Though this could be an indicator of a black hole pulling mass onto itself in the centers of these galaxies, it is unlikely, partly because the extra light comes from all over the galaxies, not just in one central point. And that is one part of this that amazes me, that even so far away, these galaxies make up more than just one tiny pixel on Hubble's camera!
You definitely want to zoom in on these gorgeous pictures.
The most likely explanation is that these are starburst galaxies. This means that they are forming stars at a prodigious rate, much faster than our galaxy is producing stars. All that light from the baby stars is what excites the gas around it, causing it to glow, not unlike a fluorescent sign.
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These galaxies are churning out a LOT of stars for their small size, something like a hundred million stars in just 30 million years. Yes, that seems like a long time, but remember, the universe is over a thousand times older than that.
Previous evidence has shown that galaxies were more productive at earlier times, but the these little galaxies far exceeded astronomers' expectations. However, it is in line with evidence from present-day dwarf galaxies that most of their stars formed in one massive starburst event. You could say that these galaxies were pretty hyper in their youth, kind of like some toddlers that I know!