3D Motions of Stars in Sculptor Dwarf Galaxy Hint at Underlying Dark Matter

Based on the combination of data taken 12 years apart by the NASA/ESA Hubble Space Telescope and ESA’s Gaia mission, astronomers have pinned down for the first time 3D motions of individual stars in the Sculptor dwarf galaxy, shedding new light on the underlying distribution of invisible dark matter that pervades the galaxy. The results are published in the journal Nature Astronomy.

This image, taken with the Wide Field Imager camera on the 2.2-m MPG/ESO telescope at ESO’s La Silla Observatory in Chile, shows the Sculptor dwarf galaxy. Image credit: ESO.

The Sculptor dwarf galaxy is located approximately 274,000 light-years away from Earth in the constellation Sculptor.

Discovered in 1937 by the American astronomer Harlow Shapley, this galaxy is one of our Milky Way Galaxy’s neighboring dwarf galaxies. However, it is much smaller and older than the Milky Way.

Dr. Davide Massari of Kapteyn Astronomical Institute in Groningen, the Netherlands, and colleagues measured the proper motions of roughly a hundred stars in the Sculptor dwarf galaxy.

For a smaller subset of ten stars, chosen among those with the smallest errors, the astronomers could also retrieve from the literature an estimate of the radial velocity, which quantifies the stellar motion along the line of sight.

Using the proper motion and radial velocity measurements, they were able to reconstruct how these stars move in three dimensions — the first time this was done for a dwarf galaxy.

“For the first time, we’ve been able to determine how individual stars are moving through a dwarf satellite of our Milky Way,” Dr. Massari said.

“This was possible by combining the exceptionally accurate measurements of stellar positions from Gaia’s first data release with equally outstanding observations taken over twelve years earlier by Hubble.”

As expected, the proper motions of stars in the Sculptor dwarf galaxy are tiny, even over the long time baseline between 2002, when the Hubble observations were performed, and the first set of publicly released Gaia data, gathered between 2014 and 2015.

“We couldn’t wait to see whether the Gaia observations could be used to measure individual proper motions in another galaxy, so we were thrilled to see this was indeed possible,” said co-author Dr. Amina Helmi, also from Kapteyn Astronomical Institute.

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According to astronomers, the Sculptor dwarf galaxy belongs to a class of galaxies called dwarf spheroidal galaxies, which are among the most dark matter dominated objects in the Universe. This makes them ideal targets for investigating the properties of dark matter.

In particular, understanding how dark matter is distributed in these dwarf galaxies allows scientists to test the validity of the currently-accepted cosmological model.

“Dwarf spheroidal galaxies like Sculptor are some of the most dark matter dominated objects we know of in the Universe,” Dr. Helmi said.

“It’s in these places that we can really ‘see’ this mysterious component at play.”

The information gathered about the 3D motion of stars in the Sculptor dwarf galaxy can be translated directly into knowledge of how its total mass — including dark matter — is distributed.

The new results show that stars in this galaxy move preferentially on elongated radial orbits. This indicates that the density of dark matter increases towards the center instead of flattening out.

These findings are in agreement with the established cosmological model and our current understanding of dark matter, taking into account the complexity of Sculptor’s stellar populations.

As a side effect of the study, the team also presented a more accurate trajectory of the Sculptor dwarf galaxy as a whole as it orbits the Milky Way.

Their results show that it is moving around our Galaxy in a high-inclination elongated orbit that takes it much further away than previously thought.

Currently, it is nearly at its closest point to the Milky Way, but its orbit can take it as far as 725,000 light-years away.

“With these pioneering measurements, we enter an era where measuring 3D motions of stars in other galaxies will become routine and will be possible for larger star samples. This will mostly be thanks to ESA’s Gaia mission,” Dr. Massari said.

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D. Massari et al. Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era. Nature Astronomy, published online November 27, 2017; doi: 10.1038/s41550-017-0322-y

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