A team of astronomers led by the Dutch Institute for Space Research SRON observed 10 times more hyper-luminous galaxies in the infrared than stars can produce according to models.
If the theory is correct, it means that stars alone cannot explain the brightness of the brightest infrared galaxies. The article was published in a special issue of Astronomy and astrophysics.
After the universe exited the Big Bang 13.8 billion years ago, galaxies filled with stars began to form relatively quickly about 3 billion years later. There was a lot of gas to go through, so a small portion of those early galaxies were able to turn into massive, hyper-luminous galaxies, with a brightness of 10 trillion suns. As gas reserves run out over time, fewer galaxies could grow at a rapid rate.
When astronomers observed the universe with the Herschel Infrared Space Telescope, they found this theory to be largely verified. However, in terms of absolute numbers, it seemed that there were an order of magnitude too many hyper-luminous infrared galaxies, both in the early universe and in more recent eras. Unfortunately, Herschel’s spatial resolution couldn’t resolve all of the individual galaxies, so they couldn’t say for sure.
An international team of astronomers, led by Lingyu Wang of SRON and RUG, have now used the LOFAR telescope – with higher spatial resolution – to distinguish galaxies individually. They found that indeed, there are more than an order of magnitude of hyper-luminous galaxies than theory predicts. With a factor of two uncertainty, they can say with certainty that we need to look for a different theory.
“We are currently studying what physical mechanisms can power such extreme galaxies,” Wang says. “Are they fueled by star formation or by the supermassive accretion of black holes?” If fueled by star formation, hyper-luminous infrared galaxies would form stars at a few thousand solar masses per year. Theoretical models cannot produce so many star-forming galaxies at such extreme rates. An alternative scenario is therefore that they are mainly fueled by accretion activity around the central black hole. We need more follow-up observations to study the true nature of these extreme objects.
The team will carry out this follow-up study using the Keck Observatory. This will give them more precise data on the redshift of galaxies and therefore their distance. Keck houses an optical telescope, providing spectra. Astronomers deduce the redshift from spectra by observing how many wavelengths the characteristic fingerprints have changed.
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