The Interstellar Medium
What is the Interstellar Medium?
The interstellar medium (ISM) is the gas and dust in interstellar space within the galaxy. Much of the ISM is very tenuous, with densities ranging from 10^3 to 10^9 particles per cubic meter. In the cores of molecular clouds, density may exceed 10^12 particles per cubic meter. Stars form within these molecular clouds and the stars replenish the ISM by expelling matter via stellar winds, envelopes and explosions.
Current research areas
Interstellar Scintillation
The twinkling of stars, called scintillation, is due to fluctuations in the density of the air along the line of sight through the Earth's atmosphere. Point-like radio sources, such as pulsars and quasars, also scintillate due to fluctuations in the electron density along the line of sight through the interstellar medium. Scintillation can lead to a change in the intensity of the source, a change in its apparent position, or both.
Jean-Pierre Macquart and Don Melrose have developed a model for scintillations of radio sources based on scattering by discrete structures in the interstellar medium. This differs from the usual model that assumes a statistical fluctuation in density. Their approach was motivated by the thought that the rather large discrete structures required to explain Extreme Scattering Events (ESEs) may be extreme examples of a range of discrete structures common in the ISM.
For more information contact Don Melrose
The Ionised Interstellar Medium
Through observational and theoretical work, SIfA researchers are pursuing questions about the nature and origin of ionised interstellar gas, and how it is related to other phases of the interstellar medium.
We work on HII regions, supernova remnants, planetary nebulae and diffuse ionised gas that permeates the Milky Way, and form part of the team that has built a specialised instrument, the Wisconsin H-Alpha Mapper (WHAM), that is dedicated to studying very faint, diffuse optical emission from the night sky. We have created the first full three-dimensional view of the ionised gas in the northern sky that has allowed us to characterise the physical conditions of the gas.
We are now leading an effort to relocate WHAM to the Southern hemisphere. Thiss will complete our view of ionised gas in the Milky Way, and allow us to address new questions about interstellar plasma in the Galactic Center and Magellanic Clouds.
For more information contact Greg Madsen.