The molecular gas content of normal galaxies atz > 4 is poorly constrained because the commonly used molecular gas tracers become hard to detect at these high redshifts. We use the [C II] 158μm luminosity, which was recently proposed as a molecular gas tracer, to estimate the molecular gas content in a large sample of main sequence star-forming galaxies atz = 4.4 − 5.9, with a median stellar mass of 10^9.7 M_⊙, drawn from the ALMA Large Program to INvestigate [C II] at Early times survey. The agreement between the molecular gas masses derived from [C II] luminosities, dynamical masses, and rest-frame 850μm luminosities extrapolated from the rest-frame 158μm continuum supports [C II] as a reliable tracer of molecular gas in our sample. We find a continuous decline of the molecular gas depletion timescale fromz = 0 toz = 5.9, which reaches a mean value of (4.6 ± 0.8) × 10⁸yr atz ∼ 5.5, only a factor of between two and three shorter than in present-day galaxies. This suggests a mild enhancement of the star formation efficiency toward high redshifts. Our estimates also show that the previously reported rise in the molecular gas fraction flattens off abovez ∼ 3.7 to achieve a mean value of 63%±3% overz = 4.4 − 5.9. This redshift evolution of the gas fraction is in line with that of the specific star formation rate. We use multi-epoch abundance-matching to follow the gas fraction evolution across cosmic time of progenitors ofz = 0 Milky Way-like galaxies in ∼10¹³ M_⊙halos and of more massivez = 0 galaxies in ∼10¹⁴ M_⊙halos. Interestingly, the former progenitors show a monotonic increase of the gas fraction with redshift, while the latter show a steep rise fromz = 0 toz ∼ 2 followed by a constant gas fraction fromz ∼ 2 toz = 5.9. We discuss three possible effects, namely outflows, a pause in gas supply, and over-efficient star formation, which may jointly contribute to the gas fraction plateau of the latter massive galaxies.

The star formation rate density (SFRD) history of the universe is well constrained up to redshift z ∼ 2. At earlier cosmic epochs, the picture has been largely inferred from UV-selected galaxies (e.g., Lyman-break galaxies; LBGs). However, the inferred star formation rates of LBGs strongly depend on the assumed dust extinction correction, which is not well constrained at high z, while observations in the radio domain are not affected by this issue. In this work we measure the SFRD from a 1.4 GHz selected sample of ∼600 galaxies in the GOODS-N field up to redshift ∼3.5. We take into account the contribution of active galactic nuclei from the infrared-radio correlation. We measure the radio luminosity function, fitted with a modified Schechter function, and derive the SFRD. The cosmic SFRD shows an increase up to z ∼ 2 and then an almost flat plateau up to z ∼ 3.5. Our SFRD is in agreement with those from other far-IR/radio surveys and a factor 2 higher than those from LBG samples. We also estimate that galaxies lacking a counterpart in the HST/WFC3 H-band (H-dark) make up ∼25% of the ϕ-integrated SFRD relative to the full sample at z ∼ 3.2, and up to 58% relative to LBG samples.

Context. Galaxy mergers are thought to be one of the main mechanisms of the mass assembly of galaxies in the Universe, but there is still little direct observational evidence of how frequent they are at z ≳ 4. Recently, many works have suggested a possible increase in the fraction of major mergers in the early Universe, reviving the debate on which processes (e.g., cold accretion, star formation, mergers) most contribute to the mass build-up of galaxies through cosmic time. Aims. To estimate the importance of major mergers in this context, we make use of the new data collected by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) survey, which attempted to observe the [CII] 158 μm emission line from a sample of 75 main-sequence star-forming galaxies at 4.4 < z < 5.9. Methods. We used, for the first time, the morpho-kinematic information provided by the [CII] emission, along with archival multiwavelength data to obtain the fraction of major mergers (f_MM) at z ∼ 5. By combining the results from ALPINE with those at lower redshifts from the literature, we also studied the evolution of the merger fraction through cosmic time. We then used different redshift-evolving merger timescales (T_MM) to convert this fraction into the merger rate per galaxy (R_MM) and in the volume-averaged merger rate (Γ_MM). Results. We find a merger fraction of f_MM ∼ 0.44 (0.34) at z ∼ 4.5 (5.5) from ALPINE. By combining our results with those at lower redshifts, we computed the cosmic evolution of the merger fraction which is described by a rapid increase from the local Universe to higher redshifts, a peak at z ∼ 3, and a slow decrease toward earlier epochs. Depending on the timescale prescription used, this fraction translates into a merger rate ranging between ∼0.1 and ∼4.0 Gyr⁻¹ at z ∼ 5, which in turn corresponds to an average number of major mergers per galaxy between 1 and 8 in ∼12.5 yr (from z = 6 to the local Universe). When convolved with the galaxy number density at different epochs, the merger rate density becomes approximately constant over time at 1 < z < 4, including values from 10⁻⁴ to 10⁻³ Gyr⁻¹ Mpc⁻³, depending on the assumed T_MM. We finally compare the specific star formation and star-formation rate density with the analogous quantities from major mergers, finding a good agreement at z > 4 if we assume a merger timescale that quickly decreases with increasing redshift. Conclusions. Our new constraints on the merger fraction from the ALPINE survey at z ∼ 5 reveal the presence of a significant merging activity in the early Universe. Whether this population of mergers can provide a relevant contribution to the galaxy mass assembly at these redshifts and through the cosmic epochs is strongly dependent on the assumption of the merger timescale. However, our results show that an evolving T_MM ∝ (1 + z)⁻² agrees well with state-of-the-art cosmological simulations, suggesting a considerable role of mergers in the build-up of galaxies at early times.

