Visibility of galaxies

Abstract

IT is well known that our counts of galaxies could be seriously biased by selection effects, largely determined by the brightness of the night sky. To illustrate this, suppose the Earth were situated near the centre of a giant elliptical galaxy, then the mean surface brightness of the sky would appear some 8–9 mag brighter than is observed from our position in the Galaxy (∼ 23 V mag (arc s)⁻² looking toward the galactic pole, discounting atmospheric and zodiacal contributions^1,2). Optical astronomers would then find extragalactic space an empty void; spiral and irregular galaxies would be quite invisible and all they would easily detect of galaxies would be the core regions of ellipticals very similar to their own. They would be blinded to much of the Universe by the surface brightness of their parent galaxy. But this blinding is clearly a relative matter and we should ask to what extent we are blinded by the spiral galaxy in which we exist, faint as it may appear by comparison. I will argue that strong indirect evidence already exists that our knowledge of galaxies is heavily biased by the sky background, and that the true population of extra-galactic space may be very different from the one we can see.