Scientists have finally been able to find the part of ordinary matter that doesn’t show up as stars and nebulae. It turned out to be hidden in the very rarefied gas and dust surrounding galaxies. It is not dark matter, but the new discovery explains a lot about it.
Searching for hidden matter
When astronomers counted the matter that makes up the stars, galaxies and gas that is everything in the Universe today, they found that it fell short of the total amount of matter formed by the Big Bang 13.6 billion years ago. In fact, more than half of the normal matter among the 15% of the universe’s matter that is not dark matter has not yet been seen by astronomers.
However, it seems that new research has discovered this missing matter in the form of very diffuse and therefore usually invisible, ionized hydrogen that forms the halo around galaxies. It turned out to be much larger than scientists had thought until now.
The data not only resolve contradictions between astronomical observations and the best, tested model of the evolution of the Universe since the Big Bang, but also indicate that supermassive black holes at the centers of galaxies are more active than previously thought and are spouting gas over distances at least 5 times greater than expected.
“We think that, once we go farther away from the galaxy, we recover all of the missing gas,” said Boryana Hadzhiyska, a doctoral student at the University of California, Berkeley, and first author of a paper reporting the findings. “To be more accurate, we have to do a careful analysis with simulations, which we haven’t done. We want to do a careful job.”
The results of the study, co-authored by 75 scientists from around the world, have been presented at recent scientific conferences, posted as a preprint on arXiv, and now they are being peer-reviewed in the journal Physical Review Letters. Hadzhiyska and Ferraro work at the Berkeley Center for Cosmological Physics in the Department of Physics at the University of California, Berkeley, and at Berkeley Lab.
How is ionized gas distributed in the Universe?
The mysterious dark matter makes up about 84% of all matter in the Universe, with the rest being ordinary matter. However, only about 7% of normal matter is in the form of stars, and the rest is in the form of invisible hydrogen gas, mostly ionized, in galaxies and filaments that connect galaxies into a kind of cosmic network.
The ionized gas and its associated electrons strung on this network of filaments is called the warm-hot intergalactic medium, which is too cold and too diffuse to be seen by the usual methods available to astronomers, and has therefore remained elusive until now.
In the new work, the researchers estimated the distribution of ionized hydrogen around galaxies by imaging about 7 million galaxies — all at a distance of about 8 billion light-years from Earth and accounting for the slight eclipse or brightening of the cosmic microwave background caused by the scattering of radiation by electrons in ionized gas, the so-called kinematic Sunyaev-Zel’dovich effect.
Creation of a 3D map of the Universe
The used images of galaxies were collected by the Dark Energy Spectroscopic Instrument (DESI) on the 4-meter Mayall Telescope at Kitt Peak National Observatory in Tucson, Arizona. The instrument, created by a team of researchers headquartered at Berkeley Lab, surveys tens of millions of galaxies and quasars to build a 3D map of the Universe that spans 11 billion light-years from Earth to measure the impact of dark energy on the expansion of the Universe.
The cosmic microwave background (CMB) around these galaxies was measured by the Atacama Cosmological Telescope (ACT) in Chile, which made the most accurate CMB measurements to date before its decommissioning in 2022.
Feedback from galaxies
Astronomers have generally believed that massive black holes at the centers of galaxies eject gas in jets of matter only during their formative years, when the central black hole absorbs gas and stars and produces a lot of radiation. This distinguishes them as what astronomers call active galactic nuclei (AGN), or quasars.
If, as the new study suggests, the halo of ionized hydrogen around galaxies is more diffuse but also more extensive than thought, it means that central black holes may become active at other times in their lives.
“One problem we don’t understand is about AGNs, and one of the hypotheses is that they turn on and off occasionally in what is called a duty cycle,” said Hadzhiyska.
Astronomers call the expulsion of gas and its subsequent fall back into the galactic disk a feedback that regulates the formation of new stars throughout the galaxy. Ferraro, Schaan and their colleagues reported hints of more extended feedback in previous work in 2020, when Shaan was a doctoral student at Berkeley Lab.
But the new work covers more galaxies and gives more accurate measurements. Subsequent work by Ried Guachalla confirmed the conclusions using the DESI spectroscopic sample and was able to study the gas in more neighboring galaxies, emphasizing that the gas is not distributed uniformly around them, but follows the “cosmic threads” that run through the universe.
Hadzhiyska noted that modern simulations of galaxy evolution should include this more powerful feedback in their models. Some new models are already accomplishing this to create stronger simulations that better fit the new data.
The discovery of missing matter, or baryons, in the universe also has implications for other aspects of cosmic evolution.
Cosmological models of the Universe
On the one hand, the expulsion of gas from the nuclei of these massive galaxies casts doubt on the assumption that gas follows dark matter, Hadzhiyska says. Underestimating this gas displacement can introduce inconsistencies into cosmological models, while the new results may actually resolve some questions about how clumpy the Universe is.
“There are a huge number of people interested in using our measurements to do a very thorough analysis that includes this gas,” she said. “People in astronomy care a lot about it for understanding galaxy formation and evolution.”
The method the team used, the kinematic Sunyaev-Zel’dovich effect, can also be used to study the early Universe, Hadzhiyska said. This could provide insight into the large-scale structure of the Universe and the laws of physics in the early Universe, as well as allowing scientists to test gravity and the general theory of relativity.
According to phys.org
