So I was reading a paper shared on the blog of Disney that @RedshiftDrift and @Mikehelland talked about. https://mjdisney.org/2024/04/galaxies-hidden-no-more/astronomy/
I haven’t read it in depth but the paper makes absolutely groundbreaking claims!
Because we live right next to a bright star the local sky is 10000 times brighter than it would be in intergalactic space so we can hardly be surprised if many cosmic structures, and in particular galaxies, are hidden from sight. Nevertheless the Surface Brightness Selection Effects are so dramatic, and so non-intuitive
Based largely on 21-cm. surveys there have been several rather persuasive claims that Hidden Galaxies (HGs) cannot exist in any numbers. But these claims, it turns out, are based on two serious misunderstandings
Indeed I have seen such claims on 21cm hydrogen surveys, e.g recently from tritonstation blog post: https://tritonstation.com/2024/04/26/the-mhongoose-survey-of-atomic-gas-in-and-around-galaxies/
Disappointing as the absence of zillions of new detections may be discovery-wise, it does teach us some important lessons.
The survey is more recent than the paper Disney share (2019) but their claims might still be relevant!
we finds Odds of about a million to one on the proposition that HGs comfortably outnumber the visible variety, and perhaps overwhelm them. That insight cannot but change our attitude to a number of key cosmological issues such as the reionisation of the universe.
Finally we argue (Sect. 9) that with a little dogged savvy, we could, exploiting advances of technology over the next decade, find unequivocal proof of the existence of this heretofore Hidden Universe.
A million to one chance than most of the galaxies in the universe are currently hidden and below the sensitivity of our instruments!! Of course this is already known what they mean is that this would apply even at low-mid redshift.
I would appreciate if e.g. @Handebrujin could check out the maths. On a related topic, for a very long time one of the most major failures of LCDM was the dwarf satellite abundance problem, meaning that e.g. simulations shows Andromeda should have at least 500! dwarf galaxies satellites and the Milky Way should have 200 while they currently have ~40 and ~60 identified. A very recent 21cm survey has discovered 2 new milky way satellite and they shows that assuming they are homogenously distributed and taking into account the 21cm camera has a very small field of view, then by extrapolation the situation is completely reversed as the dwarf galaxies problem becomes by far a too many dwarf galaxies problem (but small and ultra faint). This excess might not hold as there is tentative evidence for a bias/non homogenous distribution of dwarf satellites. It is directly related though to what the 21cm and OIII will unveil.
https://www.space.com/milky-way-satellite-dwarf-galaxies-found