Adam Lidz

Recent Atacama Large Millimeter/submillimeter Array observations reveal diffuse [C ii ] emission (“[C ii ] halos”) extending to ∼10 kpc in galaxies at 4 < z < 6. These measurements provide new insights into high-redshift galactic ecosystems and processes that drive metal enrichment on inner circumgalactic scales. To better understand the nature of [C ii ] halos, we analyze a suite of high-redshift FIRE-2 simulations at 5 ≤ z ≤ 6 in the stellar mass range of 10 ⁹ –10 ^10.5 M _⊙ . By postprocessing these simulations with three-dimensional dust radiative transfer and photoionization modeling, we generate synthetic images of [C ii ] and UV continuum emission, from which we extract one-dimensional surface brightness profiles. Our results reproduce both the galaxy-integrated and spatial distribution of [C ii ] and UV emission, capturing in particular the more extended profile of [C ii ] emission. Comparing the time evolution of [C ii ] halos with bursty star formation histories of the simulated galaxies, we find that [C ii ] emission becomes more spatially extended following the decline of star formation rate in recent starburst episodes. This implies a strong correlation between extended [C ii ] emission and bursty star formation, consistent with a key role for star formation-driven outflows in producing [C ii ] halos—though the kinematics of [C ii ]-emitting gas suggest that inflows and turbulent motions are also significant contributors. We also find a modest contribution from satellite galaxies to extended [C ii ] emission. Our framework can be readily applied to predict the observability of [C ii ] halos at higher redshifts and extended to create spatially resolved synthetic observations of other important emission lines, such as [O iii ] and H α .

We present a new approach for extracting information about cosmic magnetic fields using cross-correlations between extragalactic Faraday rotation measure (RM) catalogs and galaxy surveys. Specifically, we propose measuring the two-point cross-correlation between RM squared, RM ² , towards background sources and the projected density field of foreground galaxies, 〈RM ² × g〉, as a function of transverse separation. This statistic is analogous to the “projected fields” estimator used for the kinetic Sunyaev-Zel'dovich (kSZ) effect, 〈kSZ ² × g〉. Our estimator avoids contamination, on average, from RM contributions originating in the background source or the Milky Way, and is also free from the noise bias that arises when correlating the absolute value of the RMs with galaxies. Moreover, by binning in foreground galaxy redshifts, 〈RM ² × g〉 enables a tomographic reconstruction of the redshift evolution of large-scale cosmic magnetic fields. We model this statistic using the Illustris-TNG cosmological magnetohydrodynamic simulations and compare with approximate analytic predictions. In analogy with the kSZ case, we show that 〈RM ² × g〉 can be related to a bispectrum involving two copies of the electron-density-weighted magnetic field strength and one of the galaxy overdensity. On large scales, the correlation approximately traces the shape of the electron-galaxy two-point function, with an amplitude governed by the average electron-weighted magnetic field strength and the smoothing scale of the RM measurements. In Illustris-TNG , the effective field strength is primarily set by the magnetic field amplitudes within the inner regions of galaxy-hosting dark matter halos. It increases towards low redshift, driven by dynamo amplification and magnetized outflows. Our forecasts suggest that 〈RM ² × g〉 is detectable at high significance with current galaxy surveys and future RM catalogs from the SKA, offering a tomographic probe of large-scale magnetic fields across cosmic time.

We forecast that the Terahertz Intensity Mapper (TIM) cross-correlated with Euclid’s Deep Field Fornax (EDF-F), TIM × EDF-F, will detect the [C ii ]-galaxy cross-power spectrum at a median redshift of 1.1 with ≳7 σ confidence. The Poisson component of the cross-power spectrum at 0.1 ≤ k ≤ 10 h Mpc ⁻¹ (i.e., cross-shot noise) will be detected at ≳3 σ in four bins spanning 0.5 < z < 1.7. This measurement will constrain the mean [C ii ] specific intensity over half of cosmic history and assess the degree to which Euclid-selected galaxies account for the [C ii ] intensity observed by TIM. We find that TIM can detect the cross-power spectrum across a wide range of [C ii ] intensity models.

The “near–far” approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ( z ≳ 6), beyond the current James Webb Space Telescope detection limits ( M _UV ≳ −15). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to ∼50% of the total stellar-mass budget of the proto-MW/LG at z = 6−9. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of ∼2–3 and reduces the halo-to-halo variation in the slope by ∼20%–40%. This enables robust constraints down to at least the resolution limit of the simulations, near M _⋆ ∼ 10 ⁵ M _⊙ or M _UV ∼ −10 at z ≳ 6. We also show that “fossil-record” reconstructions—which assume each present-day system descends from a single reionization-era progenitor—are sensitive to the stellar-mass/UV-magnitude thresholds, which introduces bias in the inferred slopes at the low-mass/faint end. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with M _UV ≲ −15 to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction (∼50%) of streams with M _⋆ ≳ 10 ⁶ M _⊙ at z = 0 should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

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.

By opening up new avenues to statistically constrain astrophysics and cosmology with large-scale structure observations, the line intensity mapping (LIM) technique calls for novel tools for efficient forward modeling and inference. Implicit likelihood inference (ILI) from semi-numerical simulations provides a powerful setup for investigating a large model parameter space in a data-driven manner, therefore gaining significant recent attention. Using simulations of high-redshift 158 μ m [C II ] and 88 μ m [O III ] LIM signals created by the LIMFAST code, we develop an ILI framework in a case study of learning the physics of early galaxy formation from the auto-power spectra of these lines or their cross-correlation with galaxy surveys. We leverage neural density estimation with normalizing flows to learn the mapping between the simulated power spectra and parameters that characterize the physics governing the star formation efficiency and the Σ̇ _★ –Σ _g relation of high-redshift galaxies. Our results show that their partially degenerate effects can be unambiguously constrained when combining [C II ] with [O III ] measurements to be made by new-generation mm/sub-mm LIM experiments.