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THE MAKING OF THE MILKY WAY HALO

February 18, 1999


The brightest objects in the halo are the globular clusters. They are large groupings of stars that were formed together in the very early evolutionary phases of the Milky Way, some 12,000 - 14,000 million years ago. This happened soon after the moment when the first structures emerged in the large cloud of primordial hydrogen in which our Galaxy was born.

A popular scenario describes the first build-up of galactic structure, i.e. of stars and gas, as when normal matter began to collect inside the dark-matter halo, due to its strong gravitational attraction. The globular clusters were most probably the first denizens of this protogalaxy.

It is believed that the Milky Way Galaxy subsequently "cannibalized" other nearby dwarf galaxies and clusters, and that this process is still going on. Some astronomers have even speculated that many of the globular clusters now observed may originally have been the particularly dense, central regions ("nuclei") of unfortunate, small galaxies whose more tenuous outer structures have since been dissipated into the Galactic halo. If this is the case, then the Milky Way halo may now contain fossil structures, left over from this process (referred to as "accretion").

In order to investigate this basic issue in more detail, CCD images obtained with the Test Camera at the first 8.2-m VLT Unit Telescope (UT1) have been used to study one of the old globular clusters in the Milky Way.

NGC 6712 is an enormous swarm of stars in the southern constellation Scutum (The Shield). It is located at a distance of about 23,000 light-years, in the direction towards the Galactic Center. This cluster is of spherical form and contains somewhat fewer than 1 million stars, all of which are lighter than our Sun.

The ESO astronomers looked for possible signatures of the accretion process that is supposed to have formed the halo of the Milky Way. Thanks to the excellent observational data from the VLT, they were able to measure accurately the brightness and colours of even quite faint stars in NGC 6712. The study concentrated on cluster stars in an interval corresponding to slightly less bright than the Sun and down to about 100 times fainter. The mass of the brightest stars is about 80% of that of the Sun, while the faintest objects have about 30% of its mass.

To some surprise, the astronomers found that small and faint stars are much less numerous in NGC 6712 than what would be expected on the basis of previous studies of other globular clusters. Since the brightness of cluster stars is proportionate to their mass, this also means that NGC 6712
possesses amazingly few light stars. Indeed, all other globular clusters studied so far have been found to contain many more faint and light stars than brighter, more massive ones.

Current theories infer that, in the accretion process that formed our Galaxy, globular clusters were particularly vulnerable to disruption in the strong tidal field of the emerging Milky Way. Astronomers refer to this process as "evaporation" of stars from the cluster - the individual stars remain fully intact but are no longer gravitationally bound to the cluster. For this reason, the clusters that are still around today probably only constitute a small fraction of the original population. This also implies that a significant part, if not all, of the stars now observed in the halo might have been dislodged from such clusters at an earlier epoch.

The most natural explanation for these new VLT observations is therefore that many, perhaps most, of the small stars originally present in NGC 6712 have been lost from the cluster and are now moving around in the halo of the Milky Way.

This also seems very plausible, because the galactic orbit of NGC 6712 is such that it frequently dives deeply into the dense regions near the Galactic Center and Bulge. The cluster suffers tremendous "gravitational shocks" during such passages that have obviously contributed to the loss of
its faintest and smallest stars, which are easier to dislodge than the heavier ones.

Just some weeks from now, on April 1, 1999, the first research programmes by "visiting" astronomers will start with the FORS1 and ISAAC instruments at the VLT UT1.


European Southern Observatory (ESO)