6.2.3. A New Population of Galaxies at z = 0

An important corollary to the Cosmological Principle asserts that all observers in the Universe should construct the same catalogues of galaxies. If this were not the case, then different observers might have biased views and information about 1) the nature of the general galaxy population in the Universe, 2) the three dimensional distribution of galaxies, and 3) the amount of baryonic matter that is contained in a galactic potential. Thinking about the Cosmological Principle in terms of the homogeneity of observers' catalogues of galaxies raises an immediate and perhaps profound problem which can be stated very simply: If you see (detect) a galaxy you can catalog and classify it; if you don't see it you can't. Since galaxies in general are quite diffuse and low contrast objects with respect to the noisy background of the night sky which has finite brightness one can easily conceive of observing environments that would make galaxy detection difficult.

For instance, suppose that we lived on a planet that was located in the inner regions of an elliptical galaxy. The high stellar density would produce a night sky background that would be relatively bright and therefore not conducive for the discovery of galaxies. Similarly, if the Solar System, in its journey around the galaxy were unlucky enough to be located near or in a Giant Molecular Cloud (GMC) at the same time that evolutionary processes produced telescopes on the Earth, then our observational horizon would be severely limited by the local dust associated with the GMC. Finally, consider the poor astronomer that lives on a planet which has two moderate size moons in orbit about it, at least one of which was in the night sky at all times. Such a planet would have no "dark" time and observers would be hard pressed to discover external galaxies. Although these are extreme situations, they illustrate the basic point that for all observers, the finite brightness of their night sky acts as a visibility filter which, when convolved with the true population of galaxies, produces the population that appears in catalogs. Thus we have no guarantee that our location in the outer regions of a spiral galaxy, on a planet with 50% dark time per lunar orbital cycle, allows us to detect and catalog a representative sample of galaxies.

This potential bias in optical catalogs that was first emphasized by Mike Disney in 1976 but was known at the time of Messier. Indeed, Hubble noted this problem as well as it pertains to galaxy classification. In his 1926 paper, Hubble wrote

Subdivision of non-galactic nebulae is a much more difficult problem. At present and for many years to come, their classification must rest solely upon the simple inspection of photographic images, and will be confused, by the use of telescopes of widely differing scales and resolving powers. Whatever selection of types is made, longer exposures and higher resolving powers will surely cause a reclassification of many individual nebulae.

In this quote Hubble establishes that galaxy classification, and therefore implicitly galaxy detection, is highly dependent upon observing equipment and resolution. The essential issues are: a) how severe is the bias in terms of the potential component of the galaxy population that has been missed to date and b) how would this effect our current understanding of galaxy formation and evolution?

Galaxies are detected on the basis of their surface brightness contrast with respect to the night sky background. Mild-to-strong luminosity evolution potentially produces galaxies with rather low luminosity per unit area at z = 0. Indeed, many have suggested that the fate of the FBGs is to fade to very low surface brightness levels by z = 0. Alternatively, there could be a population of intrinsically low surface mass density systems whose evolution is quite different from "normal" galaxies but whom nevertheless are important repositories of baryonic matter. Disney suggested that such diffuse systems are hard to detect and therefore we could be missing an important constituent of the general galaxy population. For many years extragalactic astronomers lived in a comfort zone, dismissing Disney's original hypothesis as at best, applying to a limited and inconsequential population of objects. But let's examine this effect in greater detail.