New instrument will add to Gaia data

Professor of Astronomy Amina Helmi investigates how the Milky Way got its present shape. Detailed information on the movements and chemical composition of stars is vital to her work. The Gaia satellite mission, in which she plays an important role, has provided a cornucopia of data so far. Now, Helmi is involved in the building of a new instrument that will be mounted onto an Earth-based telescope and that will provide even more information.

The stars in the sky seem to have fixed positions. But in reality, they are moving around at high speeds. Some of the stars follow very peculiar paths. ‘That is because many of these stars were not born in the Milky Way, but in a different system that fused with our galaxy,’ explains Eduardo Balbinot, post-doc researcher in Helmi’s group. He studies the history of our Milky Way and is also involved in the building of 4MOST, a new instrument that will provide even more information about the way our galaxy evolved.

Fingerprint

‘Gaia measured the movements of more than a billion stars,’ Balbinot explains. ‘The most accurate measurements were carried out on relatively bright stars. 4MOST will add accurate information on the movements of fainter stars.’ Furthermore, 4MOST will be able to measure details on the chemical composition of stars. ‘This chemical composition carries the fingerprint of the birthplace of these stars.’

The movements of the stars in the halo of the Milky Way allow astronomers to calculate their origin. ‘In this way, we can discover which stars originate from the same source, such as a small galaxy or a globular cluster that has been captured by the gravitational pull of the Milky Way.’ However, when this capture took place long ago, the movements of the stars were also heavily influenced by the gravitational effects of the Milky Way, which makes it difficult to pinpoint their origin. ‘That is where the chemical composition can help us to confirm which stars belong together.’

Giant fusion

Amina Helmi is one of two principal investigators of one of the nine 4MOST core observation programmes, called the ‘Milky Way Halo Low-Resolution Survey’, a survey which she proposed. This will cover stars that form the halo, a huge cloud of stars around the Milky Way’s disk. Most of these stars have been acquired through fusions with other galaxies. Last year, Helmi and her colleagues described how one giant fusion event with a galaxy dubbed ‘Gaia-Enceladus’ occurred 10 billion year ago. ‘Helmi wants to ascertain how many galaxies have fused to form the current Milky Way,’ says Balbinot.

The University of Groningen is a partner in the development of 4MOST, together with the National Research School for Astronomy (NOVA). ‘The calibration unit is being built by the NOVA group based at ASTRON in Dwingeloo,’ says Balbinot. ‘My job is to give feedback on the development from the perspective of the future users.’

4MOST will use a special technique to study stars. The telescope receives light from a fairly large area of the sky. This light is not detected directly by a CCD camera or another optical instrument, as is usual, but sampled by an array of 2,000 glass fibres. ‘These fibres are placed at the positions given by the Gaia catalogue,’ explains Balbinot. Each fibre can therefore guide light from a specific star to the spectrometer for analysis. This is how the telescope will be able to study a large number of stars simultaneously.

Southern skies

It also means that the nine different survey programmes are going to be carried out simultaneously. An internal computer program will decide which objects in the field of view to check, based on for example observational conditions such as the weather, observability or properties of the targets themselves. Overall, the 4MOST programme will study 20 million stars, 3 million of them in high resolution. A share of 70 percent of 4MOST’s time will be dedicated to the nine simultaneous surveys, which will add up to millions of fibre-hours. For the remainder of the time 4MOST can be used for projects proposed by the general astronomy community.

At the moment, the instrument is still under construction. In about two years’ time, 4MOST will be mounted onto the European VISTA infrared survey telescope in Chile, to start its five-year observation survey of the southern skies. Just like with Gaia, Amina Helmi and her group will be at the forefront of this new observation programme – to learn even more about how the Milky Way got its present shape.

Read more on Amina Helmi's research via this web page.
See also: So many stars, so few women