers at the IAC have found the intrinsic luminosity of 24 million stars within a sphere of 6,500 light years around our Sun. Comparing their luminosities and colours with accurate models of stars they have obtained the most detailed evolutionary history of the Milky Way up to now.

Irregular rate

“We might well have expected that the Milky Way did not form stars at a constant rate throughout its history, but we didn’t expect such well defined periods of great activity”, comments Tomás Ruiz Lara, an IAC astrophysicist and the first author of the article.

Some 13 billion years ago star formation was violent and sustained, but the rate at which stars formed declined gradually as time passed.

However, superposed on this simple behaviour, there were dramatic episodes of star formation, during which the rate grew to four times the normal value. The first took place some 5-6 billion years ago, followed by others, 2 billion, one billion, and one hundred million years ago. But it wasn’t known what could provoke such violent events in a system as massive as our Galaxy.

For an answer to this question we need to know that the Milky Way, although in a relatively empty zone of the universe, is not completely isolated: together with our neighbour the Andromeda galaxy (M31) and tens of much smaller galaxies (called dwarfs) which go round it in orbit, it is part of the so-called Local Group.

Among the nearby dwarf galaxies, the Sagittarius dwarf is prominent, and currently is in full interaction with our Galaxy. But this is not happening only just now, complex simulations suggest that already some 5 to 6 billion years ago Sagittarius made its first approach to the Milky Way. This approach was repeated 2 and 1 billion years ago, coinciding exactly with the star formation events revealed in this study (as well as in the Sagittarius galaxy itself).

“Everything indicates -explains Carme Gallart, an IAC researcher and a member of the team- that these interactions between the two systems were able to stimulate the formation of new stars in our galaxy, drastically affecting its evolution. These results question some of the current models of star formation in galaxies, and put constraints on future theoretical studies”.

An unexpected implication of this work is to situate our Solar System in its context. The Solar System formed some 4,700 billion years ago, from the collapse of a large cloud of gas and dust. “It is possible -comments Tomás Ruiz Lara- that our Sun was one of the many stars formed some 5,000 billion years ago as a consequence of the interaction between our Galaxy and the Sagittarius galaxy. It could be that we are witnessing one of the key astronomical events which gave rise to the world as we know it today”.

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