Periodic Reporting for period 4 - Multi-Pop (Fulfilling the Potential of Globular Clusters as Tracers of Cosmological Mass Assembly) Reporting period: 2020-03-01 to 2021-08-31 Summary of the context and overall objectives of the project Globular clusters (GCs) are among the oldest luminous sources in the universe, bearing witness to the earliest stages of galaxy formation as well as their evolution to the present day. While GCs have played a pivotal role in our understanding of the assembly of galaxies, their full potential remains unfulfilled due to our lack of understanding of how they form. One of the largest stumbling blocks has been the anomalous chemistry (both metallicity distributions and abundance patterns) of GCs relative to field stars within galaxy. Here, we will turn the problem around and exploit these differences to understand the co-evolution of GCs and their host galaxies.Our understanding of GCs and their formation has undergone a radical change in the past two decades. First, it is now clear that while traditionally thought of as the quintessential simple stellar populations (i.e. all stars within a cluster have the same chemical abundances and age within some small tolerance), globular clusters host multiple stellar populations with spreads in He, many light elements (e.g. Na, O, Al) and even Fe in a few cases. Secondly, GCs, once thought to only be able to form in the special conditions present in the early Universe, are now known to be still forming today (known as Young Massive Clusters - YMCS). These two facts have opened up a new window into the interconnectedness of GC and galaxy formation and co-evolution.In this project we will quantitatively test current GC formation models with observations of YMCs, as well as organise what is known of the stellar populations within GCs (e.g. abundance spreads, CMD morphologies), providing, for the first time, a global view (i.e. which characteristics are specific to individual GCs and which are common to all GCs). These results, when combined with what is known about massive cluster formation in the local universe, will provide an unprecedented opportunity to use GCs to constrain the hierarchical assembly of galaxies. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far We have made substantial progress on WP1 - using young massive clusters to constrain the origin of multiple populations in the ancient globular clusters. The main outcome so far is that all the theories that have been put forward for the origin of multiple populations have now been proven wrong, or at least their predictions are not in agreement with current observations. Additionally, we have been working on our own Hubble Space Telescope Survey of massive stellar clusters in nearby galaxies (the Large and Small Magellanic Clouds) and have found a surprising result, that multiple populations are present in all clusters in the survey older than ~2 Gyr, whereas all clusters younger do not. This suggests that age plays a strong role in the onset of multiple populations and points to a possible stellar evolutionary effect as the origin. This will be a rich avenue for future studies.Along side this work, we have been implementing the inclusion of globular cluster formation and evolution in cosmological hydrodynamical simulations (E-MOSAICS). This is the first time that a self-consistent description of GC formation and evolution, along with the local and global properties of the host galaxy has been undertaken. We plan to submit the first paper of a (large) series on this in the next month or so. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) We have made substantial progress on two of the most important questions in globular cluster research, 1) what is the origin of multiple populations and 2) how to GCs form and evolve and can they be used to trace the build up of their host galaxies. The E-MOSAICS simulation
