A bright spot in the dark: Gamma and X-rays tell us the story of the Galactic Centre

The central regions of galaxies are extremely crowded places, containing up to a few hundreds of millions of stars. They are generally extremely dense environments, where a variety of phenomena occur frequently. The closest galactic nucleus within our reach is the Milky Way centre, usually referred to as Galactic Centre, which offers us a clean window on the interesting lifetime of these astrophysical objects.

Our Galactic Centre hosts a supermassive black hole called Sagittarius A* (SgrA*), weighing 4.5 million times the mass of our Sun, which is surrounded by a dense conglomerate of stars called nuclear cluster. Many detectors in the past years looked at the Galactic Centre, providing among the most beautiful observations to date at all the wavelengths.

In the last decade, the FERMI satellite revealed an intense flux of gamma-rays coming from the Galactic Centre, whose origin represents a still partly unsolved puzzle. One possible cause of the so-called “gamma-ray excess” is related to high-energy emission from a class of neutron stars, called millisecond pulsars (MSPs), that rotate extremely fast, several hundred times per second. The basic idea is that the cumulated emission of thousands MSPs could explain the observed Gamma-ray excess.

Shortly after FERMI, NuSTAR and the Chandra satellites directed the gaze toward the Galactic Centre as well, detecting a large number of sources emitting in the X-ray. The most likely candidates to explain this “X-ray excess” seem to be magnetic cataclysmic variables (CVs), a class of white dwarfs that live in binary systems and accrete gas from the companion star. Moreover, at the beginning of 2018, Chandra observations revealed the presence of several neutron star and black hole (BH) binaries also feeding from their respective Sun-like companions, all inhabiting the SgrA* neighbourhoods. Hence, it seems that compact objects are likely to live in such densely packed galactic regions, at least in our own Galaxy. Understanding how it is possible to get such a large and heterogeneous population of compact objects inhabiting galactic nuclei would unveil several aspects of galaxy formation and evolution processes.

For instance, the current paradigm for MSPs formation suggests that their number in the centre of galaxies should be much smaller than that inferred from Gamma-ray observations.

However, the discrepancy between theory and observations can be alleviated if the MSPs formation process and the galactic centre origin are put together in the same context.

Making use of state-of-the-art computer simulations of the Milky Way inner regions, a team led by Manuel Arca Sedda, researcher at the Heidelberg University, together with Bence Kocsis (Eotvos Lorand University) and Tim Brandt (University of California Santa Barbara), have shown that the observed Gamma and X-ray fluxes and the origin of the Galactic nuclear cluster are tightly connected.

These simulations show that the Milky Way centre, and in particular its nuclear cluster, formed through the so-called “dry-merger” scenario, according to which it emerges from a series of repeated collisions among massive star clusters born in the inner galactic regions.

Manuel Arca Sedda says: “Star clusters are perfect factories of MSPs, CVs and BHs, making the ‘dry-merger’ scenario an appealing mechanism to accumulate these sources into the Galactic Centre. Our models suggest that during the cataclysmic phases driving the nuclear cluster formation, the disrupting star clusters deposit their population of high-energy objects into the growing nucleus. Our results reproduce very well the observed Gamma- and X-ray fluxes (Figure 1), and allowed us to infer a number of BHs inhabiting the Galactic Centre perfectly compatible with the observational limits.”

This movie shows the Milky Way central region as seen by the Spitzer satellite (credits: NASA/JPL-Caltech) and our modelled star clusters overlapped. The big black dot in the centre of the movie represents the Galactic SMBH. The 11 conglomerates of particles with different colors represent the star clusters. Small black spots represent stellar black holes, while blue circles (white filled) represent MSPs.

Featured image credit: Twinkling stars surround the milky way over a lake and trees in British Columbia, Canada  by James Wheeler. Licensed image via Shutterstock.