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The formation of new stars in primordial galaxies, and their subsequent growth, are not directly linked to the merger of smaller galaxies, as previously thought, and this process may not even be the main trigger.
The unexpected result comes from research carried out with the Spitzer Space Telescope on a sample of 70 very remote galaxies, that inhabit the Universe when it was between 1 and 2 billion years old. Amongst the authors of the study, to be published in The Astrophysical Journal, are Ranga-Ram Chary and Hyunjin Shim, of NASA's Spitzer Science Center at the California Institute of Technology in Pasadena.
The galaxies observed are very luminous in the red and infrared region of the electromagnetic spectrum, and 70% show very strong emission in the H-alpha line of ionised hydrogen. Only 0.1% of galaxies in the local Universe have such strong H-alpha emission.
Because emission from the H-alpha line comes from electrons re-combining with protons, and it is therefore necessary to have a source of energy to ionise the hydrogen in the first place, the H-alpha line indicates regions where hot, massive stars are ionising gas via their copious ultraviolet light output. Strong H-alpha lines indicate a lot of massive star formation.
In the remote galaxies observed by Chary, Hyunjin and colleagues, there are therefore large numbers of large stars being born at a fantastic rate, up to 100 times as fast as in our own Galaxy (where no more than 10 stars are born each year). The most surprising thing about the star formation in these objects, however, is that the star formation doesn't seem to occur in sporadic, short-lived bursts, but rather over periods that can last for hundreds of millions of years.
Primordial galaxies then didn't grow during short periods of hyperactivity caused by the merger of smaller galaxies, but rather via a more continuous formation of massive stars.
These galaxies were very blue, but rather less gas-rich than in the merger hypothesis. The illustration above shows how one of these galaxies would appear today (left half) and in the early Universe (right half).
But where does all the gas come from to form these new, massive stars? The authors hypothesise that a steady flow of gas fell onto the galaxies from filaments of dark matter, though a full explanation still seems rather a long way away.
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