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

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.

In first place, the proposed research is going to benefit the academic community as it will lead to refereed publications providing results on the formation history of early-type galaxies and a new methodology for to dissecting the stellar-population properties of galaxies with multiple stellar components.

More generally, this research will also benefit the general public.

Armagh Observatory and Planetarium (AOP) is one of very few organisations in the world to have both an internationally respected research arm and a state of the art planetarium outreach arm. The result of the recent merger of the former Observatory and Planetarium institutions in 2016, AOP is core funded through the N. Ireland Executive through the Department for Communities who are keen for us to develop closer relationships with schools throughout N. Ireland, especially where astronomy can engage with topics in the curriculum. Funding opportunities arise through our sponsor Department to develop education and outreach activities around the research programmes that we conduct, which is why at the heart of AOP's corporate plan is a strategy to ensure that the Research being conducted at the Observatory is better communicated to the wider public.

To achieve this goal, the research which is being conducted at AOP is fed directly into the work of the Planetarium. It features in shows being shown at the Planetarium, within programmes providing curriculum support to teachers in schools, as part of the education work with students who come to the Planetarium, and in outreach work with communities in Northern Ireland. In addition, AOP research staff have been running Physics A-level workshops and offered the opportunity to undertake short-term research project to school-age and undergraduate students.

AOP research has traditionally concentrated on Stellar and Galactic Astrophysics, Solar-System Science and Solar Studies. With the recent appointment of Head of Research Dr. Marc Sarzi in August 2018, AOP is now expanding its research interests to also cover extra-Galactic studies. This bid seeks to support this new research direction, which will also allow to complete the range of Astronomical topics that AOP staff could cover in its outreach events or comment on in traditional media or social networks. A former holder of an STFC public engagement fellowship, Dr Sarzi also brings a great deal of expertise in organising and running outreach events and in particular with schools pupils and teachers.

More recently, AOP has been moving in the direction dictated by the advent of immersive technologies, and is currently developing in partnership with local business a series of Augmented Reality exhibitions for the general public, which will also feature research inputs from AOP and also from its new extra-galactic branch.