Galaxies, galaxies, everywhere...

This paper claims there could be a huge population of as-yet undiscovered very faint galaxies. The idea is that they have been previously missed in existing surveys because of problems with identification techniques and survey sensitivity limits.

I know (to varying degrees) all three of the authors so I was privy to an earlier draft of this last year. I already sent the first author some detailed comments many months ago, but the only response I had was, "thanks for your thoughtful contribution". As far as I can tell, my comments haven't been included in this updated version. I'm not sure whether it's worth the effort of sending a second email or not.

There are two serious problems here I want to point out :
1) The idea that neutral hydrogen (HI) surveys have missed a huge number of dark galaxies because of poor identification procedures
This can be falsified using the author's own numbers. Firstly, they claim that, for instance, the ALFALFA HI survey identify optical galaxies as much as 2 arcminutes away from the coordinates from the HI measurements. While true, the median offset is just 23 arcseconds ! So most of the time the identifications are reliable.
Secondly, the authors state the minimum physical separation between the HI and optical coordinates should be < 13 kpc for a secure identification. I looked at the numbers on this, and for the ALFALFA survey within 50 Mpc of the Milky Way just 0.6% of their detections have larger separations. So it cannot possibly have missed a large number of dark galaxies in the nearby universe.

2) Optical surveys have missed a huge population of very faint galaxies
They even give reasons why CCDs can't detect them. It's a little agonising to read because a year or so ago this might have been interesting. The trouble is that these ultra-diffuse galaxies are a hot topic at the moment with papers coming out regularly. We know they exist, and in large numbers.
What's particularly baffling is that the first author was the PhD supervisor of my PhD supervisor (thus making him my grand-supervisor), who's done a lot of work on this himself. They work in the same building. Yet apparently the one is not aware of the other. Oh deary deary me...

So, large optically faint galaxies do exist. They are definitely a thing, with certainty. But large gas-rich optically dark galaxies do not exist, with an extremely high degree of confidence. However, smaller gas-rich dark galaxies may well exist. But that's another story, aka a 26 page paper with > 200 simulations, currently in preparation. :)
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