New clues to how ancient galaxies illuminated the universe – ScienceDaily

The researchers report observations of some of the first galaxies to form in the universe, less than a billion years after the big bang (or a little over 13 billion years ago) . The data show that in a few specific wavelengths of infrared light, galaxies are considerably brighter than predicted by scientists. This study is the first to confirm this phenomenon for a large sample of galaxies from this period, showing that there were no particular cases of excessive brightness, but that even the average galaxies present at that time were much brighter in these wavelengths than the ones we see today. .

Nobody knows for sure when the first stars of our universe come to life. But the evidence suggests that between about 100 and 200 million years after the Big Bang, the Universe was mostly filled with neutral hydrogen that may have begun to merge to form stars, which then began to form the stars. first galaxies. About 1 billion years after the big bang, the Universe had become a sparkling firmament. Something else has also changed: the electrons of ubiquitous neutral hydrogen have been removed by a process called ionization. The era of reionization – the transition from a universe filled with neutral hydrogen to a universe filled with ionized hydrogen – is well documented.

Before this universe-wide transformation, long-wave light shapes, such as radio waves and visible light, traversed the more or less unencumbered universe. . But the shorter wavelengths of light – including ultraviolet light, X-rays, and gamma rays – have been interrupted by neutral hydrogen atoms. These collisions would strip the neutral hydrogen atoms of their electrons and ionize them.

But what could possibly have produced enough ionizing radiation to affect all the hydrogen in the universe? Was it individual stars? Giant galaxies? If one or the other was the culprit, these early cosmic colonizers would have been different from most modern stars and galaxies, which usually do not release large amounts of ionizing radiation. Again, perhaps something else has caused the entire event, like quasars – incredibly bright galaxies at the center fueled by huge amounts of material gravitating around holes supermassive blacks.

"This is one of the biggest outstanding questions in observational cosmology," said Stéphane De Barros, lead author of the study and postdoctoral researcher at the University of Geneva in Switzerland. "We know this has happened, but what is the cause? These new discoveries could be an important clue."

To go back to the time just before the end of the era of reionization, Spitzer observed two regions of the sky for more than 200 hours each, allowing the space telescope to capture the light that had traveled for more than 13 billion years.

Among the longest scientific observations ever made by Spitzer, they were part of an observation campaign called GREATS, the acronym for Treasury for Sparker, a vast area from the time of the re-ionization of goods. GOODS (acronym: Great Observatories Origins Deep Survey) is another campaign that made the first observations of some GREATS targets. The study also used NASA / ESA's Hubble Space Telescope archive data.

Using these ultra-deep Spitzer observations, the team of astronomers observed 135 distant galaxies and found that they were all particularly bright in two specific wavelengths of infrared light produced by ionizing radiation interacting with hydrogen and oxygen gases in galaxies. This implies that these galaxies were dominated by massive young stars composed mainly of hydrogen and helium. They contain very small amounts of "heavy" elements (such as nitrogen, carbon, and oxygen) compared to stars found in modern galaxies.

These stars were not the first stars to form in the universe (these would have been composed only of hydrogen and helium) but still belonged to a very first generation of 39, stars. The era of reionization was not an instant event. Therefore, although the new results are not enough to close the book of this cosmic event, they provide new details on how the Universe evolved at that time and on the transition.

"We did not expect Spitzer, with a mirror as large as a Hula-Hoop, to be able to see the galaxies so close to the sun," said Michael Werner, scientific lead for the Spitzer Project at Jet Propulsion NASA Laboratory in Pasadena, California. "But nature is full of surprises and the unexpected brightness of these early galaxies, combined with Spitzer's superb performance, puts them within the reach of our small but powerful observatory."

The NASA / CSA / ESA James Webb Space Telescope, which is scheduled for launch in 2021, will study the Universe at many of the same wavelengths observed by Spitzer. But where Spitzer's main mirror is only 85 centimeters in diameter, Webb's is 6.5 meters tall, 7.5 times larger, allowing Webb to study these galaxies in much greater detail. In fact, Webb will attempt to detect the light of the first stars and galaxies in the universe. The new study shows that because of their brightness in these infrared wavelengths, the galaxies observed by Spitzer will be easier to study for Webb than previously thought.

"These results from Spitzer are certainly another step in solving the mystery of cosmic reionization," said Pascal Oesch, assistant professor at the University of Geneva and co-author of the study. "We now know that the physical conditions of these early galaxies were very different from those of today's typical galaxies, and it will be the work of the James Webb space telescope to determine in detail the reasons for this situation."