Omega Centauri’s lost stars

The Milky Way, as seen by the Gaia satellite. Streams of co-moving stars are shown colored according to their motions as measured by Gaia. The “Fimbulthul” stream which is due to stars lost from the omega Centauri globular cluster (white box) has been highlighted. Credit R. Ibata.

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 stars that the team observed were quite faint for the instrument we were using,” says Dr. Nadine Manset, instrument scientist for Espadons and CFHT’s astronomy group manager. “It is great to see such challenging observations reinforce the Fimbulthul structure’s link 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.

The team’s paper appeared in the April 22nd edition of Nature.

Additional information

Nature paper (Subscription required.)
arXiv preprint (no login required).

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