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Globular clusters as living fossils of the past of galaxies

Periodic Reporting for period 2 - ArcheoDyn (Globular clusters as living fossils of the past of galaxies)

Reporting period: 2019-03-01 to 2020-08-31

Globular clusters (GCs) are the living fossils of the history of their native galaxies and the record keepers of the violent events that made them change their domicile. This project aims to mine GCs as living fossils of galaxy evolution to address fundamental questions in astrophysics:

1/ What are the seeds of supermassive black holes in the centres of galaxies? ⟷ Do GCs harbour the intermediate mass black holes?
2/ How did star formation originate in the earliest phases of galaxy formation? ⟷ Can the origin of multiple stellar populations in GCs be uncovered from distinct dynamical fossil signatures?
3/ Do satellite galaxies merge as predicted by the hierarchical build-up of galaxies? ⟷ Can we recover disrupted satellite galaxies from the GCs they left in their host?

To answer these questions we develop and apply population-dependent dynamical models to take full advantage of the emerging wealth of chemical and kinematical data on GCs.

The algorithms developed and applied to answer the above science questions are also applicable beyond astrophysics to various technological challenges presented by e.g. Big Data and super-resolution imaging in medicine, biology, and engineering. Team members will receive training through research in the top five skills needed in the 2020s landscape: complex problem solving, critical thinking, creativity, people management, and coordinating with others. The outcomes of this project and astrophysics in general can spark the interest of the public and serve as a gateway, particularly in young people, to scientific concepts in STEM.
The ERC program ArcheoDyn consists of three projects. The first and second projects employ the stellar fossil record inside globular clusters (GCs) within the Milky Way, whereas the third project uses GCs as living fossils of the hierarchical build-up of galaxies.

Project 1: Robust dynamical inference of intermediate-mass black holes in globular clusters

A set of realistic simulations of GCs have been explored and mock data created for clusters with and without a central intermediate-mass black hole (IMBH).

First results show that current dynamical models are typically unreliable to recover the presence or absence of an IMBH (Aros et al., subm.).

Next, observational limitations for the detection of IMBHs in GCs are analysed as well as extension to a larger sample of simulated GCs.

Project 2: Dynamically uncovering the origin of multiple populations in globular clusters

Three sets of results have successfully been completed related to this project.

Firstly, a detailed analysis of the stellar populations and kinematics of the GC M54 revealed the presence of a centrally concentrated metal-rich young population in the cluster. Simulation comparison led us to the interpretation that this population likely formed as the result of a strong tidal interaction between the GC and the Milky Way (Alfaro et al. 2019, 2020).

Secondly, chemo-dynamical modelling of the GC M80 suggests that the three populations must have formed with primordial kinematic differences: a novel constraint on the origin of this phenomenon (Kamann et al. 2020).

Thirdly, presentation of a novel method to perform population-orbit decomposition of stellar systems and validation against a suite of simulations (Zhu et al. 2020).
This latter novel method is more flexible than those previously used, and can take advantage of a greater range of input data. It will be applied to the latest GC datasets to uncover the origin of multiple populations.

3/ Dynamically recovering satellite galaxy mergers via their surviving globular clusters

The problem is being tackled from different and complementary sides: combining (1) theoretical studies on the information needed to recover the gravitational potential with accreted globular clusters, (2) simulations to develop, verify, as well as understand the caveats in novel methodologies, and (3) observations to gain insight into the formation histories of individual galaxies.

This multi-faceted approach already yielded several new insights.

Firstly, constraining the gravitational potential with accreted GCs only works if we have a-priori knowledge about the expected degree to which they spread in phase-space (Lilleengen, Trick & van de Ven 2019).

Secondly, uncovering the satellite merger history with GCs requires, in addition to position and radial velocity measurements of the GCs, also chemical information (Jethwa et al., in prep.). At the same time, a galaxy's accretion history can be constrained independently from
an integrated spectrum of its stellar component (Boecker et al. 2020).

Henceforth, large, high quality kinematic and metallicity datasets are essential to get a comprehensive picture of the properties and formation of galaxies. Such a data-set of velocities and metallicity measurements of hundreds of GCs in galaxies in the Fornax galaxy cluster is now available (Fahrion et al. 2020a,b) and its wealth of information will be explored to uncover the past of galaxies.
Project 1: Robust dynamical inference of intermediate-mass black holes in globular clusters

For the first time, a systematic investigation with realistic mock data from state-of-the-art simulations of globular clusters is being performed to investigate whether common modelling techniques, as well as observational limitations, allow for a robust IMBH inference.

Next steps are extending GC dynamical modelling techniques to overcome biasing assumptions and take into account systematic uncertainties in observations.

Project 2: Dynamically uncovering the origin of multiple populations in globular clusters

The combination of population plus kinematic measurements of individual stars in GCs, revealed that the chemically-identified multiple populations are also dynamically distinct. In parallel, a novel population-orbit superposition method was developed and validated which allows to disentangle the formation history of stellar systems.

Next steps are to enable the construction of these novel population-orbit superposition models through fitting of resolved stars in GCs (as part of new DYNAMITE code) and uncover the orbital properties of the multiple populations to unveil their origin.

Project 3: Dynamically recovering satellite galaxy mergers via their surviving globular clusters

In-depth investigations showed that, in addition to positions and radial velocities of GCs, information on their chemical properties is needed to uncover the merged satellite galaxies with whom they were accreted. In parallel, an innovative population-decomposition method was developed and validated to unveil a galaxy's accretion history from an integrated spectrum of its stars.

First next step is to robustly infer the gravitational potential of nearby galaxies in the Fornax cluster by simultaneously fitting available measurements of the stellar kinematics and velocities of their GCs. Second step is to enable, in the same Fornax galaxies, the above novel population-orbit superposition models to fit simultaneously the velocities and metallicity/colour measurements of the GCs in order to uncover satellite mergers. Third step, in parallel, is to enable application of the innovative population-decomposition method to the integrated-light spectrum of the stars to constrain in a complementary way the satellite mergers.
The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey
Participants of our Vienna Dynamics Workshop, 8-11 October 2019