Discovery of a primordial galaxy beginning its rotation

It dates from the beginning of the universe, more precisely about 500 million years after the Big Bang. It is the oldest rotating galaxy ever observed and provides clues to the formation of primordial galaxies.

Scientists have just discovered galaxygalaxy furthest rotation ever observed. It crossed over 13.26 billion light years before reaching us and is called MACS1149-JD1, JD1 for short. But what makes it special is its rotation rate: JD1 rotates on itself much more slowly than the rotating galaxies of a less ancient era. ” That rotation speedrotation speed of JD1 is much slower than those found in galaxies from later epochs and our galaxy, and it is likely that JD1 is at an early stage in the development of rotational motion.” explains Akio Inoue, co-author of a study published in the journal The Astrophysical Journal and professor at Waseda University.

The formation of the first galaxies, still mysterious today

There is still no consensus on the formation of the very first galaxies. The most accepted theory suggests that they were formed after the primordial fluctuations of densities that are the origin of the structure of the universe. These fluctuations gave rise to dense zones consisting of both black fabricblack fabric and off gasgas. Gradually this gas condensed and this mass of matter grew: the very first galaxies were born during genionizationgenionizationonly composed ofhydrogenhydrogen and D’heliumheliumthe lightest elements in the periodic systemthe periodic system. Star formation only comes into play after the creation of these primordial galaxies. It is when a CloudCloud of gas contained in the galaxy, called fogfogcollapses in on itself gravitygravity only one starstar forms. In a galaxy, this process comes into play many times, first at the level of the center of the galaxy, where the majority of dust and gas is, then little by little this star formation is displaced.

The setting in rotation develops from the first moments of galaxies, when they are still only halos of matter. But it then accelerates, pushed by the formation of stars, which accentuates the gravitational effects undergone on both sides of the galaxy. Thus to determine the stage of evolution of a galaxy astronomersastronomers using the properties of the stars and the gases that make them up, such as their motion or their age. For these observations they use the effect of gravitational lensgravitational lens : when a particularly massive object distorts the light of objects behind it relative to the observer. This effect makes it possible to “raise” certain objects that are too faint to be observed directly by amplifying the light they emit. However, they seem distorted, which forces the researchers to reconstruct them based on the collected data.

JD1 makes it possible to date the late rotation of primordial galaxies

But the galaxy that interests us today is indeed JD1. Discovered by the Alma telescope (Atacama Large Millimeter/submillimeter Array), located in Chile, suitable for this, it was thanks to different observation campaigns carried out between October 2018 and December 2018, for a total of almost 10 hours of observation. To find out his age, thanks to Alma, the researchers measured his redshiftred shiftalso called ” red shift “. It corresponds to the displacement of spectrumspectrum light versus larger wavelengthswavelengths when an object moves away from the observer. It is the Doppler effect, the same one that changes the perception of the tones of the ambulance siren: the sound seems louder when it is approaching (small wavelength), and lower when it is far away.

However, due to the expansion of the universe, the more a galaxy is old, and therefore distant, the more rapidly it is moving away from us. With a particularly high spectral shift (z = 9.1), the researchers assigned it a stellar population a few hundred million years old, suggesting an older formation epoch, around z = 15. “In addition to the discovery of high redshifts, that is, very distant galaxies, the study of their internal motion of gas and stars provides motivation to understand the process of galaxy formation in the most distant universe. possible old”explains Professor Richard S. Ellis, co-author of the study and researcher atUniversity College from London.

With a diameter of 3,000 light years, compared to 100,000 for the Milky Way, it turns out, as the study explains, to be dominated by rotation and not by dispersion: the dispersion of velocities is much lower than the rotation speed of the stars that make up the galaxy. This property allowed the team to model the dynamics of JD1 and to derive other characteristics from it. Especially the stars that compose it. It contains many mature stars, about 300 million years old, therefore formed more than 13.5 billion years ago. “This shows that the stellar population of JD1 formed at an even earlier point in the cosmic age,” explains Dr. Takuya Hashimoto, co-author of the study and researcher at the University of Tsukuba.

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