The cosmic web feeds the first galaxies


Galaxies in the early universe are much more mature than astrophysicists thought: their existence at such an early stage is due to their interactions with the cosmos.


A galaxy in the early universe observed by the ALMA telescope. The galaxy is considered to be already mature, because it contains large amounts of dust (yellow) and gas (red). © B. Saxton NRAO / AUI / NSF, ALMA (ESO / NAOJ / NRAO), ALPINE team.

The first galaxies formed 200 million years after the birth of the universe. These galaxies accumulated the vast majority of the stars, dust particles and metals they are made of between one and three billion years after the Big Bang, a crucial period for our understanding of how galaxies formed. Astronomers from the University of Geneva (UNIGE), based at the Geneva Observatory – together with the international consortium of astronomers of the ALPINE project – studied 118 galaxies from this period using the ALMA telescope in the Atacama highlands of Chile. totaling eight joint papers (four of which were largely made at UNIGE), astrophysicists were able to go back nearly 13 billion years to identify the gas and dust composition of galaxies. Their analyzes, published in the journal Astronomy & Astrophysics, show that the first galaxies are already mature, supporting the existence of a cosmic network capable of providing them with resources.

The ideal way to understand how galaxies are formed, including our own Milky Way, would be to follow them for life, which is impossible. “Fortunately,” begins Daniel Schaerer, astronomer in the UNIGE Department of Astronomy, “we can observe them at different times. Looking very far into space, we can go back in time and study them as they were in the past. The period between one and three billion years after the Big Bang is particularly interesting, as it corresponds to the peak growth of galaxies. “

Galactic takeover

Large galaxies, such as our Milky Way, Andromeda and others, have reached their size and mass by merging with other galaxies. In addition, they had to receive additional gas from the outside to ensure their past growth. This gas, which is probably present in the intergalactic medium, is transported to galaxies via cosmic filaments. In other words, astronomers think that “the history of galaxies is governed by what we call the ‘cosmic web’, a kind of galactic ecosystem,” continues Daniel Schaerer. Despite the fact that this theoretical model has been accepted by scientists, the presence of gas in the intergalactic medium and the existence of cosmic filaments has never been proven by concrete measurements.

Follow gas is a key component for galaxy growth, and the ALMA telescope was specifically designed to observe it using a system that captures distant infrared light. “The telescope is an interferometer with 66 antennas, each 7m to 12m in diameter, positioned at an altitude of 5,000m on a plateau in Chile. The vast number of antennas means we can collect the maximum amount of light,” he says. Miroslava Dessauges-Zavadsky – the astronomer of the Geneva Observatory – referring to a telescope unique in the world.

Excess gas

Drawing on eight studies, the international astrophysicists of the ALPINE consortium realized that the first galaxies contained much more gas than expected, which served to indicate their early development. “Some of them are very large and almost all of them already contain heavy elements, such as carbon, in the interstellar medium. This was a sign that these elements were formed from generations of stars that existed before a billion years ago, because carbon was not generated during the Big Bang, “continues Daniel Schaerer.

Spotlight on Geneva’s contribution

The quantification of the gas was carried out by a UNIGE team and is the central theme of one of the eight publications. The work helped show that distant galaxies – by extension at an early stage in their development – have far more gas than nearby galaxies. However, as the gas is consumed very quickly to form new stars, the quantities of gas detected are not sufficient to explain the observed extended growth of galaxies. “This means that galaxies must be supplied with gas from the cosmic network,” says Dessauges-Zavadsky, first author of this study.

Another study conducted by the Geneva Observatory compared the galaxies of our time (13.8 billion years) with those present 800 million years after the Big Bang. This research highlights the similarities between some of the properties of galaxies over the centuries, such as the relationship between the emission of the carbon line and the amount of stars formed. The study demonstrates the consistency of these properties over 13 billion years, an important piece of information for quantifying the growth of galaxies throughout the history of the universe.

The analysis of galactic dust is the subject of the third Geneva study. The researchers were able to show the presence of dust in 20% of the first galaxies. “This indicates that these galaxies are already mature enough and that previous generations of stars have created the elements embedded in the dust,” explains Pascal Oesch, UNIGE astrophysicist and lead researcher on this paper.

The analysis of gas movements is at the center of the fourth study conducted at UNIGE. Researchers have shown that heavy elements, such as carbon, are found not only inside but also at great distances from galaxies and moving at high speed. “The enriched gas indicates the presence of significant routes to and probably also from intergalactic space. This once again supports the cosmic web thesis, ”concludes Daniel Schaerer.

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