Cluster and Galaxy Formation

Understanding the formation of galaxies and clusters is a crucial part of cosmology, connecting our observable Universe to the underlying dark matter and dark energy.

Rich Galaxy Cluster Simulation
Galaxies and X-ray emitting gas in a simulation of a rich galaxy cluster. Credit: David Barnes/C-EAGLE project/Virgo Consortium

Galaxy formation plays a central role in modern cosmology as many cosmological probes involve galaxies, from mapping their spatial distribution to counting galaxy clusters, to studying the distorting effect of gravitational lensing on the galaxies by the intervening
dark matter. On large scales, we expect galaxies to trace the underlying dark matter distribution. We can also use lensing to infer the "clumpy-ness" of dark matter around and in-between galaxies. Measuring the galaxy and cluster distribution across cosmic time also helps us measure the growth of large-scale structure in the Universe, as the dark matter collapses under its own gravity, building up larger and larger structures such as clusters.

The Universe also contains “normal” matter, or baryons, which is a complex mixture of galaxies and intergalactic gas. Galaxies form from this gas (which is mainly hydrogen and helium), but also return material in the form of energetic winds and jets, powered by exploding stars and super-massive black holes. Understanding the role of this “feedback” is crucial to our understanding of galaxy formation as a whole, and cosmology is now reaching a level of precision where we must take these baryonic effects into account.

Our Research

Galaxies and X-ray emitting gas in a simulation of a rich galaxy cluster.
Simulated dark matter distribution showing the large-scale structure as traced by the galaxies and clusters. Credit: David Barnes/Virgo Consortium

At JBCA, we use detailed computer simulations to study how galaxies and clusters form with our main aim being to improve our understanding of baryonic physics in cosmology. Our simulations start with realistic cosmological initial conditions, contain both the dark matter and baryonic components, and include the effects of dark energy on the expansion of the universe.

We include both gravity and hydrodynamics in our calculations, as well as model the key astrophysical processes associated with galaxy formation: gas cooling and heating; star formation; growth of super-massive black holes and feedback from massive stars and active galactic nuclei. We compare our simulations to a range of observational data, from radio through optical/infrared to X-ray wavelengths, and are involved in a number of observational programmes. Our work is particularly focused on galaxy clusters, the most extreme environments for galaxy formation and also where the most massive galaxies can be found.

Research Activities

  • Galaxy Formation Physics

    Structure formation is driven by gravity but the observable properties of galaxies are also shaped by complex processes that include  radiative cooling of gas, multiphase hydrodynamics, star formation and feedback from stars and active galactic nuclei (AGN). Accurately modelling these processes is challenging since the required dynamic range in spatial scale is enormous.

    With current knowledge and computing power, we resort to modelling the physics using physically-motivated prescriptions that attempt to capture the gross behaviour  of the physics occurring below the resolution scale. At JBCA, we are particularly interested in developing new AGN feedback models, coupling the growth of black hole to the subsequent release of energy on super-galactic scales. This feedback is especially important for modelling galaxy formation on cluster scales.

  • Cluster Evolution

    Cluster cosmology fundamentally depends on our ability to calibrate the relationship between global cluster observables (such as their X-ray luminosity, Sunyaev-Zel’dovich effect, or galaxy velocity dispersion) and dark matter halo mass, since it is the latter that is most affected by the underlying cosmological model.

    Observations have verified that these observables correlate with halo mass but accurate calibration requires us to understand both the baryonic physics (which affects the observables) and our ability to estimate the mass (e.g. assuming dynamical equilibrium or using gravitational lensing). At JBCA, we are involved in a number of projects, and use our own simulations, to investigate these issues. We make predictions for a variety of observables at X-ray, optical/IR and radio wavelengths. We are also involved in several observational consortia such as the European Space Agency’s upcoming Euclid (optical/IR) and Athena (X-ray) missions.

  • Gravitational Lensing

    This is a key area in cosmology research at JBCA. It is one of the most important methods for probing the growth of structure, which consequently allows us to measure the dark matter and dark energy content of the Universe. Accurate models of lensing due to structure formation require simulations as they must include non-linear and baryonic effects.

    We are involved in a number of simulation projects that make predictions for lensing studies, with particular focus on the impact of the baryons. We use simulations to look at both strong and weak lensing. In the case of strong lensing, the signal is very much affected by the central galaxy that has condensed at the centre of the dark matter halo; simulations are vital to understand this process. For weak lensing, we are interested in both cluster mass reconstruction and cosmic-shear effects on larger scales, both for optical (e.g. for Euclid, LSST) and radio (e.g. e-Merlin, SKA) wavelengths.

  • Animation (artist's impression) showing the formation of a galaxy cluster. Credit: Klaus Dolag (MPA, Garching).

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