Supermassive black holes and nuclear stellar disks, nuclear keys to galaxy formation and evolution

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Like the pieces of a Cosmic Jigsaw, galaxies come in different shapes and colours. Understanding how galaxies evolved their present-day properties is one of the most important and longstanding issues in astronomy. Among the scientists in this field, common wisdom has it that galaxies formed from a combination of star-formation episodes, during which gas turns into young blue stars, and merging events, whereby galaxies collide and reassemble into different galactic configurations. If galaxies formed all at once, they would only become redder as their stellar population ages, without changing their shape. Whereas the star-formation history of galaxies has been measured by many observations, the pace at which galaxies merge through cosmic times remains largely unknown. In this respect, astronomers are left only with the predictions of their models. According to the widely accepted model of a Universe dominated by cold dark matter, because galaxies grow in mass over time by merging together, the largest galaxies have assembled just recently, whereas the smallest galaxies were already in place early on in the history of the Universe. Furthermore, galaxies in groups or clusters should have assembled before their counterparts in less densely populated environments. Galaxies can indeed find it difficult to merge once they crowd together. This is because they tend to swing by each other too fast for them to coalesce. As an Advanced Fellow I will test these theoretical predictions. For this investigation I will adopt as a record of the time when galaxies merged together the age of the stars in the small disks that are often found in the centre of galaxies. Numerical simulations show that such nuclear disks are easily destroyed when galaxies collides, so that their presence in a given galaxy indicates that no such a catastrophic event occurred since the disk formed. If the cold dark matter model is correct, I should find that nuclear disks are made of older stars in smaller galaxies, or in those living in the most crowded galactic environments. By the turn of the century, another puzzling component of galaxies turned out to have a key role in galaxy formation and evolution: Supermassive Black Holes. Once considered by astronomers only as a theoretical possibility, supermassive black holes have now been found to lurk at the center of many galaxies, including our Milky Way. Most importantly, the mass of the central black holes appears to scale with the mass of their host galaxies, or only to that of the bulge component of disk galaxies. Roughly speaking, central black holes have been found to be almost one thousand times less massive than their host galaxy or bulge. This striking coincidence between objects that are vastly different in size, with supermassive black holes extending at most to the size of our Solar system, strongly suggests that the evolution of supermassive black holes and galaxies is intimately related. The universality of this relationship, however, is not yet established. This is because the mass of supermassive black holes has been measured only in a limited number of galaxies, and almost exclusively in certain kinds. It would be surprising if galaxies that formed in different ways were found to share the same black-hole mass content. As an Advanced Fellow, I will complete the demography of supermassive black holes using large ground-based telescopes that can correct for the atmospheric blurring with the aid of bright reference stars or point sources created in the upper atmosphere by powerful lasers. This will allow me to measure the mass of supermassive black holes as if I were using the Hubble space telescope, and to observe precisely those galaxies for which there are very few black-hole mass measurements. Using these new data I will test the universality of the relationship between supermassive black holes and galaxies, and shed more light on the nature of this link.