The leaking star cluster

Star clusters – galactic nurseries

Star clusters are of great importance in any galaxy: they are the birthplace of new stars, often containing massive stars of 10 solar masses or more. Such massive stars often drive powerful winds; the combined action of all stars in the cluster then leads to the formation of a “superbubble” – a cavity in the interstellar medium.

Among the many star clusters in the Milky Way, Westerlund 1 stands out. It’s the closest, most massive, and most luminous star cluster in the Milky Way, located about 13,000 light years away from us.

Westerlund 1 as a cosmic-ray accelerator

Besides being stellar nurseries, young massive star clusters are also known to produce highly energetic particles known as cosmic rays. Because these particles are electrically charged, they are deflected by omnipresent magnetic fields and cannot be traced back to their origin. To study cosmic-ray productions, astronomers therefore look for high-energy gamma-ray photons that are created by the cosmic rays close to their source and that, being electrically neutral, travel in straight lines.

Previously, using the H.E.S.S. telescope system, scientists have established the presence of extremely energetic Tera-electronvolt (TeV; 10¹² eV) gamma radiation from the vicinity of Westerlund 1, marking the cluster as a powerful particle accelerator. The TeV emission appears as a ring-like structure around Westerlund 1, which has been interpreted as cosmic rays being accelerated at a shock front (the “termination shock”) formed by the collective wind from the massive stars inside the cluster. This ring-like structure, however, exhibits a “tail” protruding in one direction that was hitherto unexplained.

Astronomers at the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg and their colleagues have now conducted a detailed study of data collected with the Fermi Gamma-Ray Space Telescope, aimed to detect gamma rays at Giga-electronvolt (GeV; 10⁹ eV) energies. Their findings, published recently in the journal Nature Communications, offer unique insights into this feature, and connect it with an outflow of particles from the vicinity of Westerlund 1.

A new gamma ray source

The study reveals the existence of a new GeV gamma-ray source. Surprisingly, the new source is offset from the TeV gamma radiation found around the cluster – precisely into the direction of the tail-like structure but about 320 light years away. “However, because the GeV and TeV emission are connected smoothly in terms of their spatial appearance and their energy spectra, we believe that they share a common origin”, explains Prof. Marianne Lemoine-Goumard from the university of Bordeaux, first author of the study.

To learn more about the origin of the new source, the researchers went on to investigate the density of the interstellar medium in its vicinity. Using observations of the 21-cm emission line of hydrogen, they were able to identify a deficit in gas density that coincides with the position of the gamma-ray source. “This gave us the idea that we are dealing with an outflow – material driven by the star cluster away from the Galactic plane, creating a cavity in the interstellar medium in that direction” adds Dr. Lars Mohrmann, director of the H.E.S.S. collaboration and co-lead of the new study.

A nascent outflow from Westerlund 1

A modelling of the observed gamma-ray emission indicates that it is very likely originating from cosmic-ray electrons accelerated near Westerlund 1, via a process called inverse-Compton scattering. The consistent picture emerging from the observations is illustrated in Fig. 3: the superbubble surrounding Westerlund 1, due to the gradient in density of the surrounding medium, expands asymmetrically and begins to form a “nascent outflow”. The electrons are thought to be accelerated at the wind termination shock front. “Because high-energy electrons lose their energy quickly, the resulting high-energy gamma-ray emission measured with H.E.S.S. appears close to the star cluster, explains Lucia Härer, a doctoral student at the MPIK who developed the underlying theoretical model. “Lower-energy electrons, on the other hand, can travel much further and are transported along the outflow before they emit the gamma radiation detected with the Fermi telescope”.

While the measured gamma-ray emission is most likely due to cosmic-ray electrons, the researchers argue that these electrons are accompanied by other cosmic-ray particles, in particular protons and heavier atomic nuclei.

The scientists expect that the nascent outflow will eventually break out of the Galactic disk, thus opening a channel for the transport of cosmic rays into the surrounding Galactic halo. The new work presents the first observational indication for such a scenario and suggests that particle outflows might be a common occurrence around young massive star clusters. Because the transport of cosmic rays from the disk into the halo is a process that – although believed to be critical for galaxy evolution – is lacking direct observational confirmation, these results bear relevance in many branches of astronomy.

Future studies of gamma-ray emission from other young massive star clusters, for example with the Cherenkov Telescope Array Observatory, will be needed to answer the question if the observed outflow from Westerlund 1 is a special case or a blueprint for a common scenario.