Young massive star clusters in the Large Magellanic Cloud as cosmic-ray sources

Added: July 24, 2024.

Young massive star clusters are very compact aggregations of stars that can have masses of tens to hundreds of solar masses, with total cluster masses of more than 10,000 solar masses. The most massive one in our Galaxy, Westerlund 1, featured in a previous article on this page.

Young massive star clusters are thought to provide excellent environments for the acceleration of cosmic rays to extremely high energies. Searching for gamma-ray emission from these objects is a promising way of identifying them as cosmic particle accelerators. Unfortunately, only about a handful of young massive star clusters could so far be unambiguously linked to very-high-energy gamma-ray emission.

Together with my colleagues in the H.E.S.S. Collaboration, we have now been able to measure gamma-ray emission from two young massive star clusters – called 30 Dor C and R136 – in a satellite galaxy of the Milky Way, the Large Magellanic Cloud. This is remarkable because the Large Magellanic Cloud is located about 150,000 light years away from us, more than ten times the distance to Westerlund 1. That marks these clusters as very powerful particle accelerators.

Both star clusters are located in a region called the Tarantula Nebula. The figure below shows a gamma-ray flux map of this region, as measured with the H.E.S.S. telescopes. The left panel shows the entire emission and immediately illustrates the biggest challenge in the analysis of the data: located just in between the two star clusters is the gamma-ray source HESS J0537–691, associated with the pulsar wind nebula N 157B, which outshines the clusters. Nevertheless, through a careful modelling of the data, we have been able to subtract the emission from N 157B (see right panel), thus revealing 30 Dor C (which was already known as a gamma-ray emitter before, but could now be measured in more detail) and R136 (a newly discovered source).

Besides their detection, it is interesting that we have measured the emission from both sources as being spatially extended, with a (Gaussian) radius of around 30 parsecs, or 100 light years. This radius is indicated as a white circle in the two images below.

The left image shows 30 Dor C, with the star cluster in the centre. The light red line denotes the contour of a shell-like structure that has been discovered in X-rays. The measured extension of the gamma-ray emission matches well with this structure, suggesting a common origin. The X-ray and gamma-ray emission could for example be generated by particles accelerated at a supernova shock wave that expands very fast in the cavity that has been blown into the interstellar medium by the stellar winds of the massive stars in the cluster.

R136 is shown in the image on the right. As this cluster is younger (it is “only” about 1.5 million years old), it is possible that none of its stars has exploded as a supernova yet. Nevertheless, cosmic rays can be accelerated by the cluster, for example by the collective cluster wind that forms as a superposition of the individual stellar winds. The fact that the gamma-ray emission appears extended supports the hypothesis that it is connected to the so-called superbubble that is formed by this collective wind.

In summary, our detection of 30 Dor C and R136 strengthens the picture that young massive star clusters are major contributors to the flux of high-energy cosmic rays. With more observations of further clusters, we can learn more about how the clusters accelerate particles to such high energies.

Paper reference: Astrophysical Journal Letters 970, L21 (2024).
DOI: 10.3847/2041-8213/ad5e67
Pre-print: arXiv:2407.16219