We build a new model for the global 21-cm signal that is calibrated to measurements of the high-z galaxy luminosity function (LF) and further tuned to match the Thomson scattering optical depth of the cosmic microwave background, τ e . Assuming that the z 8 galaxy population can be smoothly extrapolated to higher redshifts, the recent decline in best-fit values of τ e and the inefficient heating induced by X-ray binaries (the presumptive sources of the high-z X-ray background) imply that the entirety of cosmic reionization and reheating occurs at z 12. In contrast to past global 21-cm models, whose z ∼ 20 (ν ∼ 70 MHz) absorption features and strong ∼ 25 mK emission features were driven largely by the assumption of efficient early star-formation and Xray heating, our new models peak in absorption at ν ∼ 110 MHz at depths ∼ −160 mK and have negligible emission components. Current uncertainties in the faint-end of the LF, binary populations in star-forming galaxies, and UV and X-ray escape fractions introduce ∼ 20 MHz (∼ 50 mK) deviations in the trough's frequency (amplitude), while emission signals remain weak ( 10 mK) and are confined to ν 140 MHz. These predictions, which are intentionally conservative, suggest that the detection of a 21-cm absorption minimum at frequencies below ∼ 90 MHz and/or emission signals stronger than ∼ 10 mK at ν 140 MHz would provide strong evidence for "new" sources at high redshifts, such as Population III stars and their remnants.

Reionization is thought to have occurred in the redshift range of 6 < z < 9, which is now being probed by both deep galaxy surveys and CMB observations. Using halo abundance matching over the redshift range 5 < z < 8 and assuming smooth, continuous gas accretion, we develop a model for the star formation efficiency f of dark matter halos at z > 6 that matches the measured galaxy luminosity functions at these redshifts. We find that f peaks at ∼ 30% at halo masses M ∼ 10 11 -10 12 M , in qualitative agreement with its behavior at lower redshifts. We then investigate the cosmic star formation histories and the corresponding models of reionization for a range of extrapolations to small halo masses. We use a variety of observations to further constrain the characteristics of the galaxy populations, including the escape fraction of UV photons. Our approach provides an empirically-calibrated, physically-motivated model for the properties of star-forming galaxies sourcing the epoch of reionization. In the case where star formation in low-mass halos is maximally efficient, an average escape fraction ∼ 0.1 can reproduce the optical depth reported by Planck, whereas inefficient star formation in these halos requires either about twice as many UV photons to escape, or an escape fraction that increases towards higher redshifts. Our models also predict how future observations with JWST can improve our understanding of these galaxy populations.

Near-infrared surveys have now determined the luminosity functions of galaxies at 6 < ∼ z < ∼ 9 to impressive precision and identified a number of candidates at even earlier times. Here we develop a simple analytic model to describe these populations that allows physically-motivated extrapolation to earlier times and fainter luminosities. We assume that galaxies grow through accretion onto dark matter halos, which we model by matching halos at fixed number density across redshift, and that stellar feedback limits the star formation rate. We allow for a variety of feedback mechanisms, including regulation through supernova energy and momentum from radiation pressure. We show that reasonable choices for the feedback parameters can fit the available galaxy data, which in turn substantially limits the range of plausible extrapolations of the luminosity function to earlier times and fainter luminosities: for example, the global star formation rate declines rapidly at z > ∼ 10, but the bright galaxies accessible to observations decline much faster than the total. Deviations from our predictions would provide evidence for new astrophysics within the first generations of galaxies. We also provide predictions for galaxy measurements by future facilities, including JWST and WFIRST.

Intensity mapping provides a unique means to probe the epoch of reionization (EoR), when the neutral intergalactic medium was ionized by the energetic photons emitted from the first galaxies. The [C II] 158µm fine-structure line is typically one of the brightest emission lines of star-forming galaxies and thus a promising tracer of the global EoR star-formation activity. However, [C II] intensity maps at 6 z 8 are contaminated by interloping CO rotational line emission (3 ≤ J upp ≤ 6) from lower-redshift galaxies. Here we present a strategy to remove the foreground contamination in upcoming [C II] intensity mapping experiments, guided by a model of CO emission from foreground galaxies. The model is based on empirical measurements of the mean and scatter of the total infrared luminosities of galaxies at z < 3 and with stellar masses M * > 10 8 M selected in K-band from the COSMOS/UltraVISTA survey, which can be converted to CO line strengths. For a mock field of the Tomographic Ionized-carbon Mapping Experiment (TIME), we find that masking out the "voxels" (spectral-spatial elements) containing foreground galaxies identified using an optimized CO flux threshold results in a z-dependent criterion m AB K 22 (or M * 10 9 M ) at z < 1 and makes a [C II]/CO tot power ratio of 10 at k = 0.1 h/Mpc achievable, at the cost of a moderate 8% loss of total survey volume.

