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The Fingerprint of a Galactic Nucleus: A Multi-Wavelength, High-Angular Resolution, Near-Infrared Study of the Centre of the Milky Way

Final Report Summary - GALACTICNUCLEUS (The Fingerprint of a Galactic Nucleus: A Multi-Wavelength, High-Angular Resolution, Near-Infrared Study of the Centre of the Milky Way)

The nuclei of galaxies are among the most extreme astrophysical environments in the local Universe. They contain massive black holes of millions to billions of solar masses, which are frequently surrounded by so-called nuclear star clusters. Nuclear star clusters are the densest and most massive star clusters in the present-day Universe. Stellar densities in their centres can be 10 million times higher than in the solar neighbourhood. Additionally, barred spiral galaxies, like the Milky Way, contain more extended star forming regions in their centres, often in the form of nuclear rings or spirals. The centre of the Milky Way is orders of magnitude closer than any comparable target. It is therefore a crucial laboratory for studying galactic nuclei and their role in the context of galaxy evolution, but extreme interstellar extinction and the high density of sources pose serious observational obstacles. The stars at the Galactic centre can therefore only be observed in the infrared and data with sufficiently high angular resolution and wavelength coverage (at least three filters) existed for only about 1% of its area before the start of this project. GALACTICNUCLEUS has been specifically tailored to overcome these observational challenges. It was made possible through a key technique, developed by the Principal Investigator, to obtain the necessary sensitive, high-angular resolution images with a stable point spread function over large, crowded fields.

In 150h of observations with the HAWK-I wide-field near-infrared camera at the European Southern Observatory's VLT telescope, we have thus surveyed the inner 0.3 square degrees, or about 60,000 square light years at the distance of the Galactic Centre, in three near-infrared filters with an angular resolution of 0.2". Confusion is about a factor of ten less than in any previous comparable survey. The GALACTICNUCLEUS survey thus provides a revolutionary new view of the centre of the Milky Way. We can detect on the order 100 time more stars than in any comparable previous work. The data have been made public for the entire astronomical community and constitute a treasure chest to improve our knowledge of the innermost region of the Galaxy and to prepare observations with the upcoming next-generation instruments and telescopes.

The novel data from the GALACTICNUCLEUS survey provide us with a new insight into the formation history of the Galactic centre: Over 80% of the stars formed very early in the Milky Way's life, more than 8 Gyr ago. Since both star formation and black hole growth require the presence of gas, the central black hole probably acquired most of its mass during this time, too. The Milky Way is a barred spiral and bars are generally efficient at transporting gas towards the centre, which can then be converted into stars. Suprisingly, hardly any star formation occurred in the Galactic centre between about 8 Gyr and 1 Gyr ago, which implies that gas transport in the bar was stalled or that the bar only formed relatively recently in the Milky Way's life. About 1 Gyr ago, about 5% of the stellar mass of the Galactic centre formed within a time of less than 100 Myr. This outbreak of star formation must have given rise to more than 100,000 supernovae that exploded within a similarly short time scale in the Galactic Centre. Conditions then calmed down again, but in the recent past, star formation was relatively high in the Galactic Centre.

Another outstanding result of our project is that we could finally identify the so-called stellar "cusp", an exponential density increase of the old stars towards the central black hole. This implies that densities in excess of one hundred million stars per cubic light year are reached within the central 12 light days of the black hole. The discovery of the stellar cusp implies the existence of similar structures in other galaxies, which will be important source regions for gravitational waves that can be observed with future space based gravitational wave observatories.