The large-scale structure of the Universe at early cosmic times (left) and today (right). The top panels show the distribution of dark matter, while the bottom panels show where galaxies are located. The side length of each map is 1.5 billion light years.
How do galaxies form? Scientists have long looked beyond our own planet to learn more about the origins of the Universe, and the next generation of telescopes will allow scientists to look back even further into cosmic history. Researchers at LMU in Munich are developing a new approach to modelling the formation and evolution of galaxies, as Dr Benjamin Moster explains. The development of new facilities like the James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT) opens up exciting new possibilities in astrophysics research, enabling scientists to probe deeper into galaxy formation at even earlier periods in cosmic history. While observations from these telescopes provide a snapshot of galaxies and their properties at a given point in time, it is necessary to build models to investigate their formation and evolution, a topic at the heart of Dr Benjamin Moster’s research. “The aim in our research is to understand how galaxies form,” he outlines. Telescope observations are an important part of this, as they provide a picture of galaxies at a particular point in time, the next step then is to investigate how their properties evolved. “We want to try and understand how these observations can be explained with models. We try to build models that connect these observations to the underlying dark matter and explain what kind of physics happen in those galaxies,” explains Dr Moster. 54
A number of different approaches are available for modelling galaxy formation, the most commonly used of which is hydrodynamical simulations. With this approach, researchers take the Universe at a very early time in cosmic history, place some initial conditions on it - derived for example from the cosmic microwave background - and then simulate the physical processes, such as gravity, gas dynamics and cooling, and star formation. “Eventually large gravitationally bound structures will form, so-called dark matter haloes – gas will then collapse in these dark matter haloes and form stars,” says Dr Moster. This approach produces cosmological volumes with galaxies that closely resemble observed galaxies, yet it can be difficult to resolve the physical processes in these hydrodynamical simulations, while Dr Moster says there are also other challenges. “You need to run a simulation for several months on these big supercomputers, and a lot of computing time is required. At the end you then get only a single simulation adopting
one chosen model that you can study,” he outlines.
Empirical galaxy formation models As Emmy Noether Group Leader at LMU in Munich, Dr Moster is now developing a different approach to modelling the formation and evolution of individual galaxies, called empirical galaxy formation models. Rather than trying to model the physics, the aim here is to introduce empirical relations between observed galaxy properties and simulated halo properties. “Unlike the gas physics, we know exactly how gravity works. Since dark matter only interacts gravitationally, we can run gravity-only simulations, which always have the same result,” says Dr Moster. These gravity-only simulations can typically run much faster, because it’s not necessary to calculate any gas physics. “If you don’t have any gas in there then you don’t need to calculate any star formation or feedback from supernovae or black holes, you just focus on gravity,” points out Dr Moster. “What we
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find in these models is that the Universe still evolves very similarly on large scales – matter again clumps in dark matter haloes.” The next step is to then introduce empirical relations between the properties of these dark matter haloes and the observed properties of galaxies, for example between the mass of a halo and the mass of the galaxy at its centre. These relations are then adjusted until the model reproduces
Hydrodynamic simulation of a dwarf disc galaxy as seen from above. The gas cools and and forms molecular clouds where stars are born (red regions). When massive stars die, they explode as supernovae, leading to large bubbles in the gas (dark regions).
reproduces the observations?” he continues. “Over time we’ve experimented with different parameters and models, so we have a degree of knowledge on the levels of uncertainty, and the uncertainty of the predictions that can be made with them.” This provides the foundations on which Dr Moster and his colleagues can then look to learn more about galaxy formation. With improved models, researchers can look at the
We want to try and understand how
these observations of galaxies can be explained with models.
We try to build models that connect these observations to the underlying dark matter and explain what kind of physics
happen in those galaxies. statistical observations, which could relate to how galaxies of a specific mass cluster with each other for example, or how many galaxies form stars in comparison to how many are passive and don’t form stars. “Our models are tuned to reproduce all these observations as accurately as possible,” says Dr Moster. A model can then be run for each dark matter halo and compared with galaxy observations, from which Dr Moster and his colleagues can then gain important insights. “So, which kind of model with which set of parameters best
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evolution of these dark matter haloes over time and address some unresolved questions in astrophysics. “For example, when do galaxies grow? In one of our papers we’ve also looked at how much mass forms in the galaxy itself, and how much comes in through satellite galaxies that merge with that galaxy,” outlines Dr Moster. Researchers have found that around 50 percent of the mass of large galaxies is formed in the galaxy itself, and the other 50 percent comes in through accreting or merging satellites; our own
galaxy is however not on this sort of scale. “The Milky Way is a fairly normal galaxy, with an average mass – only a few percent of its mass comes in through satellites,” says Dr Moster. “Another topic we are investigating is red galaxies. These are galaxies that have stopped forming stars.” The evidence suggests that these red galaxies are typically a bit more massive than blue galaxies in the same halo, now researchers are trying to understand how their properties evolved. This research holds important implications for the operation of the next generation of observational facilities such as JWST, a space telescope which is set to be launched at some point in 2021. “We can help in survey planning, so that the observers could have some initial idea of what they’re going to observe, and if it makes sense for them to look for specific galaxies,” explains Dr Moster. While ideally the observations and the model would match up, identifying any points where they differ gives Dr Moster and his colleagues valuable insights into the physics of galaxy formation and how the model could be improved. “To some extent it’s an iterative process. So as theorists we may have some good ideas of how to improve the models, but then we get interesting observational data, and we can also improve the model through that,” he says.
