[nfc]

I'm not really sure of what you mean by false positives.

After my understanding of the stacking technique as used in other contexts in astrophysics and without going into too much details :

The problem they were facing is that the signal to noise of the images of the filaments was too low to say that they had detected anything in any individual images. However by stacking (adding) images they were able to detect it because the signal grows roughly with N (N being the number of images) and the noise grows with sqrt(N). So by stacking enough images you'll get the signal to noise necessary to say you've detected sth.

[gcbw2]

Think like that: there are two bright lights in front of you. There will always be a halo between those two bright lights.

Now, my hypothesis is that "between all two bright lights, there is third, dimmer one, hidding". And then i prove it by filtering X from the two bright light halo, and prove that Y is left proving that the third light is there.

Now, how can i be sure Y is really a third dimmer light? and not just noise on the function i used to try to clean up the halo of the two bright light?

[nfc]

You are right that this could be the case, and it's mentioned in at least one of the papers [1] where they say that a better understanding of the physical state of this gas is needed to estimate its contributions to the baryonic mass.

[1] https://arxiv.org/abs/1805.04555

Now we can get to discuss the third paper ;)

[eridius]

I was wondering why stacking N different galaxies was any different than just taking one image and multiplying it by N, but your comment just provided the answer: noise.