By Julio Carballo – Bello, MAS´ and UC Astrophysical Institute postdoctoral researcher 

How was the Universe formed? And the galaxies? What are the similarities between the processes that led to the formation of the Milky Way and those that originated the 2 trillion galaxies discovered so far? These questions belong into the area of astrophysics known as Cosmology. We search for answers by using all the observational evidence gathered with our instruments.

One of the most important challenges for the researchers in that scientific discipline is to explain, resorting to a single theory, the formation and evolution of galaxies, from the distant ones to our own home, the Milky Way. Numerical simulations, involving all the physics we deem relevant, show that complex galaxies such as our own were formed via the accretion and assimilation of smaller galaxies which, little by little, are devoured by the larger ones. In this process of galactic cannibalism, the less massive galaxies are destroyed and its components become part of our Galaxy.

As in every crime, researchers hope to find some sort of clue that will lead them to understand what happened billions of years ago. The simulations predict that the galaxy that has been a victim of the Milky Way’s appetite should leave a trail of stars called tidal stream. And, in fact, thanks to surveys of large areas of the sky in the last two decades, the Galactic archaeologists have been able to demonstrate that these stellar streams do exist. Moreover, if we are able to decode the information contained in them, we will be able to find out where and when they formed, the chemical composition of their progenitor galaxies, and their orbits.

Together with the stars, the assimilated dwarf galaxies could also contribute with their globular clusters, groups composed of hundreds of thousands to millions of stars concentrated in a very small volume. Therefore, of the about 150 globular clusters in the Milky Way, an as yet undetermined number of them were possibly born in other galaxy and later accreted by our own. Recently, we have tried to confirm the hypothesis that Whiting1, a small and poorly studied cluster, was formed in the Sagittarius dwarf galaxy and was later assimilated by the Milky Way. For this purpose, we observed with  VLT (“Very Large Telescope”) a sample of 100 stars in the surroundings of the cluster and derived their velocities with respect to the Sun from the analysis of their spectra.

Our results show that a very important fraction of the observed stars did not have velocities compatible with their positions in the Milky Way, suggesting that they might belong to a cannibalized galaxy. Our main suspicion was that this cluster was formed in the interior of Sagittarius and indeed, when we compared the velocities measured by us with those predicted by the models for that dwarf galaxy, we confirmed that these stars belong to the tidal stream of Sagittarius, with a radial velocity identical to that of Whiting1. In conclusion, we were able to prove that this globular cluster is at the same distance and has the same velocity as the remnants of its parent galaxy, the Sagittarius dwarf galaxy.

Main image: Artistic view of the Sagittarius tidal stream generate by the accretion of the dwarf galaxy by the Milky Way. The approximate position of the Sun is indicated. Credits: NASA/JPL-Caltech/R. Hurt (SSC)

Position of the stars observed (coloured symbols) around the globular cluster Whiting1.

Histogram showing the number of observed stars for a specific velocity with respect to the Sun (black solid line). When we compare these numbers with the predicted velocities for the Sagittarius tidal stream (yellow and blue histograms), we find that 2 of the 3 components detected have the same velocity. The vertical line indicates the radial velocity of Whiting1.