Visualisation of a simulation showing the infall of a globular star cluster into the nuclear star cluster of the Milky Way. The colour scale shows the distribution of star densities along the lines of sight within the Galactic Centre. The globular cluster can be recognised as an isolated point that increasingly merges with the nuclear star cluster over the course of 400 million years and dissolves in the process. Despite the resulting mixing of the two star populations, certain properties of the stars of the globular cluster remain. Image: Manuel Arca Sedda et al. (ARI/ZAH)/MPIA. Hi-res image
On clear and dark nights it is still visible – the milky white, diffuse band of the Milky Way across the night sky. Since the invention of the telescope, scientists have known that this band consists of countless stars. Today, we understand that our home galaxy is mainly a large flat disc of hundreds of billions of stars, surrounded by dust and gas, and it is rotating around its centre.
The nuclear star cluster is one of the regions richest in stars in the known Universe
In the innermost centre of the Milky Way rests an extremely massive black hole. It is surrounded by one of the densest agglomerations of stars in the known Universe – a so-called "Nuclear Star Cluster" (NSC). Astronomers today assume that there are around 20 million stars in the innermost 26 light years of the Galaxy.
However, it is not visible at all without special equipment, because there are numerous dust clouds between us and the Galactic Centre that obscure the visible light. It therefore appears darker than other parts of the Milky Way. Only observations at much shorter or longer wavelengths such as infrared light reveal the structure of this region of the sky, which is actually much more massive than other regions of the galaxy.
The Milky Way is by no means unique, and astronomers now believe that most spiral galaxies could contain both a central black hole and a nuclear star cluster. However, the nuclear star cluster within the Milky Way is the only place where astronomers can resolve individual stars because of its relatively close distance, making it an ideal laboratory for studying the properties of these huge stellar clusters.
A study of the nuclear star cluster as a basis for further insights
The chemical composition of a star is an important indicator in astronomy, as it tells us something about its age. Metallicity – the abundance of heavier elements than hydrogen and helium – is an important quantity. This is because all other elements can only form in those very stars. Therefore, if a star contains a large number of heavy elements such as oxygen, carbon or iron, this means that it must have formed from the remains of a precursor star and is therefore relatively young. Conversely, a low metallicity indicates a very old star, which formed in the early days of the Universe, when there were hardly any heavy elements present in the Cosmos. The metallicity is therefore a direct indication of the age of the respective star and therefore of great importance for astronomers.
A hitherto unknown population of stars hides in the very heart of the Galaxy
An answer to this question may lie in the formation of a nuclear star cluster: according to a commonly accepted theory, they could at least partly have formed by collisions of several clusters, i.e. spatially denser collections of stars of similar ages, within a galaxy. Held together by the mutual gravitational pull, they move jointly through a bath of surrounding field stars. Stellar clusters exist in all known galaxies. Due to the phenomenon of dynamic friction, a gravitational effect of the surrounding matter, the clusters lose speed on their orbits and thus drift towards the Galactic Centre. At this point, they merge with other clusters and form the much larger nuclear star clusters. It is possible that the newly discovered population is a remnant of such an older group of stars.
Sophisticated simulations help to clarify the history of the nuclear star cluster
When a stellar cluster falls towards the Galactic Centre, the gravitational interactions with its environment cause stars to be ejected from the cluster. Once it reaches the innermost part of the Galaxy, it dissolves within a relatively short timescale and its stars become largely indistinguishable from the rest of the stars in its new environment.
Since the members of the newly discovered stellar population still have some very characteristic similarities despite their dispersal, astronomers suspect a common origin of these stars outside the nuclear star cluster. The simulations now suggest that they have entered the central area within the recent 3 to a maximum of 5 billion years.
The origin of the newly discovered stars
“Our results indicate that an infall of a rather nearby stellar cluster from the Milky Way itself is more likely,” explains Neumayer. It was probably originally formed about 10,000 to 16,000 light years away.
To support this hypothesis, the astronomers also compared the observed properties of the newly discovered stellar population with the ones of old globular clusters in the Milky Way and those that entered our Milky Way together with dwarf galaxies. They found that the properties of the newly discovered central stars matched those of globular clusters in the Milky Way much better. The calculated distances of the preceding stellar clusters also correspond well with the distances of those that have been known for a while already. “Although an extragalactic origin of the stars cannot be completely ruled out, it is rather unlikely,” Arca Sedda concludes. “This is an additional sign that the central nuclear star cluster in the galaxy is at least partly the result of the impact of smaller clusters.”
Background information
This work was carried out within the framework of subprojects Z2 and B8 of the Collaborative Research Centre SFB 881 “The Milky Way System” at the University of Heidelberg. Collaborative Research Centres are long-term projects for fundamental research, which are funded by the German Research Foundation (DFG) up to a duration of 12 years.
The SFB 881 is located at the Zentrum für Astronomie der Universität Heidelberg (ZAH) and includes scientists from the Astronomisches Rechen-Institut (ARI), the Institut für Theoretische Astrophysik (ITA) and the Landessternwarte Königstuhl (LSW). The participating non-university research institutions are the Max Planck Institute for Astronomy (MPIA) and the Heidelberg Institute for Theoretical Studies (HITS). In addition, the Haus der Astronomie (HdA) participates by making the SFB's research results available to the public.
Contacts
Nadine Neumayer
Leader Lise Meitner Group “Galactic Nuclei”
Phone:+49 6221 528-446
Max Planck Institute for Astronomy, Heidelberg
Renate Hubele
Public outreach SFB881/ZAH
Phone:+49 6221 528-291
Haus der Astronomie, Heideberg
Markus Nielbock
Press and public relations officer
Phone:+49 6221 528-134
Max Planck Institute for Astronomy, Heidelbe
Original publications
1. Manuel Arca Sedda et al.
On the origin of a rotating metal-poor stellar population in the Milky Way Nuclear Cluster
The Astrophysical Journal Letters, 901, L29 (2020)
Source / DOI
2.Tuan Do et al.
Revealing the Formation of the Milky Way Nuclear Star Cluster via Chemo-Dynamical Modeling
The Astrophysical Journal Letters, 901, L28 (2020)
Source / DOI
3. Anja Feldmeier-Krause et al.
Asymmetric spatial distribution of subsolar metallicity stars in the Milky Way nuclear star cluster
Monthly Notices of the Royal Astronomical Society, 494, 396 (2020)
Source / DOI