Smoking gun

Galaxies grow by giving birth to new stars, but Amina Helmi found evidence to support the theory that parts of the Milky Way have also grown by merging with other galaxies. She had to wait for the Gaia space mission to produce data before she was able to make this discovery.

In Professor Amina Helmi’s office, the plastic wrapper in which Nature was delivered still lies on the conference table, a silent witness to these events. At the end of October, this leading journal featured her article about a major discovery made by her research group – at an early stage in its existence, the Milky Way had absorbed another, extremely large galaxy. Without making a fuss about it, Amina Helmi describes this discovery as ‘great’. ‘One of the really big questions in astronomy is how galaxies evolve. There were no galaxies at the time of the Big Bang, so how was a galaxy like the Milky Way created? We now have a better understanding of that.’

Fingerprints

An animation of the newly-discovered merger makes it all look so simple. The gravity of a large cloud of stars, the Milky Way, attracts a smaller cloud of stars and both merge to form a single cloud. Prof. Helmi thinks there is no need embellish it any further. While the merger shown in the animation is all over in thirty seconds, the real thing actually took two to three billion years. ‘Maybe a little quicker than that. Our next job is to model that process.’ As you listen to Amina Helmi enthusiastically explaining how you can see which stars did or did not originate in the Milky Way, it’s a bit like following the plot of an episode of Crime Scene Investigation. ‘A star’s movement and chemical composition betray its origins. They are a bit like fingerprints. Most of the stars in the Milky Way move clockwise around the centre of the galaxy, but the stars from that other system move in the opposite direction. That, by itself, is evidence for something out of the ordinary. Then there is the ‘relative abundance’ of chemical elements like titanium, magnesium or calcium in these stars, which is substantially different to that of other stars in the Milky Way. That is the real ‘smoking gun’. It enables us to say that those stars really do come from a different system.’ Case closed.

Swallowed by the Milky Way

It’s worth noting that the discovery of this ancient merger hardly came as a surprise. In 2000, while still a PhD student, Prof. Helmi had presented evidence for a series of stars from yet another system that had been swallowed by the Milky Way. ‘My dissertation showed where we need to look to find evidence of galactic mergers. Remember, at that time, it was far from clear that any such mergers had ever taken place. That insight into how the merger process works prompted me to explore the catalogue of the Hipparcos mission, Gaia’s predecessor. There, I found evidence of two ribbons of stars from a small system that originated outside the Milky Way.’ At a time when almost everyone in the field was convinced that all the stars in our galaxy had been born here in the Milky Way, Amina Helmi discovered the first interloper from elsewhere. ‘I had previously developed models showing why we hadn’t discovered anything until then, so it was immediately clear to me that mergers had really taken place. The merger I discovered did not involve many stars, but that was all I could get from the available data. Since then, a great deal of effort has been invested in the search for other mergers, but it was clear to me that we had no choice but to wait for Gaia.’

Gaia

‘Gaia’, a European Space Agency satellite, was launched in 2013. What makes Gaia special is that it actually consists of two telescopes, positioned at an angle to one another, that continuously make measurements of stars. ‘Gaia observes the sky from two different directions, with a known angle between its various windows. You can measure the distance to the stars. It’s a kind of triangulation, which enables you to very accurately determine the position and velocity of the stars.’ Two years ago, data on 1.1 billion stars was published. This spring, data that Gaia has collected on a further 1.7 billion stars was made public. Prof. Helmi and her research group helped validate this data before it was released to researchers around the world. This involved checking aspects such as the quality of the space probe’s data and error measurements.

Keep on learning

Amina Helmi feels that her transition from a PhD student working on her own research to the role of a professor, who has to look far beyond her personal research interests, has greatly enriched her work. ‘This gives me an opportunity for further personal development. Thanks to my research group, I have tackled many different areas of research since completing my dissertation. I also get a real kick out of seeing our PhD students blossom and mature. If all goes well, there comes a time when they surpass me, as they come up with their own ideas and choose their own paths. As a result, I also end up learning new things. It is a never-ending learning process, and I think that’s the most important thing of all. The fact that you’re not doing the same thing, day in, day out.’

Little brains

It is difficult to say exactly what implications Prof. Helmi’s discovery might have for life on Earth. ‘The only thing you can say with any certainty is that both we and the Earth are made of stardust. It could well be that some of the iron in our bodies comes from a star that was born outside the Milky Way. Yet, even if this is true, it changes nothing, as iron is always iron, regardless of its origins. Input of this kind may well have affected the Earth’s ‘relative abundance’. However, I don’t know whether the conditions on this planet – in terms of its composition of chemical elements – are the only ones under which life can arise.’ Fascinating though this is, it was another cosmic conundrum that led Amina Helmi to take up astronomy. ‘It was at primary school that I first encountered astronomy, during a lesson about the moon. Even then, I understood why we always see the same side of the moon. I thought it was amazing that people here on Earth could work this out, using reason, without actually having to go to the moon. For me, the real fascination lies in the contrast between how vast the universe is and how small we are. I just love that contrast, because it puts everything in perspective. It is totally amazing that we – with our little brains – are able to understand the universe.’

Patience

Thanks to the continuing flow of data from Gaia, there will be plenty of things to discover and understand in the upcoming years. Anything can happen – Prof. Helmi is quite sure of that. ‘The difference between Gaia and other missions is the huge range of objects you can study – from planets and stars to galaxies beyond the Milky Way itself. Gaia observes everything with amazing clarity, so we are sure to find objects that we never knew existed. Other questions we hope to answer, with Gaia’s help, concern the distribution of dark matter and whether or not our ideas about the particles it is made of are correct. But that takes time.’ Amina Helmi is well aware of the need for patience.

Text Bert Platzer, photos Reyer Boxem. This article has been taken from our alumni magazine Broerstraat 5. December 2018.