First detection of cross-correlation between cosmic shear and X-ray background improves understanding of baryonic matter

A new study in Physical Exam Letters provides the first detection of cross-correlation between cosmic shear and the diffuse X-ray background, helping to understand the distribution of baryonic matter in the universe.

Making up about 5% of the universe, baryonic matter, composed of protons and neutrons, plays a crucial role in creating cosmological structures such as stars, planets and galaxies. It contains valuable information for understanding the large-scale structures of the universe.

Baryonic matter is attracted to concentrated regions of dark matter, called dark matter halos, by the gravitational pull of dark matter. In these halos, baryonic matter exists either in concentrated forms (such as stars and galaxies) or in diffuse forms (such as hot gas).

Detection of baryonic matter in both forms is difficult due to the complex interactions of scattered gas and dark matter effects.

Dr Tassia Ferreira and colleagues at the University of Oxford investigated the influence of baryonic physics on cosmological measurements in their PRL study. To do this, they combined data from two observational sources.

Dr. Ferreira spoke to Phys.org about the nature of baryonic matter. “I’ve always had a passion for studying the universe through an observational perspective,” Dr. Ferreira said.

“I have worked on cosmic shear for most of my research career, and at Oxford I met experts in cross-correlations using weak lensing data. So it made sense to push cross-correlations to combine effects that should intuitively be connected but had not yet been detected.”

Distribution of baryonic matter

For the cosmic shear measurements, which provide information about concentrated baryonic matter, the researchers relied on data from the Dark Energy Survey Year 3 (DES Y3).

These data contain images and measurements of galaxies, galaxy clusters and other cosmic structures.

The gravitational pull of dark matter can distort the shape of background galaxies. Cosmic shear indirectly measures the distribution of dark matter by observing how it distorts the shape of background galaxies.

This does not give direct information about baryonic matter but is useful for inferring the influence of dark matter on it.

Baryonic matter in the hot gas of dark matter halos is heated by gravitational forces, emitting X-rays. These X-ray data can be used to trace the distribution of baryonic matter in the form of hot gas.

The researchers obtained the data from the ROSAT All-Sky Survey (RASS). Conducted by the ROSAT satellite between 1990 and 1991, this survey provides an X-ray view of the entire sky.

The “halfway” mass

The correlation between the two data sets has several advantages. Dr. Ferreira explains: “The X-ray emission from hot gas in dark matter halos is governed by the temperature and density of the gas.”

“This dependence is ideal for tracking how gas is distributed. Since cosmic shear is very sensitive to mismodeling of baryonic effects, cross-correlation provides a consistent way to break up degeneracies.”

Additionally, the cross-correlation signal comes from the collective emission of all large-scale structures, making it less sensitive to modeling errors of individual objects.

Using a hydrodynamic model developed from previous research, the researchers modeled the distribution of mass and gas in halos. Cold dark matter, gravity-bound gas, ejected gas, and stars are all accounted for in this model.

Dr Ferreira said: “From X-ray observations, the bound gas fraction can be parameterized by the cosmic baryon fraction (the number of baryons relative to the total matter content of the universe), the total halo mass, the ‘halfway mass’ and the slope of the suppression of hot gas towards small halo masses.”

“X-ray observations are ideal for probing this quantity because they allow the bound gas to be followed.”

The midpoint mass of a dark matter halo is the mass at which half of the gas originally in the halo has been expelled. This value measures the extent of gas loss due to processes such as star formation or black hole formation.

The mass constraint midway through the study is a major contribution to understanding how cosmic structures lose gas over time and how this loss affects the structure of the universe.

Important and future works

Cross-correlation of the datasets revealed a significant correlation between cosmic shear and the diffuse X-ray background.

The significance of 23σ (sigma) indicates that this correlation is highly statistically significant, suggesting a strong relationship between the two data sets. This result leaves little doubt about the robustness of their conclusions.

The researchers estimated that the midpoint mass of dark matter halos was about 115 trillion solar masses.

In addition to the midpoint mass, the researchers were also able to constrain the polytropic index, which measures the relationship between temperature and the density of hot gas in dark matter halos.

The estimated value of the polytropic index agrees well with previous studies. The new constraints are tighter and more precise compared to previous cosmic shear and X-ray data.

In addition to providing a clearer view of the distribution of matter in the universe, the study also provides a new method for evaluating theories related to dark matter and dark energy.

Dr Ferreira said: “The developed procedure is a starting point for a more rigorous analysis using cross-correlations between cosmic shear and maps of the diffuse X-ray background. This is particularly relevant for future studies of weak lensing, such as those of the Vera Rubin Observatory and Euclid, with ongoing X-ray missions, such as eROSITA, which seek to obtain more precise cosmological constraints from large-scale structure data.”

In the future, Dr. Ferreira sees many opportunities to develop their findings, including validating the theoretical model before the developed methodology can be used in cosmological analyses.

“Furthermore, the residual degeneracy between cosmological and hydrodynamic parameters could be broken by including a cross-correlation with the Sunyaev-Zel’dovich Compton-y maps, given their complementary sensitivity to gas density and temperature,” she concluded.

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