Multi-Pop

The stunning Atoms for Peace galaxy was given its nickname due to its superficial resemblance to an atomic nucleus, surrounded by the loops of orbiting electrons. “Atoms for Peace” was the title of a speech given by President Eisenhower in 1953, in an attempt to rebrand nuclear power as a tool for working toward global peace. Somewhat ironically this galaxy has had anything but a peaceful past — it was formed in a catastrophic merger between two smaller galaxies nearly 1 Gyr ago. Massive star clusters were formed in the merger.

Credit: NASA & ESA, Acknowledgement: Judy Schmidt (Geckzilla)

Understanding the connection between galaxies and globular clusters Globular clusters can be found around almost all galaxies, yet questions remain about their formation and evolution and how they relate to their host galaxies. Recently, it has been found that the stars within globular clusters show chemical anomalies, not found in stars outside clusters. We spoke to Professor Nate Bastian about the Multi-Pops project’s work in studying globular clusters, which could lead to new insights into how galaxies are assembled A type of

star cluster, globular clusters can often be observed in the night sky, and continued study of them could lead to new insights into the formation of galaxies.

For around the past ten years, Professor Nate Bastian and his colleagues have been studying the formation and evolution of these clusters in nearby galaxies. “That includes the Milky Way, but also galaxies where we cannot resolve individual clusters into their constituent stars. In the local universe we can see, in detail, things like how they’re forming, how their properties depend on the local conditions, and how long they live,” he says. Globular clusters are formed fairly rapidly by astronomical standards, over a period of around one or two million years, the result of dense gases being brought together in large molecular clouds. “The star clusters that are formed could be very low-mass objects – they might have just a few hundred stars within them – or they could host 10 million stars, which would be a big globular cluster. The size of the cluster depends on the cloud properties themselves,” explains Professor Bastian.

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E-MOSAICS Researchers in the Multi-Pops project, along with LJMU colleague Dr. Rob Crain and Dr. Diederik Kruijssen and his group in Heidelberg, now aim to use the information that has been gained about globular clusters to build a deeper understanding of their formation

that are forming today are essentially the same as the clusters that formed in the early universe,” says Professor Bastian. This is by no means fully established as fact in the field, yet the results of research so far broadly bear out the initial assumption. “With E-MOSAICS, we have found that we are able to broadly

With E-MOSAICS, it is now possible to trace back the globular clusters to the extreme conditions under which they formed, and to see how globular cluster populations are shaped by the growth of their host galaxies and evolution. “We’ve taken what we’ve learned from these earlier studies, and used it to guide the development of a new suite of simulations called E-MOSAICS (Modelling Star Cluster Population Assembly in Cosmological Simulations within EAGLE). These simulations incorporate Kruijssen’s ‘MOSAICS’ model of cluster formation and evolution into the EAGLE simulations of galaxy formation, in whose development Crain played a leading role. The big leap of understanding is that the clusters

reproduce the globular cluster populations that we see today, more than 9 billion years after their formation,” continues Professor Bastian.

The simulations show that the peak of globular cluster formation occurred between 10 and 11.5 billion years ago, which was also the peak time at which stars formed in the universe, indicating that the formation of globular clusters is related to the formation of stars. “Our model suggests that globular clusters are not in fact particularly special,

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“Visualisation of the E-MOSAICS simulations. Ten Milky Way-like galaxies were chosen from the full box of the high resolution (“Recal”) EAGLE box. The main panel shows the dark matter distribution in the box, with yellow circles highlighting the positions of the galaxies resolved in the zoom-in simulations. The panels towards the right show for a single zoom-in simulation the gas density (top) and simulated optical images of the face-on (middle) and edge-on (bottom) views of the final galaxy. The five panels in the bottom row show the evolution of the gas density in the galaxy and its star cluster population from high redshift (z=10) to today (z=0).”

but rather the culmination of the average star formation process, hence globular clusters are just tracing the star formation history of the Universe” says Professor Bastian. This contrasts sharply with other theories of globular cluster formation, which suggest that they were all formed even earlier in the Universe. These theories invoke special conditions only present in the early universe to form globular clusters, so that all globulars will have ages greater than 12 or 13 billion years. Professor Bastian says the project’s simulations show a different picture. “In our simulations, no special conditions are needed, in fact we find that some globular clusters are still being formed today,” he says.

The E-MOSAICS project is jointly led by the LJMU team (including Drs. Joel Pfeffer and Rob Crain), Ms Meghan Hughes and the team of Dr. Diederik Kruijssen at Heidelberg University.

chemically homogenous, and that stars within a globular cluster have different chemical abundances; researchers in the project are also investigating the origin of these abundance variations, known as multiple populations. “In the vast majority of clusters iron is constant.

But, we see star-to-star variations in very specific elements, in particular helium, carbon, nitrogen, oxygen and sodium,” explains Professor Bastian. “These variations are not random. For example, if a star is rich in sodium,

it is poor in oxygen – if it is rich in nitrogen, then it’s also poor in oxygen.”

Another branch of the Multi-Pops project is investigating the origin of these chemical anomalies and what insights can be drawn into globular cluster formation. This part of the project is observationally driven.

