There are times when researchers are surprised by what they find. In other ocasions the surprise comes when such findings are compared with what has been reported before in the literature. There are even findings that require more and more testing as they seem “too striking to be true”. This is the story of how all the above can merge in a single piece of research. But let’s start from the beginning.
As researchers interested in unveiling the mysteries that our Universe is hiding from us, we aim at the big questions: “What is dark matter?”; “Why is the Universe expanding and accelerating?”; “What is the origin of life?”. Our question was as big as those: “How did our Galaxy, the Milky Way, form and evolve?”. A question that roots deeply into our very origin as species. Some 4.7 billion years ago, in a galaxy among the myriad of them composing the whole Universe, our Solar System emerged, and with it the seed of what we now call life.
We are currently in an unprecedented position to answer such question. As you read these lines, the Gaia space mission keeps observing and characterising the stars in the Milky Way. In its latest data release, Gaia made publicly available to the scientific community photometric measurements and positions for more than 1.7 billion stars, paralaxes and proper motions for about 1.3 billion stars, among other extremely valuable information. It was the combination of photometry, positions and paralaxes (as distance estimators) that allowed us to, via colour-magnitude diagram (CMD) fitting techniques, study the stellar populations of the Milky Way as never before. In a first episode of this series (Gallart et al. 2019, NatAst, 3, 932), we were able to unveil the early chain of events shaping our Galaxy. Here, we have been able to determine when and how stars have formed in the disc of the Milky Way since its very beginning to nowadays.
Strikingly, our Galaxy has formed its stars in a highly episodic way, with important star forming events around 5.7, 2 and 1 billion years ago (see above) and some hints of a recent starburst. Previous works had already shown a complex formation history of the Milky Way, with a clear increase in the star formation activity in the last 4 billion years, but none had studied such a large volume or was as detailed as this new determination. Why was the star formation so concentrated in such precise moments? We then set out on a search for possible drivers of star formation affecting the Milky Way at those times. The solution turned out to be the Sagittarius dwarf galaxy (Sgr). Since the 1990s, Sgr has been considered one of the main satellites of our Galaxy, with recurrent close approximations disrupting Sgr. Surprisingly, the proposed approaches coincided with the star forming events that we found. The icing on the cake came when we compared our results with studies of the star formation history in Sgr itself, both in the Saggitarius stream (debris from Sgr main body) and near its core (the globular cluster M54). Sgr experienced similar bursts of star formation at precisely the same moments (5.7, 2 and 1 billion years ago) than the Milky Way. The interpretation was straightforward: Sgr, apart from one of the main dynamical agents in the history of the Milky Way, significantly affected the way our Galaxy formed its stellar content. This was quite unexpected, given the small mass of Sgr compared to the Milky Way. The timing of these global star formation enhancements, and the precisely known characteristics of the interaction parameters of Sgr with the Milky Way, will be able to set important constraints on simulations of interaction-induced star formation in galaxies.
Little by little we are completing the puzzle of the formation and evolution of our Galaxy. Small systems collapsed and merged long ago to form the progenitor of our Milky Way. Around 10 billion years ago, such progenitor got hit by an important stellar object (a quarter of its mass) that has been named Gaia Enceladus, heating up thick disc stars to a halo configuration. Stars continued forming in a thick disc configuration for a further two billion years until a transition to a thin disc happened. Then Sgr came into play, repeateadly affecting the way stars moved in the Milky Way and triggering the formation of new stars 5.7, 2, 1 billion years ago and now, when Sgr crossed (as it is currently crossing) the disc of our Galaxy. Still, there are plenty of details to be unveiled in this puzzle, including a question directly inspired by this work: Was the formation of our Solar System, 4.7 Gyr ago, triggered by the first approach of Sgr to the Milky Way? More to come.