The science behind.

Galaxy cluster are the largest gravitationally bound systems in the universe. They span over millions of lightyears, include hundreds of massive galaxies, enormous amounts of hot, fully ionized gas and, if we trust in gravitational physics, enormous amounts of dark matter. In Fig. 1 the beautiful galaxy cluster Abell 1689 observed with the Hubble Space Telescope is shown.

Source: Hubblesite.org
Figure 1: The Galaxy Cluster ABELL 1689.
Credit: NASA, ESA, L. Bradley (JHU), R. Bouwens (UCSC), H. Ford (JHU), and G. Illingworth (UCSC)

The hot, fully ionized gas, i.e. plasma, is called Intra-Cluster Medium (ICM) and represents the major mass contribution to the direct observable mass component in these millions of light-years large structures. The mean temperature of the ICM is in the range of several million degrees and it has up to five times more mass than all galaxies in a galaxy clusters. In Fig. 2 the X-ray emission (left) and the emission in the optical regime (right) of a galaxy cluster is given.

Figure 2: Left: X-ray emission from a galaxy cluster. Right: The galaxies in the same field of view.
Source: http://chandra.harvard.edu/photo/2003/abell2029/index.html

Since the the first X-ray satellites it is evident that the ICM does contain higher order elements (e.g. Fe, Mg, O, Si, Ni), which are produced in the late evolutionary states of stars. The factories of heavy elements (stars) reside in the galaxies of a galaxy cluster, so the question arises: What are the mechanisms, that transport several 10E30 kg of mass per year into the ICM?

Basically two transport mechanisms are responsible:
- Galactic Winds
- Ram Pressure Stripping

Galactic winds are powered by supernova explosions of stars within galaxies. These explosions accelerate the surrounding gas to escape velocities and mix the gas with heavy elements. They can transport enormous amounts of gas and dust from the inner parts of galaxies in the space between the galaxies. In Fig. 3 an example of such a galactic wind is shown. The M82 galaxy has strong matter outflow perpendicular to the stellar disc.

Figure 3: A Superwind from the Cigar Galaxy
Credit & Copyright: FOCAS, Subaru 8.2-m Telescope, NAOJ

Ram-Pressure Stripping is a passive transport mechanism. As galaxies move through the ICM, they feel the pressure of the ICM on their gas. In the inner parts of galaxy clusters the densities of the ICM and the relative velocities are as high that the galaxies start to peel away their outer gas layers, which contain processed gas with heavy elements.Galactic. In Fig. 4 this effect can bes studied in detail in observations of the galaxies NGC 4522 and NGC 4402 with the Hubble Space Telescope.

Figure 4: The two ram pressure stripping galaxies NGC 4522 and NGC 4402. Credit:NASA & ESA

During my PhD and PostDoc-years I modelled the formation and evolution of galaxy clusters in complex numerical simulations with a special emphasis on such transport mechanism. The results of these simulations were then compated directly with X-ray/Optical and radio observations of galaxy cluster. With the help of these combined N-body/hydrodynamic high resolution galaxy cluster simulations the team, I worked in, was able to find that the two main transport processes are galactic winds and ram-pressure stripping.

With the help of these simulations astrophysicists are able to study the strength and the time dependence of the two processes in detail. This leads to a deep insight into the evolution and the interactions of the different processes in galaxy clusters. All the images in the art galleries give examples of the results of such simulations.


Additional information about this topic can be found in my book