Observational Galactic Chemo-dynamics: revolutionising our understanding of stars and the Milky Way galaxy

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In her research studying the physics of stars and chemical evolution of the Milky Way galaxy, Dr Maria Bergemann of the Max Planck Institute, Heidelberg, develops computer models of stellar spectra and compares them with observations. Using an established physical theory and observational data from the most advanced telescopes, she has been able to provide important observational constraints to help explain the evolutionary majesty of our Milky Way galaxy.

Chemical abundances and ages of stars in the Milky Way disc measured using the Gaia-ESO survey data.
The observed data suggest that our Milky Way galaxy has grown inside-out. © Amanda Smith

To enable us to understand how we evolved biologically, socially and technologically, we delve into our environment, digging up physical artefacts of our past. Once uncovered, this previously hidden information from the fossil record can then be used to piece the jigsaw together. Gradual completion of the puzzle then provides us with a historical narrative, and deeper insights, into our development. With the astrophysical analogue this principle is replicated, although instead of excavating rocks, fossils and soil samples, scientists collect and interpret light information from stars using current scientific technology. The Very Large Telescope (VLT, Chile), the Keck Observatory (Mauna Kea, Hawaii) and ESA’s Gaia satellite have all been combined in unison to provide the required data for studying the evolution of our Milky Way galaxy and its structure. By observing young and old stars in different parts of the Milky Way, scientists can look back in time, as stars of different ages depict our Galaxy at different stages in the past. This physical reality is an important resource for astrophysicists such as Dr Maria Bergemann, who has dedicated her career to developing theories describing the formation of starlight, analysis of chemical abundances in stars, and using relevant data to investigate the evolution of stellar populations in the Milky Way galaxy.

Obtaining new data
One critical element to consider when studying complex astrophysical systems, such as galaxies, is getting the right observational sample. To date, most stars were observed in the local solar neighbourhood, that is, in a tiny volume around the Sun. This limited target area left the scientists essentially ‘blind’ to more distant dark corners of the Galaxy.

To overcome this issue, Bergemann and her collaborators have long sought alternative strategies for observing more distant stars. To achieve this goal, there is a sensible combination of different factors such as telescope mirror dimensions, instrumental properties of the spectrograph, and the physical parameters of stars to consider. Dr Bergemann points out that this ultimately requires larger telescopes to harvest better quality data at near-ultraviolet and optical frequencies, which are home to spectral lines of most chemical elements. Additionally, useful information can also be obtained from the infrared spectra of stars, and with suitable equipment such as adaptive optics (AO) and multi-object spectrographs, one can turn the gaze to fainter more distant objects and crowded stellar regions, like the Galactic bulge. Other considerations, such as target range selection, also come into play: smaller stars, like our Sun, are intrinsically fainter, and are easier to observe nearby, whereas larger stars are more luminous and can be more easily seen at greater distances. Selecting targets to observe is a complex task – and a headache for researchers – but vitally important for research on Galactic chemical evolution theories.

We will revolutionise the understanding of stellar physics, Galactic structure and evolution

Recently, Dr Bergemann’s group has focused on optimising strategies to select the right targets for relevant scientific problems. Dr Camila Juul Hansen, who recently joined the group, plays a crucial role. To study the Galactic outskirts, the outer regions of the disc and ancient halo stars, they chose the largest telescopes: the Keck single-dish 10m telescope and the LBT 8.4m binocular facility were all utilised. Efforts have been beneficial, despite uncomfortably long integration times needed to get the required data quality. This problem will hopefully be solved using the forthcoming E-ELT facility (European Extremely Large Telescope – first light 2024). For extra-galactic work, infrared instrumentation has been selected as the most promising technique, where the largest stars in the Universe – the so-called ‘red supergiants’ – can be observed at vast distances up to and beyond tens of megaparsecs past the so-called Local Volume galaxies.

The Milky Way galaxy, perturbed by the tidal interaction with a dwarf galaxy, as predicted by N-body simulations. The locations of the observed stars above and below the disc, which are used to test the perturbation scenario, are indicated.
© T.Mueller/C.Laporte/NASA/JPL-Caltech

Improving current stellar models
Historically, modelling starlight (i.e., stellar spectra) has been carried out using simplified models that have been computed on assumptions of Local Thermodynamic Equilibrium (LTE), one-dimensional geometry and hydrostatic equilibrium. This has been a problem, as results based on these models were hindered by substantial, critical systematic errors. In a breakthrough advance, Dr Bergemann and the team have developed new modelling techniques that rectify these issues. This includes realistic physics, involving interactions between radiation fields and gas particles contained within stellar atmospheres. To help implement these novel techniques, new group member Dr Andrew Gallagher has been brought into the team. His role has been multifaceted but has predominantly involved providing the expertise on radiation transfer in 3-dimensional (3D) hydrodynamical simulations describing stellar convection. The PhD student Mikhail Kovalev is developing new spectral models based on Non-local Thermodynamic Equilibrium physics for different chemical elements. This has been a long-standing problem that has been vitally important to solve. Previous attempts to describe the electromagnetic radiation field emitted from stellar surfaces using 1D LTE modelling techniques have shown uncertainties can escalate quickly, corrupting any final description. Integrating specially developed numerical programmes, the team were able to overcome this issue. Using this upgraded technique, they were able to show the newly created models give more physically realistic descriptions of emitted starlight. The knock-on effect of this advance has been critical, providing the team with more accurate results relating to fundamental parameters of stars and their chemical makeup.

The observed high-resolution spectrum of a red giant, HD 122563, in the Galactic halo. Dr Bergemann and the team use such spectra to determine chemical composition of stars and study chemical evolution of the Milky Way.

