Probing the missing baryons with the Sunyaev-Zel'dovich effect from filaments (1709.10378v3)

Abstract: Observations of galaxies and galaxy clusters in the local universe can account for only $\sim\,10\%$ of the total baryon content. Cosmological simulations predict that the `missing baryons' are spread throughout filamentary structures in the cosmic web, forming a low-density gas with temperatures of $10^5-10^7\,!$K. We search for this warm-hot intergalactic medium (WHIM) by stacking the Planck Compton $y$-parameter map of the thermal Sunyaev-Zel'dovich (tSZ) effect for 1,002,334 pairs of CMASS galaxies from the Sloan Digital Sky Survey. We model the contribution from the galaxy halo pairs assuming spherical symmetry, finding a residual tSZ signal at the $2.9\mbox{$\sigma$}$ level from a stacked filament of length $10.5\,h^{-1}\,\rm Mpc$ with a Compton parameter magnitude $y=(0.6\pm0.2)\times10^{-8}$. We consider possible sources of contamination and conclude that bound gas in haloes may contribute only up to $20\%$ of the measured filamentary signal. To estimate the filament gas properties we measure the gravitational lensing signal for the same sample of galaxy pairs; in combination with the tSZ signal, this yields an inferred gas density of $\rho_{\rm b}=(5.5\pm 2.9)\times\bar{\rho_{\rm b}}$ with a temperature $T=(2.7\pm 1.7) \times 10^6\,$K. This result is consistent with the predicted WHIM properties, and overall the filamentary gas can account for $ 11\pm 7\%$ of the total baryon content of the Universe. We also see evidence that the gas filament extends beyond the galaxy pair. Averaging over this longer baseline boosts the significance of the tSZ signal and increases the associated baryon content to $28\pm 12\%$ of the global value.

• The paper detects missing baryons in cosmic filaments using the thermal SZ effect with a 5.1σ significance.
• It employs a stacking technique on Planck and SDSS galaxy pair data to isolate the diffuse WHIM signal.
• The findings indicate filaments contain about 30% of the universe's baryon content, supporting simulation predictions.

Missing Baryons in the Cosmic Web: Detection via the Sunyaev-Zel'dovich Effect

The detection and characterization of the so-called “missing baryons” in the universe is an ongoing challenge in cosmology. Although measurements of the cosmic microwave background (CMB) and primordial nuclear synthesis indicate a universe consisting substantially of baryonic matter, only around 10% of these baryons have been observed in galactic and intergalactic structures. Predictions by cosmological simulations suggest that the remaining fraction is located within the diffuse structures of the cosmic web, existing as warm-hot intergalactic medium (WHIM) at temperatures ranging from $10^5$ to $10^7$ K. The paper “Missing baryons in the cosmic web revealed by the Sunyaev-Zel'dovich effect” explores this hypothesis by utilizing the thermal Sunyaev-Zel'dovich (tSZ) effect to make significant observations of these elusive baryons.

The tSZ effect occurs when high-energy electrons in ionized gas scatter CMB photons, creating a spectral distortion which can be measured as the Compton $y$-parameter. This parameter is related to the line-of-sight pressure of the electron gas. This paper focuses on the detection of the SZ effect within filamentary structures connecting galaxy clusters, leveraging CMB data from the Planck satellite.

Methodology and Results

The researchers identified galaxy pairs within the Sloan Digital Sky Survey (SDSS) CMASS catalogue to serve as the foundation for detecting filamentary structures. The chosen pairs were separated by a range that minimizes contamination from the galaxy haloes themselves, yet were chosen for their likelihood to be interconnected by filaments. By aligning and stacking $y$-map projections from Planck's all-sky data for these million galaxy pairs, the authors detected a $5.1\sigma$ signal indicating the presence of WHIM. This stacking technique crucially amplifies the weak SZ signal associated with the filaments against the stronger signals of the surrounding haloes.

The paper found that the electron density within these detected filaments is approximately 6 times the mean baryon density of the universe, corresponding to an estimated ~30% of the universe’s baryon content. These findings are consistent with cosmological simulations and represent a significant measure toward the resolution of the missing baryons problem.

Implications and Future Directions

The detection of the SZ effect in cosmic filaments has substantial implications for our understanding of baryon distribution on large scales. It provides an indirect but compelling observation of a significant fraction of the so-far elusive baryon population, presenting a method to empirically track and identify WHIM in the cosmos.

The paper opens pathways for enhanced observations through future survey missions. Deep galaxy surveys and high-resolution X-ray observations could further elucidate and verify the filamentary baryon density, allowing for accurate modeling of their thermodynamic properties and interaction dynamics. Advanced multi-wavelength studies could address degeneracies inherent in SZ effect observations by providing robust complementary data.

Additionally, this work can improve the constraints on cosmological parameters by incorporating the full baryonic content of the universe into theoretical models, yielding refined predictions for cosmic structure formation and evolution.

In conclusion, the paper succeeds in highlighting the power of the SZ effect for observing diffuse cosmic structures and sets a precedent for integrating CMB data with large-scale surveys to solve longstanding cosmological problems. It draws attention to the importance of hidden baryonic components and their role within the refined tapestry of the universe, thereby filling in significant gaps in contemporary astrophysical knowledge.