Filamentary Structure Between High-Redshift Quasar Pairs in the Cosmic Web

Academic Background

The cosmic web is a core concept in modern cosmology, describing the complex network structure formed by dark matter and gas under gravitational forces. According to Cold Dark Matter (CDM) theory, the cosmic web consists of filamentary structures connecting galaxy clusters and groups. These filaments are considered fundamental components of large-scale structures in the universe, but directly observing them has been extremely challenging. Due to their extremely low surface brightness (SB), traditional astronomical instruments have struggled to capture their signals. In recent years, with the deployment of high-sensitivity spectrographs such as MUSE, scientists have begun to detect the faint radiation from these filaments.

The core objective of this study is to directly observe and quantitatively analyze the physical properties of the cosmic web through the filamentary structures between high-redshift (z ≈ 3.22) quasar pairs. Using data from the MUSE Ultra Deep Field (MUDF), the research team presented for the first time a high-resolution view of the filament connecting two quasars and conducted detailed studies on its morphology, surface brightness distribution, and density characteristics. By comparing with numerical simulations, the study validates the predicted densities of filaments in the current cold dark matter model and provides new observational evidence for understanding the formation of large-scale structures in the universe.

Source of the Paper

This paper was co-authored by more than 20 authors, including Davide Torniotti, Michele Fumagalli, and Matteo Fossati, with team members from various institutions such as the University of Milan, Durham University, the Max Planck Institute for Astronomy, and the California Institute of Technology. The paper was published in Nature Astronomy in 2024, titled “High-definition imaging of a filamentary connection between a close quasar pair at z ≈ 3.”

Research Workflow

1. Data Acquisition and Processing

The research team utilized the MUSE instrument on the European Southern Observatory’s Very Large Telescope (VLT) to conduct a total of 142 hours of deep observations in the MUDF region. MUSE is an integral field spectrograph capable of simultaneously acquiring spectral and spatial information across a wide wavelength range. The observational data underwent multi-step processing, including bias correction, dark-field correction, wavelength calibration, and sky background subtraction. A high-precision three-dimensional data cube was ultimately generated, with a resolution of 0.2 arcseconds and 1.25 Å.

2. Identification and Extraction of Filamentary Structures

To extract the low surface brightness filamentary structures from the data, the research team used the CUBEX tool to subtract continuum sources and remove the point spread function (PSF) of the quasars. By setting a Signal-to-Noise Ratio (SNR) threshold, they identified the filamentary structure connecting the two quasars. This structure extends approximately 700 physical kiloparsecs (pkpc) in projection, with a surface brightness of about 8×10^-20 erg s^-1 cm^-2 arcsec^-2.

3. Surface Brightness Distribution Analysis

The research team conducted a detailed analysis of the surface brightness distribution of the filamentary structure. By extracting the average surface brightness along the axis connecting the quasar pair, they found that the brightness distribution of the filament smoothly transitions with the circumgalactic medium (CGM) surrounding the quasars. They also measured the transverse brightness distribution of the filament, finding its thickness to be approximately 140 pkpc, with brightness decreasing as a power law with distance.

4. Numerical Simulation Comparison

To validate the observational results, the research team used Semi-Analytic Models (SAM) and hydrodynamic simulations (IllustrisTNG) for analysis. The simulation results showed that the predicted filament densities in the cold dark matter model were consistent with the observations. The study also found that when the physical distance between quasar pairs is less than 1 physical megaparsec (pMpc), they are typically connected by high-density filaments, while at distances greater than 2 pMpc, the gas density approaches the cosmic mean density.

5. Physical Property Inference

By analyzing the surface brightness of the filamentary structure, the research team inferred an internal gas density of approximately 5×10^-3 cm^-3. This low-density gas is almost fully ionized under ultraviolet background radiation, consistent with the assumption of optically thin recombination radiation. The study also considered the impact of quasar fluorescence radiation on the filament, finding its contribution to the total brightness to be limited.

Key Results

1. High-Resolution Imaging of Filamentary Structures: The study obtained for the first time high-resolution images of the filamentary structure connecting high-redshift quasar pairs, showcasing its complex morphology and branching structures.
2. Surface Brightness Distribution: The surface brightness of the filament decreases smoothly along both the connecting axis and transverse directions, consistent with predictions from the cold dark matter model.
3. Density Characteristics: The gas density of the filament is approximately 5×10^-3 cm^-3, consistent with numerical simulation results.
4. Environment of Quasar Pairs: The study shows that quasar pairs are typically connected by high-density filaments, with most physical distances being less than 1 pMpc.

Conclusion and Significance

Through high-resolution observations of the filamentary structure between high-redshift quasar pairs, this study directly validates for the first time the predicted densities of the cosmic web in the cold dark matter model. The research not only provides quantitative analysis of the filamentary structures of the cosmic web but also offers new observational evidence for understanding the formation and evolution of large-scale structures in the universe. Additionally, the study demonstrates the powerful capabilities of the MUSE instrument in detecting low surface brightness objects, laying the groundwork for future deeper investigations of the cosmic web.

Research Highlights

1. High-Resolution Imaging: For the first time, the complex morphology of the cosmic web’s filamentary structures was shown in high resolution.
2. Quantitative Analysis: Through surface brightness distribution and density characteristic analysis, quantitative observational evidence of the cosmic web was provided.
3. Numerical Simulation Validation: Predictions of the cold dark matter model were validated through comparison with numerical simulations.
4. Instrument Capability Showcase: The exceptional performance of MUSE in detecting low surface brightness objects was demonstrated.

Other Valuable Information

The research team also discovered that there may be small substructures within the filamentary structure, whose formation mechanisms and impacts on the evolution of the cosmic web warrant further investigation. Additionally, the research team plans to conduct deeper observations using larger telescopes (such as 40-meter-class telescopes) in the future to further reveal the physical properties of the cosmic web.