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The Low Surface Brightness Universe: IAU Colloquium 171

On a cool and cloudy week in July, while most of the country was focused on the World Cup finals, nearly a hundred astronomers gathered in Wales to consider the dim and overlooked contents of the universe. IAU Colloquium 171 took place July 5–10 at the University of Cardiff. The meeting marked the 60th birthday of Mike Disney, the Welsh astronomer who has been a pioneer in recognizing that the observed population of galaxies is influenced by surface brightness selection. It is now clear that many galaxies are much more diffuse or unevolved than the Milky Way. There are interesting consequences to the truism that our galaxy catalogs only contain galaxies that we can detect. “Every truth passes through three stages before it is recognized. In the first, it is ridiculed. In the second, it is opposed. In the third, it is regarded as self-evident.” So wrote Albert Schopenhauer, who was quoted by organizer Jon Davies at the beginning of the conference. The quote implies that it has been unpopular to argue that the population of galaxies has been heavily censored. (Of course, it is equally true that many unpopular ideas turn out to be wrong.) The meeting saw spirited discussions on this theme. Over the past 10 years, the locus of the debate has shifted from the existence of low surface brightness (LSB) galaxies to their cosmological significance. Most of the contributions reflected the traditional optical study of galaxies. However, much evidence was also presented on the gas content of galaxies using the 21 cm line of neutral hydrogen. Also, participants heard about attempts to learn about the low surface brightness universe using quasar absorption lines, which are sensitive to extremely small amounts of neutral gas. The following is a selective account of the meeting highlights; the full story will appear as part of the ASP Conference Series. The first speakers addressed observational selection effects. Mike Disney (University of Wales) gave a detailed history of the realization that surface brightness selection is important. His comments were incisive and acerbic in equal measure. It then became clear that there would not be consensus on the galaxy luminosity function. Among field galaxies, the volume corrections in magnitude-limited surveys are large and the