Letters of Intent received in 2022

LoI 2024-2195
Connecting via the Intergalactic Medium

Date: 5 August 2024 to 16 August 2024
Category: Focus meetings (GA)
Location: Capetown, South Africa
Contact: Emma Ryan-Weber (eryanweber@swin.edu.au)
Coordinating division: Division J Galaxies and Cosmology
Other divisions:
Co-Chairs of SOC: Emma Ryan-Weber (Swinburne University)
James Bolton (University of Nottingham)
Chair of LOC: None (None)



The focus meeting is aimed at promoting discussion and future planning between optical/near-IR observations, radio signatures and theory of the IGM from z>6 to the local Universe. We will use the South African location of the GA to connect the traditional IGM and radio communities.

Reionization and the ionizing background:
● Constraining reionization with Lyman-alpha and -beta absorption
● Metal lines as a probe of ionization and enrichment
● The thermal history of the IGM
● Inferred properties of the ionizing sources (quasars and galaxies)
● Probing the IGM with 21-cm in absorption and emission
● Role of quasars and galaxies in 21-cm tomography

Cosmology with the IGM:
● Constraints on dark matter from the IGM
● The 3D power spectrum and cross-correlations, including 21-cm
● Baryon acoustic oscillations with the Lyman-alpha forest
● Fast Radio Bursts as cosmological probes



The intergalactic medium (IGM) is the link between galaxy formation and cosmology; its spatial distribution is often referred to as the cosmic web due to the filamentary network that intergalactic gas and dark matter traces on large scales. Detailed spectroscopic observations of the cosmic web -- as seen in absorption in the spectra of distant, background quasars -- therefore play a vital role in understanding the structural, chemical and thermal evolution of baryons across cosmic time. The next decade will herald a new era with the detection of the high-redshift IGM via 21-cm radio emission and absorption.

Studies of the IGM have expanded rapidly over the past several years. Deep optical and infrared surveys are discovering some of the most distant quasars known, with over 200 at redshift z>6. These observations have yielded insight into the build-up of supermassive black holes in the high redshift Universe, but furthermore also provide many more background objects for studying the high redshift intergalactic medium (IGM). Ongoing large scale spectroscopic surveys, designed to measure the dark energy equation of state by tracking the evolution of the Baryonic Acoustic Oscillation signal, are expected to boost the number of quasars for studying the statistics of IGM density fluctuations by a factor of ten, to a million.

Recent advances in numerical models have been similarly impressive, with large scale simulations starting to routinely include radiation hydrodynamics, magnetic fields and feedback processes. There has also been a shift toward developing more efficient models for the IGM that can explore wide regions of parameter space, for example using machine learning and/or GPU acceleration. Theoretical predictions of 21-cm tomographic and forest as well as the use of Fast Radio Bursts as cosmological probes has expanded the scope of IGM models.

Given these developments, we suggest an IAU GA Focus Meeting on the IGM is timely (the closest topic to the proposed FM was “Warm and Hot Baryonic Matter in the Cosmos” in 2018 and the only previous IAU symposium on the IGM was IAUS 217 "Recycling Intergalactic and Interstellar Matter" in 2004). We will plan a programme to discuss major advances in observations and theory and plan new projects to address the key questions in IGM science.

The proposed Focus Meeting will support Education and Outreach events. The Intergalactic Medium is the material between the bright galaxies: by analogy in the night sky, the Milky Way is viewed as a lane of stars with absorbing clouds interspersed. Such dark constellations are a feature in Southern Hemisphere Cultural Astronomy, including African (Gullberg et al. 2020, Journal of Astronomical History and Heritage, 23(2), 390). In the IAU vision 2024, Astrotourism is noted as a growing industry in Africa, with dark sky sites as a destination. In coordination with the GA hosts, we will support these initiatives with public engagement events.

The proposed coordinating Division is Division J (Galaxies and Cosmology), and this Focus Meeting LoI has been endorsed by Division J President Kim-Vy Tran. The proposed Focus Meeting has been developed by the Organizing Committee of the J2 Inter-Division B-H-J Commission Intergalactic Medium, which naturally spans Divisions B (Facilities, Technologies and Data Science), H (Interstellar Matter and Local Universe) and J (Galaxies and Cosmology). The topic is particularly timely for the scientific planning of the SKA, and has clear potential for cross-division interest and discussion.