New insight into Milky Way formation

Research by a team of astronomers led by Professor Dr. Pavel Kroupa of the Argelander Institute for Astronomy at Bonn University has led to important breakthroughs in the exploration of the earliest evolutionary history of our Galaxy.

A Hubble image of M80 (NGC 6093), one of the 147 globular star clusters associated with our Galaxy and known to contain hundreds of thousands of stars. Image: The Hubble Heritage Team/AURA/STScl/NASA.

In a groundbreaking study into the Milky Way’s origins, a team of astronomers from the Argelander Institute for Astronomy at Bonn University and the Max-Planck Institute for Radioastronomy in Bonn investigated globular clusters – spherical groups of stars – located in the halo of our Galaxy. Their results show that the early Galaxy evolved from a smooth proto-galaxy to a more developed clumpy one in a matter of a few hundred million years – a short astronomical timescale.

The Milky Way's halo globular clusters contain hundreds of thousands of stars and can can be imagined as the fossilised remnants of the young galactic neighbourhood, containing clues to the early conditions under which it formed.

When the initial cloud of gas from which the Milky Way eventually formed condensed to form the stars in these clusters, there was some material left over which was subsequently redistributed throughout the star cluster by the radiation and winds emanating from the population of newly-formed stars. As a result, the globular clusters expanded and so stars at the outer rim of the clusters were lost. “This means that the present shape of the clusters was directly influenced by what happened in the early days of their existence,” explains Michael Marks, PhD student of Professor Kroupa.

The Bonn scientists also explored how the clusters were affected by the gravitational forces caused by the formation of the Milky Way itself, finding that that the more metal-rich the cluster member stars, the more influence the proto-Milky Way had on the cluster’s early formation through exertion of gravitational forces. “The amount of e.g. iron in a star is therefore an age indicator. The more recently a star cluster was born, the higher the proportion of heavy elements it contains,” adds Marks. This has important implications on the timescales involved in our Galaxy’s evolution. The clusters examined are approximately the same age but have varying levels of metal content in their member stars and thus varying intensities of early gravitational forces exerted on them by our proto-Galaxy. This infers that during a very short astronomical timescale, the structure of the early Milky Way changed significantly.

The timespan involved is linked with the collapse of the proto-galaxy gas cloud under its own gravity. This strongly affected the strength of the gravitational forces exerted on to the globular cluster stars which were simultaneously forming within the collapsing gas cloud. For the great variation in gravitational forces exerted on different globular clusters to be explained, the Milky Way must have evolved rapidly in a short timescale of a few hundred million years. The material from which the somewhat younger globular clusters formed was previously enriched with heavy elements by fast-evolving stars in the older clusters. According to the results of this investigation, this same material felt stronger attractive forces.

Professor Kroupa summarises the results of the research team: “In this picture we can elegantly combine the observational and theoretical results and understand why later forming, more metal-rich clusters experienced stronger force fields. On the back of this work, for the first time we have a detailed insight into the earliest evolutionary history of our Galaxy.”