Astronomers over the moon with ‘a box full of chocolates’

Just over four months after the data from the Gaia satellite became available, University of Groningen astronomy professor Amina Helmi has published her first analysis. The new data provided her with new insights into how our Milky Way evolved. But there is so much more: ‘The data is incredibly rich.’

Our sun is part of the Milky Way, a galaxy containing some 200 billion stars. And there are billions and billions of galaxies in the Universe. But how did they evolve? ‘The main hypothesis is that structures in the Universe have grown from small to large via mergers’, Helmi explains. But the question is where the stars that we see in a galaxy today formed: from gas clouds in the galaxy itself? Or within the structures that coalesced? ‘In the case of the Milky Way, most stars were born in the galaxy, but it is very plausible that the most pristine stars were born elsewhere.’

Helmi has devoted much of her career to investigating the origin of the Milky Way. She does this using a form of ‘stellar archaeology’, where she tries to find the remnants of mergers. These can be identified by their movement: nearly all stars in a galaxy rotate around its centre, but not in exactly the same orbit. If you look closely, you notice smallish groups of stars following the same trajectory. These groups can be what remains of a merged galaxy, their trajectories still bearing the fingerprints of their original path.

Relics

To find such relics, Helmi needs lots of data from lots of stars. Not just the position of stars in the sky, but also their movement, both through the sky and towards or away from us (radial velocity), and chemical composition. This is where the Gaia satellite comes in. Gaia was launched in December 2013. Its mission was to map the position and movement of one billion stars in the Milky Way. Helmi has combined Gaia data with data from other projects: the Hipparcos/Tycho satellite (which measured the position of a set of stars some twenty-five years ago) and the RAVE survey (which was carried out in Australia and measured radial velocity and chemical composition).

‘If stars were measured in all three surveys, it gives us a very complete picture. The first Gaia data release contained information on two million stars that had been observed by Hipparcos/Tycho, and for around 200,000 of these we also have data from the RAVE survey.’ Next, Helmi looked for pristine stars, which can be identified by their chemical composition. ‘The stars from mergers are mostly ancient and pristine in terms of their simple chemical composition. Most stars from mergers end up in the halo of the Milky Way.’ She identified some 1000 of these halo stars and plotted their movement.

Exciting

This is where things got exciting. ‘First, we found that the majority of ancient stars that come from the outer realms of the Galaxy move in an opposite direction to most of the stars in the Milky Way.’ This was unexpected. ‘We thought that satellite galaxies which eventually merged would on average move randomly or in the same direction as our Milky Way.’ Helmi doesn’t know exactly why so many stars move the other way. ‘But the data suggests that at some time in the past our Milky Way merged with a rather large object moving in the opposite direction.’

The next finding related to a large group of ancient stars that spend most of their time in the inner parts of the Galaxy. ‘We used a special algorithm to find clusters of stars with similar trajectories that hinted at a similar foreign origin. And this showed us many such clusters’. The number was consistent with most of these stars coming from mergers. ‘Stars born inside our Milky Way shouldn’t be in structures, in contrast to stars from mergers.’ Her findings will come as a surprise to some colleagues. ‘When I started in this field, most astronomers thought mergers made only a small contribution to the stars in our galaxy. Opinion has shifted but the community is still divided.’

The next step will be to analyze the chemical makeup of the different groups of stars. This will tell Helmi about their origin. ‘These results raise so many new questions. How did these mergers happen, when and why? What did those small galaxies look like? Were they anything like those we see around us today? What does this tell us about the history of the Milky Way? We need to gather more data.’ That is being done right now, by Gaia. A new release of data from even more stars is expected next year. ‘And this time, there will be radial velocity for a much larger set of stars as well, so we will be able to study a much bigger sample!’

Helmi’s analysis is published under the title ‘A box full of chocolates: The rich structure of the nearby stellar halo revealed by Gaia and RAVE.’ Why the reference to chocolates? ‘You find so many things in this data. It is of such high quality. You really have to decide which bits to analyze. Like a box of chocolates, you must pick one, because if you try to eat them all, you’ll be sick.’

Reference: Amina Helmi, Jovan Veljanoski, Maarten A. Breddels, Hao Tian en Laura V. Sales A box full of chocolates: The rich structure of the nearby stellar halo revealed by Gaia and RAVE Astronomy & Astrophysics 30 januari 2017