Dissecting the orbital and stellar-population structure of early-type galaxies with MUSE

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How galaxies form and evolve is a key open question in modern astronomy, driving billions of dollars of international research infrastructure investment over the next ten years. Modern cosmological computer simulations of how galaxies form show that the orbits of stars and their element-abundance hold as-yet untapped information on both on how star-formation proceeded over time in galaxies and how these also grew by accreting other companion galaxies.The project proposed here will uncover this `fossil record' by making full use of the unprecedented quality collected for galaxies in the nearby Fornax galaxy cluster (the Fornax3D project, led by applicant Sarzi) with the MUSE integral-field spectrograph installed on the Very Large Telescope of the European Southern Observatory. Such integral-field spectroscopic data allow indeed to measure the mean stellar spectrum observed at any particular point in these, which in turn allows to infer the mean motion of stars at that same point as well as an estimate of the mean stellar age and chemical content. This information is generally sufficient for Astronomers in the case of those galaxies where we have reasons to believe that stars formed over one single event. For most galaxies this is not the case, however, and the arduous task ahead is that of identifying from the data the different families of stars are currently present in galaxies, and understand how and when these came to be. For instance, the Milky Way and other disk galaxies is formed of three main stellar components. A central spherical bulge of old stars, a thin disk where young stars continue to form and relic, thicker disk of older stars. In the case of galaxies where star-formation is no longer occurring, the so-called early-type galaxies (ETGs, known also as elliptical or lenticular galaxies) the situation is less clear, although it evidence has been mounting that most of these objects do also harbour three distinct bulge, thin and thick components. This would also be in keeping with the idea that most ETGs descend from disk galaxies such as the Milky Way, although exactly how remains to be understood. Indeed, understanding this potential link is precisely the goal of this project, which aims understanding for instance how quickly stars ceased to form in the disk components ETGs and what role was played in particular by galactic environment, such as entering a group of galaxies like the Fornax cluster which is typical of where ETGs tend to live.To tackle the difficult problem of measuring the star-formation history of each of the disk components of ETGs, here we will draw from the unique modelling expertise within for Fornax3D collaboration to first identify the main families of stars in ETGs on the basis of their stellar orbits and models that can match simultaneously the observed stellar light distribution and stellar motions. These models will then allow to know how many stars move in which way at any point in the target galaxy for each stellar family (e.g. bulge stars, thick and thin disk stars), which will make it much more easier (and robust) to deduce from the observed stellar spectra the mean age and chemical content of the stars from each stellar family that we observe at that same point. Mapping the stellar age of the faded stellar disks of ETGs will already inform us on the processes that led to the cessation of star formation, as some processes will imprint different age trend with distance from the galaxy centre.