I am generally interested in galaxies, how they form, how they evolve and how they fit into the universe around them. One big problem preventing astronomers from answering these questions is how to deal with interstellar dust. Interstellar dust within galaxies can reduce their observed brightness and alter their shape and color.
Take a look at the picture of the spiral galaxy NGC 6946 below which was taken in visible light. The most obvious thing to notice is the bright sprial pattern that gives these objects their name. What causes these bright areas? They could either be areas with more stars or areas with less dust. Most theories of star formation suggest the former but we cannot know for sure until we can measure the effects of dust extinction.
Interstellar dust is believed to consist of small grains of graphite and/or silicates with sizes ranging from a billionth to a millionth of a metre. Dust makes up a tiny fraction of the total mass of a galaxy but still manages to play an important role in many of the physical processes that occur in the Interstellar Medium (ISM); for instance, it is likely that the major mechanism for the formation of molecular hydrogen is on the surface of dust grains, they hold the atoms long enough to allow interactions in the rarefied environment of the ISM.
My research interests however, are more concerned with the remarkable ability of dust to interact with starlight. Despite its small mass fraction, the small size of the dust particles gives it an enormous collective surface area. This is coupled with a close match of the grain sizes to the wavelength of optical light and favourable electrical properties, resulting in prodigious interaction. The dust interacts with the starlight in two ways: scattering, a change in the direction of propagation of a photon; and absorption, a photon's energy being transferred to the particle in the form of heat. The removal of energy from a beam of light by these processes is called extinction. The reduction in the amount of light we receive from an external galaxy by the dust in its own interstellar medium is referred to as internal extinction.
In astronomy the only means we have to investigate the physics of galaxies is to measure the electro-magnetic radiation they produce. Any radiation produced in the ultra-violet, visible and near infrared bands, such as that produced by stars, will potentially interact with dust before being detected. The effect of starlight passing through a dusty medium is to reduce its intensity. The amount of reduction depends on the distance the light has to travel through the dust, the density of dust along that path and the physical properties of the grains. For example, travelling through 1kpc of the interstellar medium in the vicinity of the sun will remove approximately 80% of the energy from a beam of light at visual wavelengths. The distance from where the sun is positioned to the centre of the Galaxy is about 8.5kpc. The amount of light that manages to sucessfully avoid dust over this distance is tiny making the centre of our Galaxy virtually invisible!
It is clear that we need to correct for the effects of dust extinction if we want to study the physics of objects. Astronomers observing stars in our own Galaxy are only too aware of this need and dust has been under constant study since its discovery (in 1930) to the present day in an attempt to determine its distribution, extinction strength and physical properties. Various techniques have been developed to correct the brightness of stars for the effects of interstellar extinction, and this can now be done reliably and relatively simply.
It has proved difficult, however, to find such simple methods to correct the observed properties of galaxies for the effects of dust, as yet, no straightforward diagnostic for dust extinction exists even though all areas of extra-galactic astronomy that rely on measurements of optical brightness could be influenced by dust. Not measuring the full brightness of galaxies might not be our only problem. If the dust isn't distributed evenly throughout spiral galaxies (which seems likely from looking at dust in our own Galaxy) then it may profoundly alter their appearance. If we see a dark patch, how are we to know whether it is caused by a lack of stars (a hole) or if the stars are still there but covered up by dust?