Line intensity mapping (LIM) is a promising approach to study star formation and the interstellar medium (ISM) in galaxies by measuring the aggregate line emission from the entire galaxy population. In this work, we develop a simple yet physically-motivated framework for modeling the line emission as would be observed in LIM experiments. It is done by building on analytic models of the cosmic infrared background that connect total infrared luminosity of galaxies to their host dark matter halos. We present models of the H I 21 cm, CO(1-0), [C II] 158 µm, and [N II] 122 and 205 µm lines consistent with current observational constraints. With four case studies of various combinations of these lines that probe different ISM phases, we demonstrate the potential for reliably extracting physical properties of the ISM, and the evolution of these properties with cosmic time, from auto-and cross-correlation analysis of these lines as measured by future LIM experiments.

Line intensity mapping (LIM) provides a unique and powerful means to probe cosmic structures by measuring the aggregate line emission from all galaxies across redshift. The method is complementary to conventional galaxy redshift surveys that are object-based and demand exquisite point-source sensitivity. The Tomographic Ionized-carbon Mapping Experiment (TIME) will measure the star formation rate (SFR) during cosmic reionization by observing the redshifted [C II] 158 µm line (6 z 9) in the LIM regime. TIME will simultaneously study the abundance of molecular gas during the era of peak star formation by observing the rotational CO lines emitted by galaxies at 0.5 z 2. We present the modeling framework that predicts the constraining power of TIME on a number of observables, including the line luminosity function, and the auto-and cross-correlation power spectra, including synergies with external galaxy tracers. Based on an optimized survey strategy and fiducial model parameters informed by existing observations, we forecast constraints on physical quantities relevant to reionization and galaxy evolution, such as the escape fraction of ionizing photons during reionization, the faint-end slope of the galaxy luminosity function at high redshift, and the cosmic molecular gas density at cosmic noon. We discuss how these constraints can advance our understanding of cosmological galaxy evolution at the two distinct cosmic epochs for TIME, starting in 2021, and how they could be improved in future phases of the experiment.

The cosmic near-infrared background (NIRB) offers a powerful integral probe of radiative processes at different cosmic epochs, including the pre-reionization era when metal-free, Population III (Pop III) stars first formed. While the radiation from metal-enriched, Population II (Pop II) stars likely dominates the contribution to the observed NIRB from the reionization era, Pop III stars — if formed efficiently — might leave characteristic imprints on the NIRB thanks to their strong Lyα emission. Using a physically-motivated model of first star formation, we provide an analysis of the NIRB mean spectrum and anisotropy contributed by stellar populations at z &gt; 5. We find that in circumstances where massive Pop III stars persistently form in molecular cooling haloes at a rate of a few times 10−3 M⊙ yr−1, before being suppressed towards the epoch of reionization (EoR) by the accumulated Lyman-Werner background, a unique spectral signature shows up redward of 1 μm in the observed NIRB spectrum sourced by galaxies at z &gt; 5. While the detailed shape and amplitude of the spectral signature depend on various factors including the star formation histories, IMF, LyC escape fraction and so forth, the most interesting scenarios with efficient Pop III star formation are within the reach of forthcoming facilities such as the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx). As a result, new constraints on the abundance and formation history of Pop III stars at high redshifts will be available through precise measurements of the NIRB in the next few years.

We investigate the dust properties and star-formation signature of galaxies in the early universe by stacking 111,227 objects in the recently released COSMOS catalogue on maps at wavelengths bracketing the peak of warmed dust emission. We find an elevated far-infrared luminosity density to redshift 8, indicating abundant dust in the early universe. We further find an increase of dust temperature with redshift, reaching $\sim 100\pm 12\ \rm K$ at z ∼ 7, suggesting either the presence of silicate rich dust originating from Population II stars, or sources of heating beyond simply young hot stars. Lastly, we try to understand how these objects have been missed in previous surveys, and how to design observations to target them. All code, links to the data, and instructions to reproduce this research in full are located at https://github.com/marcoviero/simstack3/.