Professor Bastian and his colleagues are using data gathered on globular clusters from the Hubble Space Telescope and the Very Large Telescope (VLT) in Chile, with the aim of testing different scenarios that have been put forward to explain these variations in abundance. “Certain predictions are derived from these scenarios, which we then test against the data,” he outlines. Some exciting results have been gained from this work. “The first is that none of the models work. So we have essentially ruled out all of the scenarios that have been put forward in the literature.

This might seem disappointing, but it’s also exciting, as it opens up new avenues of research,” says Professor Bastian.

“The other exciting finding is that we’ve seen a surprising age effect in the presence of these multiple populations. We see these chemical anomalies within all of the clusters above about 2 billion years old (Gyr), whereas we do not find this in clusters younger than 2 Gyr. However, the finding of chemical anomalies in a cluster as young as 2 Gyr is one of the strongest pieces of evidence that we have linking young and ancient globular clusters.”

This part of the Multi-Pop project is being led by Ms. Silvia Martocchia (a PhD student at LJMU) and Drs. Ivan Cabrera-Ziri (Harvard) and Carmela Lardo (EPFL, CH).

Future Directions A lot of the detail around these findings still needs to be filled in, and researchers continue to investigate the underlying causes behind the differences that have been observed.

Abundance variations Another radical change in our understanding of globular clusters is that they have traditionally been 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). However, it’s now known that globular clusters are not in fact

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Part of the Multi-Pop team (from left to right): Dr. Joel Pfeffer, Dr. Ivan Cabrera-Ziri, Dr. Carmela Lardo, Dr. Chris Usher, Ms. Silvia Martocchia, Dr. Sebastian Kamann and Prof. Nate Bastian (missing Ms. Meghan Hughes, Ms. Hannah Dalgleish, and Drs. Maria de Juan Ovelar and William Chantereau).

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Multi-Pops Fulfilling the Potential of Globular Clusters as Tracers of Cosmological Mass Assembly

Project Objectives

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.

This project turns the problem around to exploit these differences to understand the co-evolution of GCs and their host galaxies.

Project Funding

The project is funded by an ERC Consolidator Grant (PI N. Bastian), Royal Society University Research Fellowship (PI N. Bastian). The work on “E-MOSAICS” is being carried out in collaboration with Dr Diederik Kruijssen, who is funded through an ERC Starting Grant and an Emmy Noether Independent Group Award and Dr Rob Crain who is funded by a Royal Society University Research Fellowship.

Contact Details

Professor Nate Bastian Royal Society University Research Fellow Head of Research - Astrophysics Research Institute Liverpool John Moores University T: +44 151 231 2933 E: N.J.Bastian@ljmu.ac.uk W: http://www.astro.ljmu.ac.uk/~njb/MultiPops_ERC.html W: http://www.astro.ljmu.ac.uk/~astjpfef/emosaics/

This will form an important part of Professor Bastian’s future research agenda, alongside running further simulations.

“Together with Dr. Kruijssen’s group, we’ve been trying to simulate a Milky Waytype galaxy, and we’ve run well over 300 simulations, in which we have changed the parameters. We’ve explored parameter space, and how we implement the physics of cluster formation and evolution,” he outlines. “Every time we run a new simulation for E-MOSAICS, we can turn off different physical processes, in order to highlight which process is dominant. That’s really what we’re looking for here – what’s the major factor behind the observations that we see?”

The next-generation of cosmological simulations of galaxy formation will adopt higher spatial resolution and more detailed treatments of the interstellar medium.

By incorporating these developments in E-MOSAICS, Prof. Bastian’s team and colleagues in LJMU and Heidelberg will be able to probe deeper into the origin and evolution of globular clusters. “We’ll use those models, and insert our model on the top of it,” he says. The wider goal in this research is to place globular cluster formation and evolution within the wider cosmological context, which is a long-held ambition in the field. “This has been tried many times in the past, with varying degrees of success, but this

study is breaking new ground,” says Professor Bastian. “We are following the formation of globular clusters – depending on the local conditions – then we follow them through time. We trace where the globular clusters actually end up in the galaxy, so we have spatial information as well.”

A number of collaborations have been established to investigate these predictions and compare them with observed data.

While in the ideal case the simulations exactly correspond with observed data, inaccuracies help researchers to improve models further. “We’re trying to really push the models to their breaking point – to figure out exactly what’s going wrong and what’s going right – and then insert new physics,” explains Professor Bastian. On the observational side of the project, Professor Bastian plans to investigate the age effect that has been observed in the composition of globular clusters in more detail. “Our application for time with the Hubble space telescope next summer has been approved, and we also hope to get some time with the VLT in Chile,” he continues. “We have two relatively large proposals to explore the age effect in more detail. For example, what is the exact age? So we are going to sample that age range more finely, and try to figure out exactly when this multiple populations phenomenon comes in.”

©NASA, ESA, and Martino Romaniello (European Southern Observatory, Germany).

Professor Nate Bastian

Professor Nate Bastian gained his PhD from Utrecht University (The Netherlands) in 2005. He has held Postdoctoral positions in University College London, Cambridge and Exeter Universities. He’s been awarded the STFC Advanced Fellowship (now known as the Rutherford Fellowship), and the Royal Society University Research Fellowship. He was a Senior Scientist at the Excellence Cluster Universe in Garching, Germany for 2 years.

He is currently a Professor of Astronomy at Liverpool John Moores University.

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