Missing baryons found in far reaches of galactic halos

Researchers have channeled the universe’s earliest light – a vestige of the universe’s formation known as the cosmic microwave background (CMB) – to solve a mystery of missing matter and learn new things about galaxy formation . Their work could also help us better understand dark energy and test Einstein’s general theory of relativity by providing new details about the speed at which galaxies move toward or away from us.

Invisible dark matter and dark energy account for about 95% of the total mass and energy of the universe, and the majority of the 5% considered ordinary matter is also largely invisible, like the gases at the edge of galaxies from which their planet exists. called halos.

Most of this common matter is made up of neutrons and protons – particles called baryons that reside in the nuclei of atoms such as hydrogen and helium. Only about 10% of the baryonic matter is in the form of stars, and most of the rest resides in the space between galaxies in strands of hot, scattered matter known as the warm-hot intergalactic medium, or WHIM.

Because baryons are so scattered in space, it has been difficult for scientists to get a clear picture of their location and density around galaxies. Because of this incomplete picture of where ordinary matter resides, most of the universe’s baryons can be considered “missing.”

Now an international team of researchers, with significant contributions from physicists at Lawrence Berkeley National Laboratory (Berkeley Lab) and Cornell University at the U.S. Department of Energy, have mapped the location of these missing baryons using the best measurements to date. of their location and density around groups of galaxies.

It turns out that the baryons are in galaxy halos after all, and these halos extend far beyond popular models predicted. While most of the stars of an individual galaxy are typically in a region about 100,000 light-years from the center of the galaxy, these measurements show that for a given group of galaxies, the farthest baryons may extend about 6 million light-years away. . Centre.

Paradoxically, this missing matter is even more challenging to map than dark matter, which we can observe indirectly through its gravitational effects on normal matter. Dark matter is the unknown material that makes up about 27% of the universe; and dark energy, which is pushing matter apart at an accelerated rate in the universe, makes up about 68% of the universe.

“Only a few percent of ordinary matter is in the form of stars. Most of it is in the form of gas, which is generally too faint and diffuse to detect,” said Emmanuel Schaan, Chamberlain Postdoctoral Fellow at Berkeley Lab’s Physics Department and lead author for one of two articles on the missing baryons, published March 15 in the journal Physical assessment D

The researchers used a process known as the Sunyaev-Zel’dovich effect that explains how CMB electrons get a boost in energy through a scattering process as they interact with hot gases around galaxy clusters.

“This is a great opportunity to look beyond the positions of galaxies and the velocities of galaxies,” said Simone Ferraro, a Divisional Fellow in the Physics Division at Berkeley Lab who participated in both studies. “Our measurements contain a lot of cosmological information about how fast these galaxies are moving. It will complement the measurements of other observatories and make them even more powerful,” he said.

A team of researchers at Cornell University, consisting of research associate Stefania Amodeo, assistant professor. Professor Nicholas Battaglia and graduate student Emily Moser led the modeling and interpretation of the measurements, examining their implications for faint gravitational lenses and galaxy formation.

The computer algorithms the researchers developed should prove useful in analyzing “weak lensing” data from future experiments with high precision. Lens phenomena occur when massive objects such as galaxies and galaxy clusters are roughly aligned in a particular line of location, so that gravitational distortions actually bend and distort the light from the more distant object.

Weak lensing is one of the main techniques scientists use to understand the origin and evolution of the universe, including the study of dark matter and dark energy. Learning the location and distribution of baryonic matter puts this data at your fingertips.

“These measurements have profound implications for low lens performance, and we expect this technique to be very effective in calibrating future low lens performance studies,” Ferraro said.

Schaan noted, “We are also getting information relevant to galaxy formation.”

In the latest studies, researchers relied on a dataset of galaxies from the ground-based Baryon Oscillation Spectroscopic Survey (BOSS) in New Mexico, and CMB data from the Atacama Cosmology Telescope (ACT) in Chile and the Planck telescope in space of the European Space Agency. . Berkeley Lab played a leading role in mapping BOSS and developed the computational architectures required for Planck’s data processing at NERSC.

The algorithms they created benefit from analysis using the Cori supercomputer from Berkeley Lab’s DOE-funded National Energy Research Scientific Computing Center (NERSC). The algorithms counted electrons, allowing them to ignore the chemical composition of the gases.

“It’s like a watermark on a banknote,” Schaan explained. “If you place it in front of a backlight, the watermark appears as a shadow. To us, the backlight is the cosmic microwave background. It serves to illuminate the gas from behind so we can see the shadow as the CMB light travels through it. gas.”

Ferraro said, “It’s the first really important measurement that really determines where the gas was.”

The new image of galaxy halos, provided by the “ThumbStack” software that researchers created: huge, fuzzy spherical regions extending far beyond the starlit regions. This software is effective in mapping those halos even for groups of galaxies that have low mass halos and for those that are moving very quickly away from us (known as high redshift galaxies).

New experiments that should benefit from the halo mapping tool include the Dark Energy Spectroscopic Instrument, the Vera Rubin Observatory, the Nancy Grace Roman Space Telescope, and the Euclid Space Telescope.