A large part of the hidden ordinary matter in the Universe finally seems to be better localized.
Beware of possible confusion: this is not dark matter. This matter is not exotic, it is composed of the same particles as stars, planets, and our own bodies. And yet, it remained invisible until now.
Simulated distribution of gas (in blue, green, and yellow) around galaxies (white dots in the image). The study reveals that the gas in our Universe extends farther from galaxies than most simulations predict, indicating strong activity from galaxies that have expelled gas far from galaxy groups.
Credit: IllustrisTNG
The problem has existed for decades. Measurements of the young Universe indicate the amount of baryonic matter, that composed notably of protons and neutrons. Yet stars, galaxies, and visible clouds represent only a small fraction in the current Universe.
The team associated with the CHIME/FRB collaboration relied on fast radio bursts. These brief and very bright radio flashes come from distant galaxies. When passing through gas, their different wavelengths arrive with a slight measurable delay.
The more ionized matter a signal passes through, the more pronounced this temporal spreading. The researchers analyzed 2,870 fast radio bursts detected by the Canadian radio telescope CHIME. They then compared these measurements with the position of millions of galaxies cataloged by the DESI instrument.
This comparison reveals a clear correlation. Regions where galaxies are numerous are also surrounded by more diffuse baryonic matter. The gas is not concentrated near the galaxies. On the contrary, it extends far beyond their visible boundaries.
According to the study, this matter can be observed up to about 4 million light-years around galaxies. This distance exceeds the predictions of many simulations. It indicates that energetic processes inside galaxies expel gas farther than expected.
Black hole jets and stellar explosions can contribute to this phenomenon. These mechanisms propel gas out of galaxies and modify their environment. The result does not resolve all questions about cosmic matter, but it informs about its distribution.
The method could become more precise with the increase in the number of detected radio bursts. It already allows mapping of otherwise almost invisible gas. Understanding this diffuse matter also helps to better describe galaxy formation and their exchanges with the space surrounding them.