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Omega Centauri’s lost stars

Apr 23 2019

Stellar streams detected by STREAMFINDER, overlaid on an all-sky map computed using the fluxes measured in the Gaia DR2 catalogue. Stars belonging to the various streams are color-coded by their proper motion in the galactic latitude direction. Credits: Rodrigo Ibata, Observatoire astronomique de Strasbourg.

Stars belonging to the Fimbulthul structure: the colour of the symbols represents their proper motion in the galactic latitude direction. The current position of Omega Centauri and its probable trajectory through our Galaxy are indicated. Credits: Rodrigo Ibata, Observatoire astronomique de Strasbourg, TRAPPIST/E. Jehin/ESO.

A team of researchers from the Strasbourg Astronomical Observatory, Bologna Observatory and the University of Stockholm has identified a stream of stars that was torn off the globular cluster Omega Centauri. Searching through the 1.7 billion stars observed by the ESA Gaia mission, they have identified 309 stars that suggest that this globular cluster may actually be the remnant of a dwarf galaxy that is being torn apart by the gravitational forces of our Galaxy.

In 1677, Edmond Halley gave the name “Omega Centauri” (ω Cen) to what he thought was a star in the Centaurus constellation. Later in 1830 John Herschel realized that it was in fact a globular cluster that could be resolved into individual stars. We now know that Omega Centauri is the most massive globular cluster in the Milky Way: it is about 18,000 light years from us and contains several million stars that are about 12 billion years old. The nature of this object has been the subject of much debate: is it really a globular cluster, or could it be the heart of a dwarf galaxy whose periphery has been dispersed by the Milky Way?

This last hypothesis is based on the fact that ω Cen contains several stellar populations, with a large range of metallicities (i.e. heavy element content) that betray a formation over an extended period of time. An additional argument in favor of this hypothesis would be to find debris from the cluster scattered along its orbit in the Milky Way. Indeed, when a dwarf galaxy interacts with a massive galaxy like our own, stars are torn off by gravitational tidal forces, and these stars remain visible for a time as stellar streams, before becoming dispersed in the vast volumes of interstellar space surrounding the massive galaxy.

By analyzing the motions of stars measured by the Gaia satellite with an algorithm called STREAMFINDER developed by the team, the researchers identified several star streams. One of them, named “Fimbulthul” (after one of the rivers in Norse mythology that existed at the beginning of the world), contains 309 stars stretching over 18° in the sky.

By modeling the trajectories of the stars, the team showed that the Fimbulthul structure is a stellar tidal stream torn off ω Cen, extending up to 28° from the cluster. Spectroscopic observations of 5 stars of this stream with the Canada-France Hawaii Telescope show that their velocities are very similar, and that they have metallicities comparable to the stars of ω Cen itself, which reinforces the idea that the tidal stream is linked to ω Cen.

The researchers were then able to show that the stream is also present in the very crowded area of sky in the immediate vicinity of the cluster. Further modeling of the tidal stream will constrain the dynamical history of the dwarf galaxy that was the progenitor of ω Cen, and allow us to find even more stars lost by this system into the halo of the Milky Way.


Link to the paper:  https://arxiv.org/pdf/1902.09544.pdf


Rodrigo Ibata, rodrigo.ibata@astro.unistra.fr, +33 3 68 85 23 91

Michele Bellazzini, Michele.bellazzini@inaf.it, +39 051 635 73 26

Khyati Malhan, khyati.malhan@fysik.su.se, +46 72 085 22 05

Nicolas Martin, nicolas.martin@astro.unistra.fr, +33 3 68 85 24 67

Paolo Bianchini, paolo.bianchini@astro.unistra.fr, +33 3 68 85 24 02



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