Two of NASA's Great Observatories, the Spitzer and Hubble Space Telescopes, have teamed up to "weigh" the stars in several very distant galaxies. One of these galaxies, among the most distant ever seen, appears to be unusually massive and mature for its place in the young universe. This comes as a surprise to astronomers because the earliest galaxies in the universe are commonly thought to have been much smaller agglomerations of stars that gradually merged together to build large majestic galaxies like our Milky Way.
The galaxy is named HUDF-JD2. (HUDF referes to the Hubble Ultra Deep Field Survey, in which the galaxy was discovered.) The galaxy is the red smudge in the top center of the image above. The image was made by Hubble's near infrared camera. HUDF-JD2 is not visible at all in optical wavelengths, due to the extreme redshift.
The distance to the galaxy, almost 13 billion light years, can be estimated from the redshift of spectral lines in its light. The redshift also indicates that we are seeing the galaxy as it was only 800 million years after the big bang.
As faint as HUDF-JD2 appears to us, its actual brightness must be about 8 times that of the Milky Way today, so the galaxy must have about 8 times as many stars. It would therefore rank as one of the largest of galaxies even at the present age of the universe (about 13.7 billion years).
It has generally been assumed that galaxies of any size did not start to form until several hundred million years after the big bang, so HUDF-JD2 could be only perhaps 400 or 500 million years old itself. That's a very short time for such a massive galaxy. A further standard assumption is that large galaxies form by the merging of smaller ones, which would be even more surprising for such a young galaxy at such an early time.
More detailed analysis suggests that HUDF-JD2 contains few luminous young blue stars, which is another surprise, since young stars should continue to form in galaxies for several billion years. (Many luminous new stars are still forming in the Mily Way, even after 13 billion years.) The implication is that HUDF-JD2 ran out of star-forming interstellar matter in a very short time. The lack of bright young stars means that the average star in HUDF-JD2 is less luminous than in galaxies which are still forming new stars. Consequently, there must be more stars than expected in order to account for the galaxy's observed brightness.
Clearly, some "standard" assumptions can't be entirely correct. Galaxies may have started to form a lot earlier than assumed, or large galaxies could have formed directly instead of through mergers. Or perhaps both.
Other references: here, here, here, here.
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