The first 3D view of the formation and evolution of globular clusters

ESO Very Large Telescope (VLT) during observations (credits: ESO/S. Brunier)

A study published on 1 November in the journal Astronomy & Astrophysics adds to the understanding of the dynamical evolution of multiple stellar populations in globular clusters – spherical and very compact stellar agglomerates populated by around 100 to 1,000 million stars.

A team of researchers led by the Italian National Institute for Astrophysics (INAF) and the University of Bologna has conducted the first 3D kinematic analysis of these stellar aggregates for a representative sample of 16 galactic globular clusters. The study provides a pioneering empirical description of their kinematic properties and their evolution from formation to present day.

Emanuele Dalessandro, lead author of the article and researcher at INAF in Bologna, explained: “The results of this analysis provide the first solid evidence that globular clusters formed through multiple star formation episodes. This sets fundamental constraints on the dynamical path followed by the clusters throughout their evolution.”

The study also emphasises the kinematic differences between the first and second stellar populations and it helps to gain a better understanding of their formation and evolution mechanisms.

Co-author of the study Mario Cadelano, researcher at the Department of Physics and Astronomy of the University of Bologna and INAF member, added: “We have known for about two decades that globular clusters consist of multiple stellar population: a primordial one, with chemical properties similar to other stars in the galaxy, and one with anomalous chemical abundances of light elements such as helium, oxygen, sodium, and nitrogen. Despite numerous observations and theoretical models tried to shed light on their characteristics, the processes behind the formation of these stellar populations still represent one of the greatest mysteries in stellar astrophysics.”

Dating back to the dawn of cosmos, 12-13 billion years ago, globular clusters are among the first systems to have formed in the universe and represent a typical population of all galaxies. They are compact systems – with mass of several hundred thousand solar masses and spanning just a few parsecs – but can be observed even in distant galaxies.

“Their astrophysical significance is immense. They not only help us test cosmological models of the formation of the universe, but also provide natural laboratories to study the formation, evolution, and chemical enrichment of galaxies.” Dalessandro explained.

The analysis shows that stars with different abundances of light elements are characterised by different kinematic properties, such as rotational speed and distribution of velocities.

Cadelano added: “We studied in detail how thousands of stars move within the cluster and we discovered that stars from different populations exhibit different kinematic properties – stars with anomalous chemical composition tend to rotate around the cluster faster than the others and gradually spread from the from the central to the outer regions.”

ESA’s Gaia Satellite mapping the stars of the Milky Way (credits: ESA/ATG medialab; background: ESO/S. Brunier)

The intensity of these kinematic differences is directly proportional to the dynamical age of the globular clusters. These results align with the long-term dynamical evolution of globular clusters, in which stars with anomalous chemical abundances form more centrally concentrated and rotate more rapidly than the standard ones.

According to Dalessandro, this suggests that globular clusters formed through multiple star formation episodes. Such findings are important to understand the physical processes behind the formation and evolution of massive star clusters.

The study combines the proper motions and radial velocities collected using the ESA’s Gaia telescope, along with data gathered mainly using the ESO VLT as part of the Multi-Instrument Kinematic Survey (MIKiS). Conducted by the University of Bologna, the MIKiS is a spectroscopic survey that focuses on exploring the internal kinematics of globular clusters. The use of these telescopes, from space and on the ground, has resulted in an unprecedented 3D view of the velocity distribution of stars within the selected globular clusters.

Francesco Ferraro, professor at the Department of Physics and Astronomy of the University of Bologna and the principal investigator of the MIKiS survey, concludes: “This represents one of the most significant achievements of the MIKiS survey. Through several Large Programmes led by the University of Bologna, we have obtained over 300 hours of observation with the ESO VLT to study the internal kinematics of a sample of globular clusters in our galaxy.”

Despite over a century of research on star clusters, recent observational results show that our knowledge of these systems is still incomplete. In the last twenty years, the discovery of multiple populations – subpopulations characterised by different abundances of several light elements (such as helium, sodium, oxygen, and nitrogen) – has challenged our understanding of the physical mechanisms governing the formation of globular clusters. The key question still stands: is there a single formation mechanism for all clusters?

Dalessandro suggests: “Our study shows that stars with anomalous chemical abundances formed in a more centrally concentrated subsystem and with a faster rotation than the other stars in the cluster. The results align with previous predictions about the initial differences in the dynamical properties of multiple populations.”

This new 3D view of stellar dynamics provides a new and fascinating perspective on the dynamical evolution of globular clusters, shedding light on some of the most complex mysteries surrounding their origin.