By combining data from the Hubble Space Telescope and the Gaia mission, University of Groningen astronomers have been able to measure the proper motion of 15 stars in the Sculptor Galaxy, the first such measurement of stars in a small galaxy outside the Milky Way. Analysis shows an unexpected preference in the direction of movement, which suggests that the standard theoretical models used to describe the motion of stars and dark matter halos in other galaxies might be invalid. The results will be published on 27 November in Nature Astronomy.
Astronomers have long been able to measure the movement of stars in our 'line of sight' (i.e. the movement towards or away from us) by measuring the redshift, which is caused by the Doppler effect. However, measuring the movement in the plane of the sky, what is known as the proper motion, is much more difficult. To detect this, you need multiple precise measurements of a star's position over the course of several years. The immense distances involved mean that many stars in our galaxy move very little across the sky when seen from Earth. For stars outside the galaxy, this movement is even less.
The European Gaia mission, which is currently underway, was designed to measure the exact location of more than one billion stars, mostly in our own galaxy. 'But Gaia also measures star positions in nearby galaxies', explains University of Groningen astronomer Davide Massari. 'And for some of these stars, we also have their location as measured by the Hubble Space Telescope, some 12 years ago.'
Massari and his colleagues from the Kapteyn Astronomical Institute set out to combine the two data sets. This is not an easy task, as the two missions measure the location in different ways. The team managed to combine the data by using background galaxies which did not change position in the 12 years. 'We had to be very careful to rule out any systematic errors', says Massari. But they succeeded, and out of 120 stars in the Sculptor Galaxy that were measured by both Hubble and Gaia, they found extremely accurate paired observations for 15.
'Next, we determined how the stars move in this small galaxy, which is quantified by the anisotropy parameter', Massari explains. If they are high, the stars have very elongated trajectories and if they are very small, they have circular orbits. 'Knowing this allows us to pin down the properties of the dark matter halo in which the galaxy is embedded. But our measured value was very surprising, as the standard models didn't allow it.' This means that some of the assumptions on which these models are based must be wrong.
'So far, we have only been able to test models by using the line-of-sight movement. That seemed fine, but now, with proper motion, the standard models are breaking down', Massari explains. 'One possible explanation is that the models assume all stars to be in a single population of stars'. But we know Sculptor is complex with at least two stellar populations (one more compact and one more extended). There is a model that includes this and does predict the anisotropy which Massari and colleagues observed, as long as most of the stars they measure belong to the most compact population.
The movement of stars depends mostly on the invisible dark matter halo around a galaxy. This is why it is so important to determine the anisotropy parameter, because it can be used to pin down the distribution of dark matter in this galaxy, which in turn depends on the nature of dark matter itself. Massari: 'Our results show that by using the Gaia data, combined with other data sets, we can measure the proper motion of stars outside the Milky Way and thus improve the models which describe how dark matter is distributed in these other galaxies.'
A second major result is a more precise measurement of the orbit of the Sculptor Galaxy around the Milky Way. 'This orbit is much wider than expected. Previously, it was believed that the current spheroidal shape of Sculptor was in part the result of some close passages, but our measurements show that this is not the case.' Massari and the team from the Kapteyn Institute are looking forward to extending their sample of stars outside the Milky Way with known proper motion after the new Gaia data release early next year.
Reference: D. Massari, M. A. Breddels, A. Helmi, L. Posti, A. G. A. Brown and E. Tolstoy: Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era. Nature Astronomy 27 November 2017.