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Catching stars to reveal the secrets of the darkness: ERC Consolidator for Elena Maria Rossi

The centre of our Galaxy is so dark and dense that it is almost impossible to observe what is inside. By catching the rare hypervelocity stars that are ejected from it, Elena Maria Rossi aims to unveil the mysterious environment around the supermassive black hole inside. But she’ll also be solving another mystery. For this extent, Rossi receives a €2 million ERC Consolidator Grant.

Dr Elena Maria Rossi

The Galactic Centre

Supermassive black holes, enormous energy levels and the densest stellar systems in the Universe. According to Rossi, galactic centres are the most fascinating objects in space and our own Galactic Centre the best chance to study them. However, because it is so crowded with dust and stars, it’s challenging to observe directly what’s going on inside it, she explains. ‘You have to be creative and find complementary ways to get your information.’ Rossi’s method is a great example: she searches for hypervelocity stars that are emitted from the Galactic Centre into the much emptier galactic halo. ‘I’m investigating what’s coming out of it, to learn more about what’s going on inside!’ 

Superfast stars

Hypervelocity stars travel through the galactic halo with a velocity high enough to leave the galaxy forever. Rossi: ‘All other stars in the Galaxy are bound, but these ones somehow received a huge amount of kinetic energy to be in escaping orbits. The mystery is how they reach such velocities.’ 

Scientists expect that these stars are being kicked out of the Galactic Centre by black holes of different sizes. ‘Black holes are the only objects that can transfer so much momentum,’ Rossi explains. ‘By looking at the spatial and velocity distributions of these hypervelocity stars, I can find out how many black holes and stars there are in the immediate vicinity of the supermassive black hole and how they’re distributed in space and mass. This will help me understand how our Galactic Centre —and therefore galactic centres similar to ours —formed and how stars and black holes interact.’

Dark matter

On top of that, Rossi noticed that her stars could solve another mystery: the extent and distribution of dark matter, the main ingredient of the Galactic halo. ‘The ejected stars will fly through the halo, and their trajectory will bear the imprint of the intervening dark matter. From these trajectories I can derive the mass profile of dark matter.’ This way, Rossi is tackling two questions that don’t seem connected in the first place. ‘It is wonderful to see that my stars are connecting these two fields of science.’

The wide net called GAIA

Until recently, Rossi’s experiment could not be executed, because hypervelocity stars are very rare and a wide net is needed to catch them. However, this net now exists and is called GAIA, a whole sky survey that builds the most detailed map of the stars in our halo so far. It will contain over a billion stars. 

For every one million normal stars, there will be only one hypervelocity star. The challenge is to find these rare objects in this huge database. Rossi: ‘I need GAIA, because I need all those one billion stars in order to find a few thousand hypervelocity stars. I have designed efficient ways to data mine this huge catalogue and to pick out the stars that I need.’

Not-so-worst-case scenario

Rossi will use her ERC grant to hire two postdocs and two PhD candidates. ‘I feel very fortunate. With their help, I will advance more quickly on this very exciting quest.’ She hopes to find at least a couple of hundred hypervelocity stars. But also the worst-case scenario: not finding any hypervelocity stars at all, would be an exciting outcome. ‘That would mean that our understanding of the Galactic Centre is faulty! Then we need to go back and change our hole theory. Would there be less solar mass black holes than we thought? And why? A whole new set of problems would unroll. So by either means: this project will be very exciting!’

See also the press release by ERC: Consolidator Grant recipients 2020

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