Early galaxies may have been larger and more complex than previously thought

A1689-zD1 is the name that astronomers give to a young galaxy located about 13 billionlight years of our Milky Way. a galaxy “normal” on which astronomers pointed the Large Millimeter/Submillimeter Antenna Array of the Atacama (Alma, Chile) in the hope of unlocking its secrets. And as they report on the occasion of the congress of the American Astronomical Union, they were not disappointed.

They have indeed discovered a halo of carbon cold that extends, surprisingly, far beyond the center of A1689-zD1. As well as flows of gas hot and ionized from the heart of the young galaxy. Such discharges are usually the result of violent activity, such as supernova. All of this could constitute proof that stars are formed there, continuously. Especially since the carbon halo is found in the same regions as thehydrogen neutral interstellar known to concentrate in the regions of star formation. So A1689-zD1 could turn out to be much larger than astronomers thought.

It is also possible that this carbon halo is the remnant of past galactic activity. A1689-zD1 could thus have already experienced mergers. Of the gravitational forces complexes would then have exerted themselves on the galaxy leading to the ejection of a colossal quantity of gas over these great distances. But in both cases, A1689-zD1 probably underwent a rather active early evolution. “And even if it had never been observed, it could be what happens most of the time, for the first galaxies formed in our Universe»comments Hollis Atkins, the lead author of the study, in a statement of National Radio Astronomy Observatory (United States).

Rififi in the primitive Universe

Seiji Fujimoto, another author of these works, confirms this: “The carbon halo in A1689-zD1 is much more extensive than what was observed with the Hubble Space Telescope, and that could mean that the first galaxies aren’t as small as they seem. This would have a major impact on the theory of galaxy formation and evolution in thePrimitive universe. »

Why didn’t the Hubble Space Telescope see anything? Because he does not see in the field radio , available in Alma. Because here, the astronomers also took advantage, quite by chance, of a gravitational lensing effect which made it possible to enlarge the target galaxy more than nine times. Enough to reveal critical details that usually remain hidden for such distant galaxies.

Looking at it from another eye – and closer too -, therefore, it would seem that the first galaxies that emerged in our Universe were already very complex. That the early Universe was not quite as astronomers had drawn it. Thus, researchers now await with undisguised impatience the observations planned by the James Webb Space Telescope(JWST). From January 2023, they should take an even more complete look at A1689-zD1.

A very young galaxy from the beginning of the universe and already dusty

The universe was then only 700 million years old, but this young galaxy A1689-zD1, very distant, therefore, and spotted by the VLT, was already rich in dust. Its detailed observation by Alma was made possible by a gravitational lensing effect and confirms the rapid evolution of galaxies born shortly after the big Bang .

Article ofESO published on 09/03/2015

A team of astronomers led by Darach Watson from the University of Copenhagen used the instrument X shooter installed on the VLT of the Eso as well as Alma – vast (sub-)millimeter array of the Atacama – in order to observe one of the galaxiesthe youngest and most distant. To their surprise, the target appeared to them very advanced. Indeed, the proportion of dust it contains is similar to that of a mature galaxy like the Milky Way . This essential component contributes to the formation of planets, moleculescomplexes and normal stars.

Galaxy A1689-zD1, identified in earlier space telescope snapshots Hubble, was the target of their observations. A gravitational lensing effect produced by Abell 1689 a spectacular cluster of galaxies located on the line of sight, increased its brightnessapparent by a factor of 9. In the absence of this effect, the young galaxy, of low luminosity, could not have been detected.

To observe A1689-zD1 is to scrutinize a portion of the universe which was then only 700 million years old (i.e. 5% of its current age). This system is modest in appearance – much less massive and luminous than many other objects of the young universe previously studied – and therefore more representative of galaxies dating from this period, that of the reionization . All first stars then generated a dawn cosmic, illuminating for the first time a vast and transparent universe, thus ending the period of stagnation known as the dark age.

The chemical complexity as well as the abundance of interstellar dust within this galaxy surprised the observers, who expected to discover a very young system. “After the VLT confirmed the galaxy was moving away, we realized that it had already been observed using Almasays Darach Watson. We didn’t expect to find much, but I can tell you that we were all very excited at the idea that Alma had taken a precise snapshot. One of the main objectives of this observatory is to detect emissionsof cold gas and dust from galaxies in the young universe – and we had one! »

This young galaxy quickly generated stars

This galaxy is a child of cosmos , but a precocious child. At her age, she should be deficient in chemical elementsheavy (in metalsas we designate in astronomy the elements heavier than hydrogen andhelium ). These metals are produced in the hearts of stars and then dispersed into space when, for example, at the end of its life, thestar explode. Repeated over many generations of stars, this process leads to the significant enrichment of the universe in heavier elements such as carbon,oxygenand thenitrogen .

Surprisingly, the galaxy A1689-zD1 seemed to emit intense radiation in theinfrared distant (the initial radiation, transformed into millimeter waves by the expansion of the Universe, can be detected by Alma). This suggested that it had already given birth to many stars and produced significant amounts of metals. Thus, not only did it contain a good proportion of dust, but it was characterized by a dust/gas ratio similar to that of more mature galaxies. “Although the exact origin of galactic dust remains unknown, our observations suggest that its production began very early, within 500 million years of the start of star formation in the universe – a time frame very short on the cosmological scale, knowing that most stars live for billions of years”specifies the researcher who led this study published in the March 2, 2015 edition of Nature(also available here on the ESO website).

The results suggest that the galaxy A1689-zD1 either continuously created new stars at a moderate rate beginning 560 million years after the Big Bang, or went through a very intense stellar creation phase followed by of a period of decline.

Prior to this study, astronomers feared they would not be able to detect galaxiesso far away that way. The case of A1689-zD1 shows that it is possible to discover it by means of brief observations carried out with Alma.

“This incredibly dusty galaxy seems to have been in a hurry to give birth to its first generation of starscomments Kristen Knudsen of the Chalmers University of Technology (Sweden) who co-authored the article. In the future, Alma will allow us to discover other galaxies of this type and will reveal to us the secret of their precocity. »