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For the star density to make the galaxy opaque, the surface brightness of that region of the galaxy would have to be as bright as the surface of the sun.
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Really? Why is that, why not the surface brightness of, say, Jupiter?
I think it should be calculated differently, the real question is how bright must a quasar at the z-distance (2.11) be to be able to shine through the dense part of a galaxy. I don't know the numbers on that, but it surely is easy to see that not many (if any) background objects are visible through a galaxy core.
Anyway, the quasar's spectrum shows that it isn't behind the galaxy (no reddening).
Cheers.