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From dark to light The connection between galaxies and their dark matter haloes through cosmic time
The Galaxies and Dark Matter research group at the University Observatory at the LMU Munich. Left-to-right: Benjamin Moster (group leader), Joseph O’Leary (PhD Student), Ulrich Steinwandel (PhD Student), Abhishek Malik (Master Student), Aura Obreja (Postdoc), Jannik Bach (Master Student), Alina Stephan (Bachelor Student), Jelena Petereit (Bachelor Student)
Project Objectives
The aim of our research is to study how galaxies form within dark matter haloes, and to better understand the various physical processes that shape their properties. For this we use empirical models, which relate the observed galaxy populations to the underlying dark matter distribution in a statistical manner that is as independent as possible of any model assumptions.
Project Funding
The Dark to Light project is funded through the Emmy Noether Programme of the Deutsche Forschungsgemeinschaft (DFG). The research is hosted at the University Observatory of the LMU Munich.
Project Partners
• Thorsten Naab • Simon White Max-Planck-Institute for Astrophysics, Garching bei München, Germany
Contact Details
Project Coordinator, Dr. Benjamin Moster University Observatory Munich Ludwig-Maximilians-University of Munich Scheinerstraße 1 81679 Munich Germany T: +49-89-2180-9284 E: moster@usm.lmu.de W: www.usm.lmu.de/people/moster W: www.usm.lmu.de/GDM https://ui.adsabs.harvard.edu/ abs/2018MNRAS.477.1822M/abstract Dr. Benjamin Moster
Dr. Benjamin Moster is an Emmy Noether Group leader at Ludwig Maximilians University (LMU) in Munich, where he conducts research into galaxy formation. He is also a guest researcher at the Max-Planck-Institute for Astrophysics in Garching. Previously he was a Kavli Fellow at the Institute of Astronomy at Cambridge University in the UK, while he has also held research positions in Germany and the US.
Astrophysics questions A further important aspect of the project’s research is the possibility of comparing these empirical models with much more complex hydrodynamical simulations of a single galaxy. This enables researchers to assess whether the mass of a simulated galaxy is in the right range for example, or if the star formation rate is too high or low. “It’s possible to essentially zoom in on a single galaxy at very high resolution in these hydrodynamical simulations. Then you can make a direct comparison,” explains Dr Moster. The aim here is to simulate a galaxy at very highresolution, then compare it to empirical predictions. “The empirical model effectively gives you observational constraints. We can then compare those to an individual galaxy, and see if the physics that we’ve simulated leads to the right result, or if it leads to something different,” continues Dr Moster. “If it’s the latter, then we should think about including more physical processes in the simulation that weren’t included previously.” This could help researchers understand the physical processes responsible for the galaxy properties shown in the empirical models, and from that learn more about the physics that drives the formation of those galaxies. This could mean looking at whether supernovae explosions play a major role in galaxy formation, or investigating the
importance of black holes. “We are looking at how black holes affect galaxy growth,” says Dr Moster. Another major avenue of investigation in Dr Moster’s group is whether a change in the background cosmology away from that described in the lambda cold dark matter (ΛCDM) model would affect galaxy properties. “We ran these dark matter simulations not with cold dark matter, but with warm dark matter,” he outlines. “Warmer particles travel faster, which means it’s harder for them to form small structures, so small blocks of dark matter can’t really form. If we take this as the basis for the empirical model then we can ask - how do galaxy properties change?” The development of ever-more sophisticated telescopes will allow scientists to probe deeper into the issues around these kinds of questions. For example the Euclid mission, which is set for launch in 2022, will accurately measure the positions of galaxies in the early Universe, from which Dr Moster and his colleagues could learn more about the nature of dark matter. “If we find that galaxies cluster somewhat differently in the warm dark matter simulation than in the CDM simulation, then we have a prediction and then can say; ‘ok, with this next generation telescope you could observe it and try to nail down whether dark matter is cold or warm,” he says.
Hydrodynamic simulation of a dwarf disc galaxy as seen from its centre.
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