Simulating the ISM and its processes

Author: Sebastian Vider

The physics of the Interstellar Medium (ISM) is the subject of current research. Discovering its laws and understanding its nature is an important piece of the puzzle to further our understanding of the universe. The processes in the ISM connect the small^* scales, where star formation takes place, to the evolution of entire galaxies.

What is the ISM?

The ISM is all the stuff that fills the space between the stars inside a galaxy.
It consists predominantly of hydrogen and helium gas. Other important components are dust particles, radiation, cosmic rays, magnetic fields, and others. The dynamics of the gas in the ISM is governed by large-scale movement like the rotation of the galaxy and more local forces such as self-gravity, which is the gravitational force that the gas feels due to its own mass. One can imagine that the rotation of the galaxy stirs the gas, while self-gravity tends to gather the gas into denser structures called molecular clouds (MCs). The densest parts of the MCs collapse further and hence become the birthplaces of new stars.

Feedback - How stars affect the ISM

The term feedback includes all processes by which stars affect the ISM. Some notable processes are supernovae, jets, stellar wind, and radiation. Stellar feedback is essential for regulating star formation. If the feedback is strong enough, it stops the star formation process by heating and dispersing the gas cloud. Without feedback, all the gas in the ISM would quickly collapse and turn into stars. This is not what we observe in the Milky Way and other galaxies.

Simulating processes of the ISM

3D hydrodynamics codes such as Flash help us to model the ISM. We are constantly working on refining existing and developing new Flash modules, which are used to model certain properties of the gas in the ISM, of stars, and feedback. Two new developments are the multi-band OnTheSpot module, which is a reverse ray-tracing algorithm for ionizing radiation, and the Outflow module, which models collimated outflows (also called jets) of accreting protostars. More information on the new modules will follow in the next monthly highlight.

 

* Small in this context means scales of individual stellar systems and has to be understood in relation to the scale of the whole galaxy or even galaxy clusters.