Unravelling the Cosmic Web with fluorescent emission

Project Information

CosmicWeb

Grant agreement ID: 864361

DOI

10.3030/864361

EC signature date 6 April 2020

Start date 1 September 2020

End date 31 August 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 000 000,00

EU contribution

€ 2 000 000,00

Coordinated by

UNIVERSITA' DEGLI STUDI DI MILANO-BICOCCA
Italy

Periodic Reporting for period 3 - CosmicWeb (Unravelling the Cosmic Web with fluorescent emission)

Reporting period: 2023-09-01 to 2025-02-28

Summary of the context and overall objectives of the project

Our cosmological model predicts that most of the matter in the universe is distributed in a network of filaments - the Cosmic Web – in which galaxies form and evolve. Because most of this material is too diffuse to form stars, its direct imaging has remained elusive for several decades leaving fundamental questions still open, including: what are the morphological and kinematical properties of the Cosmic Web on both small (kpc) and large (Mpc) scales? How do galaxies get their gas from the Cosmic Web?
The CosmicWeb program tackle these questions with an innovative method and technology that allows us to directly detect in emission the gaseous Cosmic Web before the peak of galaxy formation, when the universe is less than 3 billion years old: using bright quasars and galaxies as “cosmic flashlights” to make the gas “fluorescently” glow.
The Program is divided into two, interconnected, main Projects. Project 1 aims at the detection of Cosmic filaments on large scales (Scientific Objective, SO, 1), the study of their relation with galaxies (SO2) and the comparison with models to constrain dark matter properties (SO3). Project 2 aims at the high-spatial resolution study of filaments using non resonant lines such Hydrogen H-alpha in order to reveal their small scale distribution (SO4) and their kinematics compared with galaxies (SO5) to inform new CGM models (SO6). The success of these SOs require: i) deep MUSE observations in order to reveal Cosmic filaments in Ly-alpha emission on large scales, ii) multi wavelength information to detect and study the associated galaxy properties, iii) JWST observation to detect H-alpha emission at high-spatial resolution, and, iv) numerical models on both large and small scales (with different dark matter properties).

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

During this period all actions associated with the Scientific Objectives of the Program have seen large progresses in some cases have been completed.
The most important result achieved so far has been the discovery of the largest Cosmic Web filaments detected to date in a field called MQN01 centered on a bright distant quasar . This discovery was possible thanks to a deep MUSE mosaic follow-up of 40 hours in a large area around a previously found nebula in a shallower survey. The discovered filaments are exceptional, extending beyond the currently surveyed volume and converging into a massive node of the Cosmic Web close to a large concentration of galaxies and Active Galactic Nuclei (AGN).
The exceptionality of the discoveries in the MQN01 field allowed the CosmicWeb PI to secure observing time on top facilities for a multi-wavelength campaign which started in 2021, including: i) JWST spectroscopic observations to study the kinematics of filaments and galaxies; ii) ALMA deep observations of molecular gas and dust in galaxies; iii) X-ray observations with Chandra to detect AGN; iv) HST observations to study the morphology of star forming galaxies and their relation with filaments; v) multi-wavelength photometry with the VLT to derive the Star Formation Rate and Stellar Masses of galaxies and study how these depend on the properties of filaments feeding them.
The results of these observations have been spectacular and resulted so far in 4 published or accepted articles led by team members supported by the ERC during the reporting period. These include the discovery of the largest overdensity of galaxies associated with a massive node of the Cosmic Web as traced by multi-wavelength observations from the sub-mm to the X-ray (Travascio et al. 2024; Pensabene, Cantalupo et al (including all team members of the “CosmicWeb” ERC-supported team) 2024; Galbiati et al. 2025 (in press); Wang et al. 2025 (Nature Astronomy)). These galaxies are embedded into a Cosmic Web structured as traced by MUSE and JWST observations which is now, for the first time, also detected in H-alpha emission (results which will be submitted soon). This dataset allows us to successfully fulfil all the Scientific Objectives of the Project by August 2026. Among the most important results, the JWST observation revealed the presence of the largest rotating galactic disk found in the early universe (Wang et al. 2025). The result, accepted in Nature Astronomy in January 2025 (and published in March 2025) attracted considerable attention from the press and the public after a very successful Press Release. In addition, a large number of collaborative publications (~20) on topics related to the Project have been published in the reporting period.
Important results have been obtained also on the modelling side. Through the use of state-of-the-art cosmological simulations, a new empirical method has been derived that allows us to derive the mass and environment of the haloes from the observed line emission profiles. This new method, published in de Beer et. al. 2024, allow us to break the main model degeneracy in order to derive from the detected emission the properties of diffuse gas such as density and temperature. These properties are fundamental in order to understand how galaxy form and grow through intergalactic gas feeding.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The discovery of the Cosmic Web filaments in emission in the MQN01 and its associated galaxy overdensity represent major breakthroughs in the field for at least three reasons: i) they give us the unique opportunity to study the morphology and properties of Cosmic Web filaments in emission on both cosmological scales (several Mpc) with MUSE and small scales (a few kph or smaller) with JWST observations; ii) it is a unique field for which the properties of the filaments, in terms of density distribution, morphology and kinematics, could be correlated to the galaxy properties (e.g. SFR, stellar masses, molecular gas content, orientation, and morphology) on a statistical sample given the large galaxy overdensity; iii) the central regions of the structure represents a unique environment discovered so far at high redshift with the presence of one of the largest overdensity of galaxies and AGN, the likely progenitor of today’s massive galaxy clusters. These unique characteristics will allow us to significantly advance our understanding of the processes regulating structure and galaxy formation and evolution in different environments providing a new laboratory to test new models of both structure and galaxy formation, including the original goal of the program to test dark matter properties.

A giant rotating disk galaxy ("Big Wheel", center) discovered in a massive Cosmic Web node at z=3.25