Darkness Revealed: the realm of the low-surface-brightness sky

Tuesday 16th April, 2019

We were pleased to welcome as our speaker Prof David Valls-Gabaud from the Observatoire de Paris, which celebrated its 350th anniversary in 2017. He came to talk to us about "The Realm of the Very Low Surface Brightness Universe". He is also a member of the International Astronomical Union and the Institute of Astronomy, which is a department of the University of Cambridge and his work has taken him all over the world.

He began by saying that our galaxy, the Milky Way, is surrounded by a spherical halo of stars and gas. By analysing the structure of this halo astronomers can piece together a historical record of the Milky Way's interaction and mergers with other galaxies in the past. Further examination has revealed streams of stars within the halo as well as the detection of low surface brightness dwarf galaxies around our galaxy such as Segue 1 and Crater 2.

The term low surface brightness is used to describe an object whose light is spread out over a large area, effectively making it difficult to see or photograph. This is in contrast to nearby stars which tend to be point-like sources of light making them very bright and easy to see.

He continued by saying that the detection of these dim dwarf galaxies has solved part of the puzzle relating to a theory about the origin and evolution of galaxies in the Universe. This theory, known as Lambda Cold Dark Matter (LCDM) model, predicted more small dwarf galaxies than had been observed.

The "Lambda" in the theory's name relates to the idea that some form of invisible energy, called dark energy, is causing the expansion of the Universe to accelerate. The "Cold Dark Matter" terms refer to theoretical cold particles that are invisible as they do not emit or scatter light but they do interact gravitationally with normal matter and each other. Using computer simulations the LCDM model predicts that galaxies began small and grew larger merging with other small galaxies.

Prof Valls Gabaud then explained that professional telescopes are usually designed to look at a very small area of the sky in great detail, which makes them poor instruments for investigating dim galaxies that take up a relatively large area of the sky. It is also difficult to gain permission to use these large telescopes for many nights as they are in great demand by astronomers all over the world and are heavily oversubscribed. He added that professional systems also have systems of lenses which scatter or absorb the very light that makes up an image. Sometimes the optical system can even introduce "ghosts" or false images.

He is very keen for amateur astronomers to help with research in this area of astronomy and in his talk he listed a number of specific technical steps amateurs could take to capture images of low surface brightness galaxies and their surroundings. These included choosing a dark site away from light pollution, using a telescope with "fast optics" i.e. a short focal ratio, reducing any straylight in the telescope, and using computer software to add together multiple images of the same object into a single image (also known as stacking exposures).