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Galaxy formation

Clumps that survive to tell a tale

Nature volume 402, pages 29–30 (1999)Cite this article

How did the Milky Way system form? The present consensus1 is that the region inside the orbit of the Sun was mainly formed by rapid collapse2, whereas the outer Galactic halo assembled more slowly by the capture of many smaller bits and pieces3. Two papers published in this issue4,5 bear on the origin of the halo. The first by Helmi et al.4 on page 53 shows that about 8% of metal-poor halo stars near the Sun started life in a single coherent structure (perhaps a dwarf galaxy) that was disrupted during (or soon after) the Milky Way formed. The second paper by Lee et al.5 on page 55 significantly strengthens the view that the brightest globular cluster of stars in the Milky Way may actually be the remnant nucleus of another spheroidal dwarf galaxy.

The first hard evidence6 for the idea that the Galactic halo might have a clumpy structure was that the nearby high-velocity star Groombridge 1830 belongs to a small group that is currently passing through the Galactic disk. More recent observations7 discovered a clump of stars, roughly 4.6 kpc above the Galactic plane (1 pc = 3.26 light years), which are apparently moving together through the Galactic halo. From a survey8 of an area in the Galactic halo known as Selected Area 57 it was concluded that the stars in this halo field show a high degree of clumping, and that “the halo field star population may derive predominantly from accretion of stellar agglomerations, most likely dwarf galaxies”.

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Figure 1: Clumping of metal-poor stars in the Galactic halo.

References

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  1. the Dominion Astrophysical Observatory, The National Research Council of Canada, 5071 West Saanich Road, Victoria, V8X 4M6, British Columbia, Canada

    Sidney van den Bergh

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  1. Sidney van den Bergh
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Correspondence to Sidney van den Bergh.

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van den Bergh, S. Clumps that survive to tell a tale. Nature 402, 29–30 (1999). https://doi.org/10.1038/46922

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