Recently the James Webb space telescope detected six massive ancient galaxies. They are very old and very far away. But these galaxies must still exist today and be even heavier now. Why can't we see any such huge galaxies near us now?
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2$\begingroup$ Why would they be heavier? How massive do you consider our galaxy to be? $\endgroup$– Jon CusterOct 2 at 14:34
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$\begingroup$ I thought the old galaxies discovered were much larger than the Milky Way. But maybe I misunderstood. $\endgroup$– Tony HägerOct 2 at 14:45
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8$\begingroup$ The largest of the galaxies found is though to have been 60% more massive than the current Milky Way, and that was 13 billion years ago so it has presumably spent the last 13 billion years growing by consuming smaller galaxies (as the Milky Way had done). So this seems an entirely reasonable question and I can see no justification for the downvote or the snarky comment. $\endgroup$– John RennieOct 2 at 15:05
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$\begingroup$ I agree with @JohnRennie on the fact that it's a legit question. I am not an expert but (maybe) if they are big they are rare, so it's unlucky to find one close by (if you look distant the enclosed volume increases, so even "rare things" tend to show up). It's worth checking this nice answer, it could give some good ideas: physics.stackexchange.com/a/153865/226902 $\endgroup$– QuilloOct 2 at 15:05
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$\begingroup$ @TonyHäger I'm reluctant to post an answer since this isn't my area, but giant galaxies might be more common than we think. $\endgroup$– John RennieOct 2 at 15:07
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1 Answer
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ESO 383-76 has a diameter of about 540.89 kiloparsecs and a mass of $2.3\times 10^{14} M_\odot$; it is around 200 megaparsecs away from us. There are many massive galaxies visible in the vicinity/present era.
It is just that the galaxies that were massive in the early universe (and presumably have accreted even more mass now by infalling smaller galaxies) have been moved to large distances: the redshift-scale factor conversion $1/a=1+z$ means that a $z=13.2$ galaxy is now 14.2 times further away, and the density of such early galaxies is $1/14.2^3=0.00035$ of what it was. So it is not surprising that we mostly see more recent, smaller galaxies.
answered Oct 2 at 15:07