New clues about how ancient galaxies illuminated the Universe – ScienceDaily



NASA's Spitzer Space Telescope revealed that some of the first galaxies in the Universe were brighter than expected. Excess light is a byproduct of galaxies that release incredibly high amounts of ionizing radiation. The finding offers clues to the cause of the Reionization Era, a major cosmic event that transformed the universe from being almost opaque to the bright star landscape seen today. The new work appears on a paper in Monthly notices from the Royal Astronomical Society.

The researchers report observations of some of the first galaxies that formed in the universe, less than a billion years after the Big Bang (or just over 13 billion years). The data shows that at some specific wavelengths of infrared light, galaxies are considerably brighter than scientists anticipated. The study is the first to confirm this phenomenon for a large sample of galaxies of this period, which shows that these are not special cases of excessive brightness, but even the average galaxies present at that time were much brighter in these lengths. wave that the galaxies we see today. .

No one knows for sure when the first stars of our universe come alive. But evidence suggests that between about 100 million and 200 million years after the Big Bang, the Universe was filled primarily with neutral hydrogen gas that may have just begun to coalesce into stars, which then began to form the first galaxies. About a billion years after the Big Bang, the Universe had become a bright firmament. Something else had also changed: the electrons in the omnipresent neutral hydrogen gas had been removed in a process known as ionization. The time of reionization, the change from a universe filled with neutral hydrogen to one filled with ionized hydrogen, is well documented.

Before this transformation in the whole Universe, the forms of light of long wavelength, like the radio waves and the visible light, crossed the universe more or less without restrictions. But the shorter wavelengths of light, including ultraviolet light, X-rays and gamma rays, were interrupted by neutral hydrogen atoms. These collisions would strip the neutral hydrogen atoms of their electrons, ionizing them.

But what could have produced enough ionizing radiation to affect all the hydrogen in the Universe? Was it individual stars? Giant galaxies? If either were the culprit, those early cosmic colonizers would have been different from most modern stars and galaxies, which generally do not release high amounts of ionizing radiation. On the other hand, perhaps something else caused the event, such as quasars, galaxies with incredibly bright centers driven by huge amounts of material orbiting supermbadive black holes.

"It's one of the biggest open questions in observational cosmology," said Stephane De Barros, lead author of the study and a postdoctoral researcher at the University of Geneva in Switzerland. "We know what happened, but what caused it? These new findings could be a great clue."

To look back in time, just before the end of the Reionization Era, Spitzer looked at two regions of the sky for more than 200 hours each, which allowed the space telescope to collect light that had traveled for more than 13 billion years. of years to reach us.

As some of the longest scientific observations ever made by Spitzer, they were part of an observation campaign called BIG, short for Treasury of the wide area of ​​Spitzer's reionization era of ASSETS. PROPERTY (in itself, an acronym: Grand Observatory of the Deep Survey of origins) is another campaign that made the first observations of some BIG objectives. The study also used data from the NASA / ESA Hubble Space Telescope.

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

These stars were not the first stars to form in the Universe (they would have been composed only of hydrogen and helium), but they were still members of a very early generation of stars. The Reionization Era was not an instantaneous event, so while the new results are not enough to close the book on this cosmic event, they do provide new details about how the Universe evolved at this time and how the transition developed.

"We did not expect Spitzer, with a mirror that was not bigger than a Hula-Hoop, to be able to see galaxies so close to the beginning of time," said Michael Werner, Spitzer project scientist at the Jet Propulsion Laboratory. of NASA in Pasadena, California. . "But nature is full of surprises, and the unexpected brightness of these early galaxies, coupled with Spitzer's magnificent performance, puts them within reach of our small but powerful observatory."

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

"These Spitzer results are certainly another step in solving the mystery of cosmic reionization," said Pascal Oesch, an badistant professor at the University of Geneva and co-author of the study. "We now know that the physical conditions in these early galaxies were very different from those of typical galaxies today, and the work of the James Webb Space Telescope will be the work of solving the detailed reasons why."


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