While the night sky often appears empty between the stars and galaxies, a team of astronomers recently revealed that these seemingly dark regions are actually permeated by a vast diffuse light. This kind of luminous 'sea' connects the bright islands of the Universe.
This discovery comes from a three-dimensional map of the young Universe, developed using data collected by the Hobby-Eberly Telescope Dark Energy Experiment. Scientists examined a particular ultraviolet light, called Lyman-alpha, produced by hydrogen when it is excited by radiation from young, hot stars. The mapped period, dating from 9 to 11 billion years ago, corresponds to a peak era of star formation.
A section of the new 3D map from HETDEX data, showing concentrations of excited hydrogen (Lyman-alpha light) in the space between galaxies, indicated by stars.
Credit: Maja Lujan Niemeyer/Max Planck Institute for Astrophysics/HETDEX, Chris Byrohl/Stanford University/HETDEX To achieve this result, the researchers used an original approach called line intensity mapping. Instead of cataloging each galaxy one by one, they measured the combined light from hydrogen over large celestial areas. This method captures not only bright objects, but also the faint radiation from diffuse gas and small galaxies that escape traditional observations.
The Hobby-Eberly Telescope, located at the McDonald Observatory in Texas, provided an impressive amount of data, with over 600 million spectra analyzed. Using supercomputers, the team reconstructed the distribution of hydrogen in an immense cosmic volume. Gravity, which assembles matter, allowed them to interpret this background glow by using the positions of already known galaxies.
This map reveals the luminous framework that connects cosmic structures. It thus provides a new tool to examine how galaxies formed and evolved within their environment, interacting with intergalactic gas.
The work, presented in
The Astrophysical Journal, marks a turning point in the way of mapping the cosmos. It paves the way for a broader use of intensity mapping to probe not only the brightest objects, but also the entire cosmic network. This helps to better understand the processes at work during the most active era of the Universe.
Lyman-alpha light, a signature of hydrogen
Lyman-alpha light is an ultraviolet emission produced when hydrogen atoms, the most abundant element in the Universe, are excited. This excitation generally occurs under the influence of intense radiation emitted by young, very hot stars. When these atoms return to a lower energy state, they release this characteristic light, which can travel over cosmological distances.
In astronomy, this luminous signature serves as a valuable marker to trace the presence of hydrogen, even when it is in the form of diffuse and low-density gas. It is particularly visible in the young Universe, during the so-called 'cosmic noon' period, when star formation was at its maximum. Modern telescopes can detect this glow despite the expansion of the Universe, which stretches light towards longer, redder wavelengths.
The study of this emission allows scientists to reconstruct the distribution of ordinary matter, which makes up only a small part of the Universe's content. It helps to understand how gas assembled to form the first galaxies and how it flows between them, fueling the birth of new stars over cosmic time.