Unknown Population of Stars Discovered

Central region of the Milky Way in infrared light. | © NASA/JPL-Caltech/S. Solov
Central region of the Milky Way in infrared light. | © NASA/JPL-Caltech/S. Solovy (Spitzer Science Center/Caltech)

Near the centre of the Milky Way there exists a heretofore unknown population of stars that exhibits characteristic properties. It was discovered by an international research team under the direction of Dr Manuel Arca Sedda of Collaborative Research Centre "The Milky Way System" (CRC 881) of Heidelberg University. The scientists identified as the origin of these stars a globular cluster that made its way into the centre of our home Galaxy a long time ago.

There is a supermassive black hole at the innermost centre of the Milky Way. It is surrounded by one of the densest collection of stars in the known universe - a nuclear star cluster (NSC). It contains up to 20 million stars within what for astronomers is a small range of only 26 light years. While observing this region of space through the Very Large Telescope in Chile, the researchers were able to identify a conspicuous population of stars. "A small portion of the stars inside this nuclear star cluster, about seven percent, had a distinctively different chemical composition and movement pattern than the surrounding stars. The characteristics these stars share, however, is surprisingly similar, which indicates that they have a common origin," explains Dr Arca Sedda from the Centre for Astronomy of Heidelberg University (ZAH).

According to Dr Sedda, nuclear star clusters could have originated at least in part from collisions of several smaller star clusters within the Galaxy. Such globular clusters are spatially more dense collections of stars of similar age. Their respective gravities influence each other and they thus move together through space. They lose speed over time and drift to the Galactic Centre. Here they collide with other star clusters to form considerably larger star clusters. "The population we just discovered might be the remnant of just such an older star cluster from further outlying regions of the Milky Way," states Manuel Arca Sedda.

To test this theory, the researchers used powerful computer simulations, enabling them to virtually map how a star cluster could make it into the Galactic Centre. Their simulations suggest that there must have been an infall of this type in the region of the nuclear star cluster within the last three to a maximum of five billion years. The original star cluster probably formed 10,000 to 16,000 light-years away from the centre of the Milky Way.

The astronomer also compared the characteristics of the newly discovered star population with those of known globular clusters in the Milky Way and discovered that they match up pretty well. This is another indication that the central nuclear star cluster in the Galaxy developed at least in part from the infall of smaller star clusters.

The research work - published in the journals "The Astrophysical Journal Letters" and "Monthly Notices of the Royal Astronomical Society" - was conducted within the framework of "The Milky Way System" Collaborative Research Centre of Heidelberg University. The researchers there are studying the origin and evolution of the Milky Way and its surroundings. The CRC 881 is located at the Centre for Astronomy of Heidelberg University und includes scientists from the

Institute for Astronomical Computing, the Institute of Theoretical Astrophysics, and the Königstuhl State Observatory. The participating non-university research institutions include the Max Planck Institute of Astronomy, the Heidelberg Institute for Theoretical Studies, and the House of Astronomy.

M. Arca Sedda, A. Gualandris, T. Do, A. Feldmeier-Krause, N. Neumayer, D. Erkal: On the origin of a rotating metal-poor stellar population in the Milky Way Nuclear Cluster, The Astrophysical Journal Letters (2020),

T. Do, G. D. Martinez, W. Kerzendorf, A. Feldmeier-Krause, M. Arca Sedda, N. Neumayer, A. Gualandris: Revealing the Formation of the Milky Way Nuclear Star Cluster via Chemo-Dynamical Modeling, The Astrophysical Journal Letters (2020),

A. Feldmeier-Krause, W. Kerzendorf, T. Do, F. Nogueras-Lara, N. Neumayer, C. J. Walcher, A. Seth, R. Schödel, P. T. de Zeeuw, M. Hilker, Nora Lützgendorf, H. Kuntschner, M. Kissler-Patig: Asymmetric spatial distribution of subsolar metallicity stars in the Milky Way nuclear star cluster, Monthly Notices of the Royal Astronomical Society (2020),