Line-Intensity Mapping

TL;DR

This review surveys Line-Intensity Mapping (LIM) as a three-dimensional probe of large-scale structure and the high-redshift universe, detailing the LIM framework, key tracers, line-emission physics, and modeling approaches. It presents power-spectrum and cross-power-formalisms, discusses refinements including redshift-space distortions and halo-model terms, and surveys simulations, statistics, and cross-correlation potentials. The article surveys the current experimental landscape, foreground challenges, and mitigation strategies, while outlining current measurements and forecasts across 21 cm, CO, [CII], Ly-α, and other lines. It argues that LIM, especially when used with multi-line and cross-correlation techniques, can illuminate cosmic history from the Cosmic Dawn through reionization, constrain cosmology (including dark energy, inflation, and neutrino masses), and map the evolution of gas in galaxies and the intergalactic medium. Overall, LIM promises economical, tomographic access to vast cosmological volumes and the gas content of the universe, with cross-probe validations and sophisticated statistical tools guiding future discoveries.

Abstract

Line-Intensity Mapping (LIM) has emerged as a powerful technique for studying large-scale structure and the high-redshift universe, enabling three-dimensional maps of line emission across vast cosmological volumes. In this review, we summarize the LIM framework, its key scientific goals, and its future prospects. We describe the landscape of emission line tracers, theoretical modeling approaches, anticipated signals, and data-analysis methodologies. We also discuss experimental challenges, particularly those posed by astrophysical foregrounds, and review possible mitigation strategies. Further, we highlight a range of cross-correlation science cases, linking LIM with other cosmological surveys. Finally, we summarize current and upcoming experiments and early results, including recent first detections, while outlining the outlook for future discoveries. Specifically, LIM may offer new insights into galaxy formation and evolution and cosmology, while revealing the Epoch of Reionization, Cosmic Dawn, and possibly the Cosmic Dark Ages. LIM enables cosmological measurements that complement other probes and provide unique access to the high-redshift universe, potentially shedding light on dark matter, dark energy, and cosmic inflation.

Figures (40)

• Figure 1: Illustration of the line-intensity mapping approach. Left panel: The gray points show a model for the CO(1-0) emission from simulated galaxies near $z \sim 3$ across a 2.5 deg$^2$ field. The red points show the sources that would be sufficiently bright to observe with a traditional galaxy survey (using an extremely long integration with the VLA). Right panel: For contrast, this panel shows the (smoothed, yet noise-free) CO(1-0) brightness temperature fluctuations from the same region, as one would measure in a LIM survey. The line-intensity fluctuations trace the total clustered emission, including the collective impact of galaxies that are too faint to detect individually in even an ambitious traditional galaxy survey. From Patrick Breysse, Kovetz:2017agg.
• Figure 3: A model SED from the THESAN simulation suite, illustrating some of the possible line targets for LIM surveys. The blue curve shows the intrinsic emission line spectrum from the simulated galaxy, while the red curve is reprocessed by dust extinction/emission. Rest-frame optical transitions from [OII] ($3726, 3729 \, \hbox{\AA}$), [OIII] ($4959, 5007 \, \hbox{\AA}$), H-$\alpha$ ($6563 \, \hbox{\AA}$), and H-$\beta$ ($4861 \, \hbox{\AA}$) are indicated, along with fine-structure emission lines from [OIII] ($52 \mu$m, $88 \mu$m), [NII] ($122 \mu$m), and [CII] ($158 \mu$m). Other promising line-emission targets for LIM surveys include CO rotational transitions, Ly-$\alpha$ (included here, but unmarked on the left hand side of the SED), HeII recombination lines, and others. In this section, we discuss many of these emission lines and their scientific utility. From Kannan:2021ucy.
• Figure 4: Illustration of some of the key ISM phases and associated emission lines. In the HII region phase, newly formed O and B stars -- i.e. massive stars with high surface temperature which emit copious numbers of hydrogen ionizing photons -- photoionize surrounding neutral hydrogen gas. Here transitions between energy states in various ions are collisionally excited, leading to long wavelength fine-structure transitions as well as optical and ultraviolet emission lines. Examples include emission from CII, NII, OII, and OIII ions, and many additional lines. Further, recombination cascades from residual neutral hydrogen in HII regions lead to Lyman-series and Balmer-series lines (among others) at rest-frame optical and ultraviolet wavelengths. In the PDR phase hydrogen is largely neutral, but ultraviolet photons singly-ionize carbon atoms and dissociate molecular gas. Further out, in the cold molecular gas phase, CO molecules are shielded from dissociating UV radiation by dust grains. Finally, outside of the dense cloud (the "GMC" in the diagram), lies -- along with other gas and dust -- warm/cold atomic gas, which mainly consists of neutral hydrogen and is best traced by the 21 cm transition. From Sun19.
• Figure 5: Energy levels of OIII. The right-most column specifies the spin, orbital, and total angular momentum of each state. The next column gives the degeneracy of states, with $g=2J+1$ distinct states in each level. The third column is the energy relative to the ground state in temperature units, while the arrows indicate transitions between different states and the associated emission wavelengths. The three lowest levels are split by the spin-orbit fine-structure interaction; transitions between these levels lead to relatively long wavelength emission and their small energy splitting makes the results insensitive to the gas temperature. From Yang2020, adapted from Draine11.
• Figure 6: Models of redshifted 21 cm fluctuations during the Epoch of Reionization and the Cosmic Dawn. The panels show a lightcone through a 21cmFAST Mesinger11 simulation model. The thickness of each slice (i.e., the dimension into and out-of the page) is 1.5 co-moving Mpc, while the height is 750 co-moving Mpc. The top panel shows the neutral hydrogen fraction, $x_{\rm HI}$, across the lightcone, while the bottom panel is the 21 cm brightness temperature (relative to the CMB). The 21 cm signal appears in absorption against the CMB at $z \sim 10-20$ in this model (blue regions in the bottom panel), and then emission (red regions), until the universe is progressively ionized (dark regions in top panel and white regions in the bottom panel). From Munoz:2021psm.