“In a sense, the galaxy hardest for us to see is our own,” wrote Issac Assimov. “For one thing, we are imprisoned within it, while the others can be viewed as a whole from outside… . Furthermore, we are far out from the center, and to make matters worse, we lie in a spiral arm clogged with dust. In other words, we are on a low roof on the outskirts of the city on a foggy day.”
A Mystery in Infrared
Not so for the “infrared eyes” of the Herschel Space Observatory used by a team of astronomers led by the Netherlands Institute for Space Research (SRON), which has observed galaxies far beyond the Milky Way that there are 10 times more hyper-luminous galaxies in the infrared than stars can produce according to the models. If the theory is correct, it means that stars alone cannot account for the brightness of the most luminous infrared galaxies that emit more energy in the infrared than at all other wavelengths combined
Most Luminous Objects in the Universe
After the universe emerged from the Big Bang 13.8 billion years ago, galaxies filled with stars began to form relatively quickly around 3 billion years later. There was plenty of gas to go around, notes SRON, “so a small portion of these early galaxies were able to grow into massive, hyper-luminous galaxies considered to be some of the most luminous persistent objects in the Universe, exhibiting extremely high star formation rates, and most of which are known to harbor Active Galactic Nuclei (AGN) shining with a brightness of 10 trillion suns. As the gas reserves depleted with time, fewer galaxies could grow at a fast pace.” Infrared galaxies emit more energy in the infrared than at all other wavelengths combined.
The Extremely Luminous Infrared Galaxy WISE J224607.57-052635.0, with a luminosity of 300 trillion suns, discovered in May, 2015 by NASA’s Wide-field Infrared Survey Explorer (WISE), is the most luminous galaxy found, belonging to a new class of objects discovered by WISE, extremely luminous infrared galaxies, or ELIRGs. The black hole at its center was already billions of times the mass of our sun when our universe was only a tenth of its present age of 13.8 billion years.
Black Holes Billions Times Mass of the Sun When Universe was 1/10th its Present Age
The 2015 NASA study reports three reasons why the black holes in the ELIRGs could have grown so massive. First, they may have been born big. In other words, the “seeds,” or embryonic black holes, might be bigger than thought possible. The other two explanations, reports NASA’s WISE team “involve either breaking or bending the theoretical limit of black hole feeding, called the Eddington limit. When a black hole feeds, gas falls in and heats up, blasting out light. The pressure of the light actually pushes the gas away, creating a limit to how fast the black hole can continuously scarf down matter. If a black hole broke this limit, it could theoretically balloon in size at a breakneck pace. Black holes have previously been observed breaking this limit; however, the black hole in the study would have had to repeatedly break the limit to grow this large.”
Fast Forward to 2020
Fast forward to 2020 –When the SRON astronomers observed the universe with the Herschel infrared, they found that in terms of absolute numbers, it looked like there are over an order of magnitude too many hyper-luminous infrared galaxies, both in the early universe and more recent epochs. Unfortunately, Herschel’s spatial resolution couldn’t resolve all individual galaxies, so their results were inconclusive.
Enter international team of astronomers, led by Lingyu Wang from SRON and University of Groningen, using the LOFAR telescope—with higher spatial resolution—to distinguish galaxies individually. They found reports SRON, “that indeed, there are over an order of magnitude more hyper-luminous galaxies than the theory predicts. With an uncertainty of a factor two, they can say for sure that we need to look for a different theory.”
“We are now studying what physical mechanisms can power such extreme galaxies,” says Wang. “Are they powered by star formation or by supermassive black hole accretion? If powered by star formation, hyper-luminous infrared galaxies would be forming stars at a few thousands solar masses per year. Theoretical models cannot produce that many galaxies forming stars at such extreme rates. So an alternative scenario is that they are predominantly powered by accretion activity around the central black hole. We need more follow-up observations to study the true nature of these extreme objects.”
The SRON team says they will perform this follow-up study using the Keck observatory. It will give them more accurate data on galaxies’ redshift and therefore their distance. Keck harbors an optical telescope, providing spectra. Astronomers deduce the redshift from spectra by looking at how many wavelengths the characteristic fingerprints have shifted.
Image credit: artist’s concept depicts the current record holder for the most luminous galaxy in the universe. The galaxy, WISE J224607.57-052635.0, is erupting with light equal to more than 300 trillion suns. NASA/JPL-Caltech