Webb reveals secrets of galaxy mergers after the Big Bang

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Webb reveals secrets of galaxy mergers after the Big Bang
This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen from, among other things, excited hydrogen atoms — Lyman-α emission. Credits: ESA/Webb, NASA & CSA, S. Finkelstein (UT Austin), M. Bagley (UT Austin), R. Larson (UT Austin), A. Pagan (STScI), C. Witten, M. Zamani (ESA/Webb)

One of the key missions of the James Webb Space Telescope is to study the early Universe. Now, thanks to the unmatched resolution and sensitivity of the NIRCam instrument, scientists have seen for the first time what the local environment of galaxies in the very early Universe looks like.

This solved one of the most enigmatic mysteries in astronomy – why astronomers detect light from hydrogen atoms that should have been completely blocked by the primordial gas formed after the Big Bang.

Exploring the primordial universe

Webb’s new observations have revealed small, faint objects surrounding the same galaxies that show “unexplained” hydrogen emission. Combined with state-of-the-art simulations of galaxies in the early Universe, the observations showed that the chaotic merger of these neighboring galaxies is the source of the hydrogen emission.

Light travels at a finite speed (300,000 kilometers per second), which means that the farther away a galaxy is, the longer it took for light from it to reach our solar system. Thus, observations of the most distant galaxies not only explore the far corners of the Universe, but also allow us to study the Universe as it was in the past.

To study the early Universe, astronomers need extremely powerful telescopes capable of observing very distant – and therefore very faint – galaxies. One of Webb’s key capabilities is its ability to observe these very distant galaxies, and thus probe the early history of the Universe. An international team of astronomers has made excellent use of Webb’s amazing ability to solve a long-standing mystery in astronomy.

Secrets of galaxy mergers

The earliest galaxies were places of vigorous and active star formation, and therefore were rich sources of light emitted by hydrogen atoms, called Lyman-α radiation. However, during the reionization era, these active star formation zones (also known as stellar nurseries) were surrounded by a huge amount of neutral hydrogen gas. Moreover, the space between galaxies was filled with more of this neutral gas than it is today.

Gas can absorb and scatter this type of hydrogen radiation very efficiently, so astronomers have long predicted that the abundant Lyman-α radiation emitted in the very early Universe should not be observed today. However, this theory has not always stood up to scrutiny, as astronomers have observed examples of very early hydrogen radiation before. This has created a mystery: how is it that this hydrogen emission, which should have been absorbed or scattered long ago, is being observed?

Callum Whitten, a researcher at the University of Cambridge and the principal investigator of the new study, explains:

“One of the most mysterious problems of previous observations was the detection of light from hydrogen atoms in the very early Universe, which should have been completely blocked by the pure neutral gas formed after the Big Bang. Many hypotheses were previously proposed to explain the large escape of this ‘unexplained’ radiation.”

Випромінювальна галактика Lyman-α EGSY8p7 (зображення NIRCam)
This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light is emitted from, among other things, excited hydrogen atoms – Lyman-α radiation. The Webb’s high sensitivity highlights this distant galaxy along with its two companion galaxies, where previous observations have seen only one larger galaxy in its place.
Authority: ESA/Webb, NASA & CSA, C. Witten, M. Zamani (ESA/Webb)

Important galaxy mergers

The team’s breakthrough was made possible by Webb’s extraordinary combination of angular resolution and sensitivity. Observations with Webb’s NIRCam instrument were able to distinguish between smaller, fainter galaxies surrounding the bright galaxies from which the “unexplained” hydrogen emission was detected. In other words, the neighborhoods of these galaxies turn out to be a much busier place than we previously thought, filled with small, faint galaxies.

It is very important that these small galaxies interacted and merged with each other, and Webb found that galaxy mergers play an important role in explaining the mysterious radiation from the earliest galaxies.

Sergio Martin-Alvarez, a team member from Stanford University, adds:

“Where Hubble saw only a large galaxy, Webb sees clusters of smaller interacting galaxies, and this discovery has had a huge impact on our understanding of the unexpected hydrogen emission from some of the earliest galaxies.”

Computer simulations

The team then used state-of-the-art computer simulations to investigate the physical processes that could explain their results. They found that the rapid build-up of stellar mass as a result of the merger of galaxies caused powerful hydrogen emission and facilitated the escape of this radiation through channels purified from large amounts of neutral gas. Thus, the high rate of mergers of previously unobserved small galaxies has become a convincing solution to the long-standing mystery of the “unexplained” early hydrogen emission.

The team plans further observations of galaxies at different stages of merger to continue to develop their understanding of how hydrogen is released from these changing systems. Ultimately, this will allow them to improve our understanding of the evolution of galaxies.

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