Mike Disney has worried about selection effects in detection of galaxies (and their results on what we know about the galaxy population) for about 35 years. Here's his latest, no doubt deliberately provocative but a set of conclusions to be seriously wrestled with:
The galaxy ancestor problem, Mike Disney and Huw Lang:
There are a lot of wrinkles I plan to ping him about - gains from the very blue colors of young stellar populations, detection of galaxies from their UV-bright compact regions at substantial redshift, dusty-galaxy detections in the FIR and submm when we have accurate enough coordinates - but these visibility limits are important issues. In the most extreme cases they consider, much of what we think we know about galaxy evolution is instead a reflection of selection effects operating on a galaxy population much broader than we can easily detect at any single redshift.HST finds galaxies whose Tolman dimming should exceed 10 mag. Could evolution alone explain these as our ancestor galaxies? Or could they be representatives of quite a different dynasty whose descendants are no longer prominent today? We explore this latter hypothesis and argue that Surface Brightness Selection Effects naturally bring into focus quite different dynasties from different redshifts. Thus the HST z=7 galaxies could be examples of galaxies whose descendants are both too small and too choked with dust to be recognizable in our neighborhood easily today. Conversely the ancestors of the Milky Way and its obvious neighbors will have completely sunk below the sky at z>1.2 although their diffuse light could account for the missing Reionization flux. This Succeeding Prominent Dynasties Hypothesis (SPDH) fits the existing observations both naturally and well,including the bizarre distributions of galaxy surface brightnesses found in deep fields, the angular size ~ inverse (1+z) law,'Downsizing' which turns out to be an 'illusion' in the sense that it does not imply evolution, 'Infant Mortality', i.e. the discrepancy between stars born and stars seen, and finally the recently discovered and unexpected excess of QSOAL DLAs at high redshift. If the SPDH is true then a large proportion of galaxies remain sunk from sight, probably at all redshifts. We show that fishing them out of the sky by their optical emissions alone will be practically impossible, even when they are nearby. More ingenious methods will be needed to detect them. It follows that disentangling galaxy evolution through studying ever higher redshift galaxies may be a forlorn hope because one will be comparing young apples with old oranges, not descendants with their own ancestors.