In this work, we analyse the connection between gas availability and the position of a region with respect to the spatially resolved main-sequence (MS) relation. Following the procedure presented in Enia et al. ( 2020), for a sample of five face-on, grand design spiral galaxies located on the MS we obtain estimates of stellar mass and star formation rate surface densities (Σ_⋆ and Σ_SFR) within cells of 500 pc size. Thanks to H i 21cm and ¹²CO(2–1) maps of comparable resolution, within the same cells we estimate the surface densities of the atomic (Σ_H i) and molecular (⁠|$\Sigma _{\rm {H_2}}$|⁠) gas and explore the correlations among all these quantities. Σ_⋆, Σ_SFR, and |$\Sigma _{\rm {H_2}}$| define a 3D relation whose projections are the spatially resolved MS, the Kennicutt–Schmidt law and the molecular gas MS. We find that |$\Sigma _{\rm {H_2}}$| steadily increases along the MS relation and is almost constant perpendicular to it. Σ_H i is nearly constant along the MS and increases in its upper envelope. As a result, Σ_SFR can be expressed as a function of Σ_⋆ and Σ_H i, following the relation log Σ_SFR = 0.97log Σ_⋆ + 1.99log Σ_H i − 11.11. We show that the total gas fraction significantly increases towards the starburst regions, accompanied by a weak increase in star formation efficiency. Finally, we find that H₂/H i varies strongly with the distance from the MS, dropping dramatically in regions of intense star formation, where the UV radiation from newly formed stars dissociates the H₂ molecule, illustrating the self-regulating nature of the star formation process.

It is widespread opinion that hydrogen reionization is mainly driven by primeval star-forming galaxies, with high-z active galactic nuclei (AGNs) having a minor role. Recent observations, however, challenge this notion, indicating a number of issues related to a galaxy-driven reionization scenario. We provide here an updated assessment of the space density of relatively faint (M ₁₄₅₀ ∼ −22.5) AGNs at z _spec ∼ 5.5 in order to improve the estimate of the photoionization rate contribution from accreting supermassive black holes. Exploiting deep UV rest-frame ground-based spectra collected at the Very Large Telescope on the CANDELS/GOODS-South field and deep Chandra X-ray images in the CANDELS/GOODS-North and EGS areas, we find two relatively bright (M ₁₄₅₀ ∼ −22.5) AGNs at z _spec ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10⁻⁶ cMpc⁻³ at z ∼ 5.5 and M ₁₄₅₀ ∼ −22.5 by simply dividing their observed number by the cosmological volume in the range 5.0 < z < 6.1. Our estimate does not consider corrections for incompleteness; therefore, it represents a lower limit, although uncertainties due to cosmic variance can still be significant. This value supports a high space density of AGNs at z > 5, in contrast with previous claims mostly based on standard color selection, possibly affected by significant incompleteness. Our estimate for the AGN photoionization rate at z ∼ 5.5 is in agreement with the observed values at similar redshifts, which are needed to keep the intergalactic medium highly ionized. Upcoming James Webb Space Telescope and giant ground-based telescopes observations will improve the study of high-z AGNs and their contribution to the reionization of the universe.

Context. There is ample evidence that dust is already present in abundance at high redshift. However, given the faintness of distant galaxies in the optical and the near-infrared, the data are limited and the understanding of how the dust affects the emerging radiation of galaxies at very high redshift is still limited. Aims. Using the ALMA Large Program to INvestigate [C II] at Early times (ALPINE), our objective is to quantify the dust attenuation properties in galaxies at z = 4.4–5.5, particularly with regard to the shape of their attenuation curve. Methods. Using the CIGALE spectral energy distribution (SED) code, we modeled the stellar populations and their interactions with the dust to measure some of the physical properties of the ALPINE sample. We selected a subsample of 23 main-sequence galaxies requiring a detection in at least six bands in the rest-frame UV and optical, as well as in the dust continuum around 158 μm or the [C II] fine-structure line. The inclusion of this line is important for improving the constraints on the physical properties, while achieving a reasonably low uncertainty on the slope of the attenuation curves. Results. We find that the attenuation curves span a broad range of properties – from curves that are much steeper than the SMC extinction curve to those that are shallower than the starburst attenuation curve. The shape of the attenuation curves strongly depends on the V-band attenuation. Galaxies with the lowest attenuation also present the steepest curves. The steepness of such curves probably results from the combination of the intrinsic physical properties of the dust, the relative distribution of stars and dust in the interstellar medium, and the differential reddening, with an important fraction of the dust concentrated in star-forming regions. The broad range of attenuation curves found at z ∼ 5 shows that no single attenuation curve is appropriate for main-sequence galaxies. When SED modeling is not feasible, assuming a fixed curve can lead to large errors, for instance, in the interpretation and application of the IRX-β diagram. Conclusions. Considerable caution should be exercised when correcting high redshift galaxies for the presence of dust using the UV slope β as it can affect the estimation of both star formation rates and stellar mass even at low V-band attenuation due to the steepness of the attenuation curve. However, when SED modeling can be used, the impact of the choice of the attenuation curve on the star formation rate and the stellar mass is limited.