Scientists have explained the unusual brightness of the first galaxies

In the first hundreds of millions of years after the Big Bang, there were already unexpectedly many bright galaxies in the universe. Observations by the James Webb Space Telescope (JWST) have raised the question for astronomers: why were these objects so noticeable, even though the early universe was filled with gas and dust capable of absorbing light? The researchers proposed an explanation related to the powerful radiation of young stars. How it could clear the space around star clusters is described in the Izvestia article.

The mystery of the first galaxies

The JWST observations showed that just a few hundred million years after the Big Bang, there were more bright galaxies in the universe than some theoretical models suggested. In these objects, stars formed in particularly dense and compact clusters. At the same time, it remained unclear why the light from young stars could so well escape through the surrounding matter.

Researchers from the Center for Computational Astrophysics at the Flatiron Institute and the University of Texas at Dallas have studied the role of Lyman alpha radiation, an ultraviolet light that occurs during electron transitions in hydrogen atoms.

The results of the work are published in The Astrophysical Journal (AJ). The authors suggested that the radiation from young stars could act like a powerful stream of air: to expel less dense gas and dust from the space between the stars. As a result, it became easier for light to leave the galaxy and reach observers.

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Light creates pressure

Lyman alpha radiation differs from many other types of light in how it interacts with hydrogen. When an atom absorbs such a photon, it almost immediately emits it again with the same wavelength. The photon can then absorb another hydrogen atom, after which the process repeats. Due to repeated absorption and re-emission, light interacts longer with the surrounding matter and transmits an impulse to it.

When gas or dust absorbs radiation energy, they receive a boost and can move outward from the light source. This creates radiation pressure that can affect the structure of star formation regions. However, it was not easy to assess its effect in the early Universe: for this it is necessary to take into account the huge number of repeated interactions of photons with matter.

Gravity held the dense gas in place

To assess the effects of radiation, Menon and his colleague Aaron Smith used images from pre-existing radiation-hydrodynamic cloud simulations in which dense star clusters formed. The scientists then calculated the pressure of Lyman-alpha radiation using the COLT program, designed to simulate the transfer of such light. The researchers also took into account the different dust content in the surrounding matter.

Calculations showed that in the densest regions, the radiation could not overcome the gravitational pull. The gas fibers from which the stars were formed remained associated with the star formation region. Therefore, the pressure of Lyman alpha should not have significantly prevented the formation of stars in these clusters.

This is important because scientists previously assumed that powerful radiation could push matter out of stellar regions and thus limit the formation of new stars. The results indicate a more complex picture: dense gas is held by gravity, while less dense matter can leave the area under the influence of radiation.

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How the cosmic wind cleared space

In the rarefied regions between dense gas structures, gravity is weaker. There, the pressure of the Lyman-alpha radiation could be stronger than gravity and accelerate the gas to high speeds. According to the researchers' calculations, such flows can occur quickly enough — within a few million years — and carry matter out of stellar regions, and potentially beyond the boundaries of galaxies.

Removing the rarefied gas and dust could make star clusters more visible. The light had to pass through a smaller amount of matter that could scatter or absorb it. Therefore, the galaxies could look brighter to observers, although the mechanism itself did not necessarily mean that more stars had formed in them.

What remains to be seen

The work helps explain why some galaxies in the early universe turned out to be so bright in the JWST observations. However, the authors do not claim that the pressure of Lyman-alpha radiation fully explains their properties. The study shows one of the possible mechanisms that could influence the visibility of young star clusters and the movement of matter around them.

The analysis has limitations. Scientists calculated the effects of radiation based on images from other simulations, rather than simulating all the processes simultaneously — light transport, gas movement, and star formation. For a more complete picture, calculations are needed in which these phenomena will be taken into account together.

According to the researcher, in the future, the development of computational methods and the growth of computer power will allow us to move to more complex three-dimensional models. They will help to more accurately assess the role that radiation played in the formation of the first galaxies and the propagation of light in the young universe.