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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ByoPiC (The Baryon Picture of the Cosmos)

Teaser

Summary

The cosmological paradigm of structure formation is both extremely successful and plagued by many
enigmas. Not only the nature of the main matter component, dark matter, shaping the structure skeleton in
the form of a cosmic web, is mysterious; but also half of the ordinary matter (i.e. baryons) at late times of the
cosmic history, remains unobserved, or hidden! ByoPiC focuses on this key and currently unresolved issue in
astrophysics and cosmology: Where and how are half of the baryons hidden at late times? ByoPiC will
answer that central question by detecting, mapping, and assessing the physical properties of hot ionised
baryons at large cosmic scales and at late times. This will give a completely new picture of the cosmic web,
added to its standard tracers, i.e. galaxies made of cold and dense baryons. To this end, ByoPiC will perform
the first statistically consistent, joint analysis of complementary multiwavelength data: Planck observations
tracing hot, ionised baryons via the Sunyaev-Zeldovich effect, optimally combined with optical and near
infrared galaxy surveys as tracers of cold baryons. This joint analysis will rely on innovative statistical tools
to recover all the (cross)information contained in these data in order to detect most of the hidden baryons in
cosmic web elements such as (super)clusters and filaments. These newly detected elements will then be
assembled to reconstruct the cosmic web as traced by both hot ionised baryons and galaxies. Thanks to that,
ByoPiC will perform the most complete and detailed assessment of the census and contribution of hot
ionised baryons to the total baryon budget, and identify the main physical processes driving their evolution
in the cosmic web. Catalogues of new (super)clusters and filaments, and innovative tools, will be key
deliverable products, allowing for an optimal preparation of future surveys.

Work performed

During the reporting period, many achievements were realised within the ByoPiC project. We have worked on several aspects of the project and the results are in published or submitted articles.
We have performed a full multi-wavelength analysis (optical, X-rays and Sunyaev-Zeldovich) of a pair of clusters, A399-A401. We have exhibited and characterised a filament of matter between the pair of clusters, using the Sunyaev-Zeldovich (SZ) effect from Planck satellite. We have shown its fossile nature and have computed its pressure, density and temperature.
We have systematically analysed all pairs of clusters in the Planck data from catalogues of SZ, X-rays and optical clusters in order to search for filaments. We have constructed a sample of a few tens of such cases. A couple of them show significant signal in the Planck SZ map. In particular, the merging system A2029-A2033 is being analysed in details (higher resolution data such as those of ACTPol would be needed).
Filaments between clusters can be identified by stacking stacking of pairs of galaxies. This was done using a large sample of galaxies from the SDSS survey. It has shown an SZ signal interpreted as the gas between galaxy clusters.
With stacking technique of 580 superclusters of galaxies, we have detected for the first time the SZ signal from hot gas in superclusters. With a simple model, we derive that this signal may account for ~30% of missing baryons.
We have performed the cosmological analysis of cluster counts and power spectrum from Planck satellite. We show that the tension between clusters and cosmic microwave background (CMB) is no more significant but that the mass estimate of clusters as preferred by CMB is still incompatible with most other astrophysical probes. This result was subject of a Press release from CNRS-INSU (http://www.insu.cnrs.fr/node/9479). We have applied our formalism to perform the analysis that was published by the Planck collaboration.

Final results

The ByoPiC team is not yet fully established (a last call for position will be opened for recruitment in 2019) but the project has significantly gained in efficiency. Therefore, several short term expected important results of the ByoPiC project are foreseen. We have obtained preliminary results showing the detected SZ signal from hot gas in cosmic filaments detected in galaxy surveys. If confirmed this would represent a major achievement of the project. We are studying the connectivity of the massive clusters (nodes of the cosmic web) by exploring how the Coma cluster (and the full Coma supercluster) connect to large scale filaments of the cosmic web. We have started similar studies to the inter-cluster filaments in superclusters such as Shapley. We plan estimate the gas properties using the Planck SZ map and exisiting archival X-ray data. In parallel, the research activities on the data analysis and statistical approaches for the detection, classification and reconstruction of the comsic web have started. We are currently investigating the use of machine learning methods to focus on the classification and reconstruct the full cosmic web. Such an approach if sucessful will be a major result. Furthermore, we are investigating different approaches, including machine learning, to improve the Planck SZ map and to optimise it at small angular scales, necessary for the ByoPiC project.