400, 350, 300, 220… During the last two weeks millions of years have passed like so many sports results in a summer competition. Last week, works announced with great fanfare the observation of a galaxy that existed only 300 million years after the big bang. Record broken on Wednesday, July 27, by an international team, including French astrophysicists, with a galaxy dating from 220 million years after the Big Bang, according to the estimate.
This enthusiasm is due to the James-Webb Space Telescope, the JWST. Shipped on December 25, 2021, more than ten years late, the telescope delivered its first shots on July 11. “It was a big bet, but the preliminary results are excellent, confesses François Hammer, a specialist in the evolution of galaxies at the Paris Observatory. The first image was taken in just 12 hours of exposure time, compared to 240 hours for its predecessor Hubble. And it is much more accurate! »
A view of hundreds of galaxies.
Published images have amazed the public, such as the Carina Nebula, where one would almost believe one could see sand dunes under a starry sky, or the Southern Ring planetary nebula, a ring of blue and orange lace. For specialists, the most important of the photos bears the less poetic name of “Smacs 0723”, a cluster of galaxies in the constellation of the Flying Fish.
On a black background, there is a multitude of white, blue and red-orange dots. “This view actually flattens multiple planes, describes David Elbaz, research director of the department of astrophysics at CEA Saclay. Very bright six-pointed tips represent stars, the closest ones. The rounder white dots are the galaxies of the observed cluster, and finally, all behind, the red and orange dots are as many very distant galaxies. » If some of them appear completely distorted, the fault lies in the so-called “gravitational lensing” effect of light.
“Distant objects in the background emit rays of light towards the telescope. Along the way, these rays meet the galaxy cluster. The gravity of this cluster bends space-time and bends light, deciphers the astrophysicist Jean-Pierre Luminet. Sometimes we even witness duplications, gravitational mirages. » This magnification effect, which multiplies the background and the luminosity, allows us to observe very distant and faint galaxies.
Galaxies just after the dark ages of the Universe
However, it is not even this impressive view that hosts the record announced on Wednesday, July 27. The CEERS international team has discovered the oldest galactic ancestor in other data collected by the James-Webb telescope and not released to the public. This galaxy, nicknamed “Maisie”, would date back to 220 million years after the Big Bang, or three times nothing on the scale of the age of the Universe, almost 14 billion years.
“We’re going back to a time when we weren’t even sure there were galaxies.” explains David Elbaz, a member of the CEERS team. Fast rewind. First the big bang, then some adventures, and then darkness. The Universe experiences a dark age from its 380,000th anniversary. “It became opaque, before lighting up with the formation of the first stars and the first galaxies, due to gravitation, François Hammer vulgarises. Ideally, James-Webb should give us more information about when the lamp turns on.»
For this, the telescope can count on its extra-large mirror (read the markings) and its infrared instruments. “To reach us, light travels great distances, David Elbaz begins. The space expands, like an unfolding accordion. Therefore, the light is also stretched, the wavelengths increase and we are witnessing a red shift. »
Observation in the infrared then makes it possible to see this distant and very old light, which has become invisible. “Infrared allows us to see colder and more distant objects”, says Jean-Pierre Luminet. The second advantage is that infrared can “pierce” clouds of dust and gas, making it easier to observe the areas where stars are born.
Analysis of the atmosphere of exoplanets
Another too often forgotten mission, James-Webb will also take a closer look at the atmosphere of exoplanets, the planets outside the solar system. In addition to the first four shots, the space agencies involved have also published the composition of Wasp-96 b’s atmosphere. The results show the presence of water molecules. It is not enough to advance on possible life forms, because Wasp-86 b is a gas giant with the image of Jupiter, completely inhospitable.
“Water can be very present in the Universe and this is not a sufficient clue to prove the presence of life”, recalls Francois Hammer. The astrophysicist at the Paris Observatory calls for caution in the face of too loud announcements: “It is a new telescope and there are some calibration problems. »
Refining our theories about the formation of the Universe
On the CEERS side, David Elbaz readily acknowledges that the age of the “Maisie” galaxy requires confirmation, by spectroscopy and not just observation. “In the best of cases, this data should arrive in December, he waits. The probability of seeing a galaxy of this size in this region was zero. Therefore, we wonder if this is an exceptional case or a challenge. Are there more primordial galaxies in the JWST images than our models predict? »
“The standard cosmological model has difficulties in explaining the formation of very early galaxies in the Universe, recognizes Jean-Pierre Luminet. Until a few years ago, they were thought to date back, at best, a billion years after the Big Bang. » If the insights provided by James-Webb confirm the theories, the boundaries of our Universe will only be better defined. And, on the contrary, if this highlighting does not correspond, new models will have to be determined.
In any case, the scientific enthusiasm does not wane. Close to fifty studies are already available in pre-publication, the stage before peer review, on the ArXiv platform. And each day brings its new batch. Who knows, maybe next week a new study will present an even more distant galaxy…
An exceptional telescope
6.5 meters in diameter for the main mirror that collects light from the Universe, compared to 2.4 meters in diameter for the previous Hubble Space Telescope. It is the largest mirror ever deployed in space.
four scientific instruments are aboard the James-Webb, two of which are part French (Miri and NIRSpec). These instruments allow infrared observations of the first galaxies and stars.
1.5 million kilometers separates the Earth from the telescope, located at Lagrange point 2. This distance, which prevents any repair, is necessary to scan the distant Universe in the infrared.
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