[Paper Review] Line-Intensity Mapping: 2017 Status Report

Ely D. Kovetz, M. Viero+46 more|arXiv (Cornell University)|Sep 26, 2017

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy136 references161 citations

TL;DR

A comprehensive overview of line-intensity mapping as of 2017, detailing theory, instrumentation, detections, and prospects for CO, [CII], Lyα, 21-cm, and other lines across cosmic history, with emphasis on methodologies and experimental landscape.

ABSTRACT

Following the first two annual intensity mapping workshops at Stanford in March 2016 and Johns Hopkins in June 2017, we report on the recent advances in theory, instrumentation and observation that were presented in these meetings and some of the opportunities and challenges that were identified looking forward. With preliminary detections of CO, [CII], Lya and low-redshift 21cm, and a host of experiments set to go online in the next few years, the field is rapidly progressing on all fronts, with great anticipation for a flood of new exciting results. This current snapshot provides an efficient reference for experts in related fields and a useful resource for nonspecialists. We begin by introducing the concept of line-intensity mapping and then discuss the broad array of science goals that will be enabled, ranging from the history of star formation, reionization and galaxy evolution to measuring baryon acoustic oscillations at high redshift and constraining theories of dark matter, modified gravity and dark energy. After reviewing the first detections reported to date, we survey the experimental landscape, presenting the parameters and capabilities of relevant instruments such as COMAP, mmIMe, AIM-CO, CCAT-p, TIME, CONCERTO, CHIME, HIRAX, HERA, STARFIRE, MeerKAT/SKA and SPHEREx. Finally, we describe recent theoretical advances: different approaches to modeling line luminosity functions, several techniques to separate the desired signal from foregrounds, statistical methods to analyze the data, and frameworks to generate realistic intensity map simulations.

Motivation & Objective

• Introduce line-intensity mapping as a technique for mapping aggregate line emission from galaxies and the IGM.
• Survey science goals spanning cosmology, galaxy formation, reionization, and dark energy.
• Summarize first detections and the experimental landscape of current and upcoming instruments.
• Discuss theoretical modeling, foreground separation, and analysis techniques for intensity maps.
• Highlight opportunities, challenges, and future directions for multi-line intensity mapping.

Proposed method

• Present the basic formalism for the intensity mapping power spectrum Pk(z) with clustering and shot-noise components (Eq. 1.1).
• Relate the mean line intensity ⟨I(z)⟩ and shot power Pshot(z) to the luminosity function Φ(L,z) via its moments (Eq. 1.2).
• Outline the experimental landscape and capabilities of planned/apparent instruments (e.g., COMAP, AIM-CO, TIME, SPHEREx, HERA, SKA).
• Discuss strategies to separate signal from foregrounds and interlopers, including cross-correlations and masking techniques.
• Describe theoretical modeling approaches for line luminosity functions and simulations combining hydrodynamics with halo-patch methods.

Experimental results

Research questions

• RQ1 What cosmological information can line-intensity mapping extract from the large-scale structure and expansion history across different redshifts?
• RQ2 How can intensity mapping of CO, [CII], Lyα, and 21-cm lines illuminate star formation, galaxy evolution, and the IGM?
• RQ3 What are the most effective methods to mitigate foregrounds and line interlopers in intensity maps?
• RQ4 How can cross-correlations between different lines and with optical surveys enhance detections and constrain astrophysical quantities?
• RQ5 What modeling and simulation frameworks best enable rapid, realistic forecasts and data interpretation for upcoming surveys?

Key findings

• There are preliminary detections of CO, [CII], Lyα, and low-redshift 21-cm, with multiple experiments planned or online.
• Line-intensity mapping can map large cosmic volumes without resolving individual galaxies, enabling access to faint/extended emission.
• Cross-correlations and multi-line approaches provide robust detections and rich astrophysical information even in the presence of foregrounds.
• A diverse experimental landscape (ground-based and satellite concepts) targets a wide redshift range, from EoR to z≈0–2.5, with various lines (CO, [CII], Lyα, 21-cm).
• Theoretically, modeling luminosity functions, foreground mitigation, and advanced simulations (hybrid hydrodynamics + halo methods) are key to maximizing science return.