Using data to explore our Milky Way Galaxy
The strongest observational constraint on the evolution of our Galaxy comes from the studies of the chemical composition of stars and their space motions. With realistic models of stellar spectra at hand, and a suitably selected sample of stars, researchers are well equipped to look for signatures of various events and processes in galaxy formation. Processes such as the redistribution of gas, outflows, variability of star formation activity, and mergers with satellite galaxies have all been defined for investigation. The various questions that have long been asked: How does the interaction with nearby dwarf galaxies shape the structure of the Milky Way? When did it happen? And how? In one of the first studies with the Gaia-ESO survey data, Dr Bergemann and the team explored the abundances and ages of hundreds of stars, showing that our Galaxy has grown “inside-out”, with inner regions assembling faster than the outer regions. This study brought their attention to the structure of the outer parts of the Milky Way. In a subsequent study (Nature, in press), the team observed more distant stars at the outer disc and the Galactic halo using the Keck 10-metre telescope. They were able to provide the cleanest evidence to date for theoretically proposed oscillations of the Milky Way disc caused by interactions with a passing dwarf galaxy. The Gaia-ESO survey and the Gaia space mission act as the primary data source, supplying parallaxes and hundreds of thousands of stellar spectra to aid with the team’s ongoing investigations. Additionally, most stars they observe are expected to host exoplanets, and some may even have Earth-like worlds. The star-planet connection is important, and an area the team actively engage with within their research. As Dr Bergemann highlights, star-planet relationship data will be collected by the forthcoming ESA PLATO which is planned for launch in 2026.

My ambition is to make the new 3D NLTE modelling a new standard in modern observational astrophysics

The future
Stars and their radiation fields are an important, abundant resource that astrophysicists rely on to provide deep insights into the makeup of the universe. Stars themselves are extreme natural laboratories, whose internal physical conditions span many orders of magnitude, both in temperature and gas pressure, allowing unique tests to be carried out on fundamental physical theories.

Historically, a research area that has generally skewed modelling outcomes has been the determination of stellar chemical abundances. Introducing more sophisticated Non-Local Thermodynamic Equilibrium analysis into the research has helped to resolve these issues. This is an area Dr Bergemann has personally developed in her research. Presently they are expanding on this area, creating new models that rely on structures from 3D hydrodynamical simulations of stellar convection, instead of the traditional 1D hydrostatic models. ‘My ambition is to make the new 3D NLTE modelling a new standard in modern observational astrophysics’. As Dr Bergemann concludes, describing her co-lead involvement in the forthcoming high-resolution survey within the 4MOST international project, investigating the Milky Way’s disc and galactic bulge: ‘We will revolutionise the understanding of stellar physics, Galactic structure and evolution.’

To what extent has theoretical astrophysics benefitted from the advancement in computing power, particularly supercomputers?
Computing power is essential for our research. We are running sophisticated calculations of radiation transport in stellar atmospheres, which must resolve extreme variations in the physical state of matter in stars. Stars are our labs and supercomputers provide the platform for carrying out experiments at the ever-increasing level of physical reality.

How do you expect your area of work to benefit from the next generation of ground-based telescopes?
Our models of stellar surfaces must be as close to what we see in nature as possible. Next generation facilities, such as the ground-based 40m E-ELT telescope, will offer unique observational data to constrain our models, but also to break the distance barriers, and observe stars at the extremes of the cosmic distance ladder.

Will your research have a benefit towards the ongoing search for the dark universe – matter and/or energy?
Understanding the properties of dark matter and dark energy is essential for building models of cosmic structure and galaxy formation. As such, advances in this field will bring revolutionary insights and may open a new window into the history of our cosmic origins. No one knows whether dark matter does anything to the interior of stars, but if it does, even stellar physics will undergo a major revision.

4MOST is an exciting project. Are you enjoying working on its implementation and what spectroscopic advances can we expect when it eventually comes online in 2022?
Preparing the operations of the 4MOST facility is a major challenge. It involves the collaboration of hundreds of researchers, engineers, and instrument-builders, and this remarkable tandem is truly fascinating, as people with different backgrounds and expertise unite to achieve one goal – building a versatile multi-object spectroscopy facility to address basically every topic in astronomy, from individual stars in the Milky Way to cosmology.

As an inspirational character to the enquiring mind, what advice can you give to any young, aspiring researchers?
My advice would be to keep experimenting, be creative, not stumble in front of scary maths and physical theory, and never ever give up.

Research Objectives
Dr Maria Bergemann of the Max Planck Institute, Heidelberg develops computer models of stellar spectra and compares them with observations. Using an established physical theory and observational data from the most advanced telescopes, she has been able to provide important observational constraints to help explain the evolutionary majesty of our Milky Way galaxy.

Funding
Independent Research Group/SFG

Collaborators
Martin Asplund, Giuseppe Bono, Remo Collet, Ben Davies, Gerry Gilmore, Rolf-Peter Kudritzki, Bertrand Plez, Hans-Walter Rix, Martino Romaniello, Ralph Schönrich

Bio
Maria completed her PhD under the supervision of Prof Thomas Gehren at Ludwig Maximilians University in Munich. She then moved to the Max Planck Institute for Astrophysics in Garching for her 1st Postdoc with Prof Martin Asplund, followed by her 2nd Postdoc at the Institute of Astronomy (University of Cambridge, UK) to work with Prof Gerry Gilmore. In 2014, she received a large grant from the Max Planck Society and took it to the Max Planck Institute for Astronomy in Heidelberg to establish her own independent research group: ‘Stellar Spectroscopy and Stellar Populations’.