by Staff Writers
Groningen, Netherlands (SPX) Nov 30, 2017
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.
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.
'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.
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.
Paris (ESA) Nov 23, 2017
It's the perfect meeting of old and new. Astronomers have combined the latest data from ESA's Gaia mission with a simple analysis technique from the 18th century to discover a massive star cluster that had previously escaped detection. Now, subsequent investigations are helping reveal the star-forming history of our Galaxy, the Milky Way. In the latter years of the 18th century, astronomer ... read more
University of Groningen
Stellar Chemistry, The Universe And All Within It
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