Search

Context. Near- to mid-infrared observations (from Spitzer and JWST) have revealed a hidden population of galaxies at redshift z = 3 − 6 called optically dark objects, which are believed to be massive and dusty star-formers. They contribute substantially to the cosmic star-formation rate (SFR) density at z ∼ 4 − 5 (up to 30 − 40%). Aims. While optically dark sources are widely recognized as a significant component of the stellar mass function, the history of their stellar mass assembly (and the evolution of their interstellar medium) remains unexplored. However, they are thought to be the progenitors of the more massive early-type galaxies found in present-day groups and clusters. It is thus important to examine the possible connection between dark sources and merging events in order to understand the environment in which they live. Methods. Here, we report our search for close companions in a sample of 19 optically-dark objects identified in the SMACS0723 JWST deep field. They were selected in the NIRCam F444W band and undetected below 2 μm. We restricted our analysis to the reddest (i.e., F277W–F444W > 1.3) and brightest (F444W < 26 mag) objects. Results. We identified KLAMA, an optically dark source showing a very close companion (angular distance < 0.5″). The spatially resolved SED fitting procedure indicates that all components lying within 1.5″ of who is it the dark source are indeed at z ∼ 5.7. Tidal features (leading to a whale-shaped morphology) corroborate the hypothesis that KLAMA is the most massive (log(M⋆/M⊙) > 10.3) and dusty (AV ∼ 3 at the core) system of an ongoing merger with a mass ratio of ∼10. Thus, around ten similar merger events would be required to double the stellar mass of KLAMA. Merging systems with properties similar to KLAMA are identified in the SERRA simulations, allowing us to reconstruct their stellar-mass assembly history and predict their molecular gas properties (in particular, the [CII] emission for the simulated system). Conclusions. The discovery of mergers within dark galaxies at the end of the Epoch of Reionization highlights the importance of conducting a statistical search for additional candidates in deep NIRCam fields. Such research will aid in our understanding of the significance of merging processes during the obscured phase of stellar-mass accumulation. [ABSTRACT FROM AUTHOR]

The study of gas-phase metallicity and its spatial distribution at high redshift is crucial to understand the processes that shaped the growth and evolution of galaxies in the early Universe. Here we study the spatially resolved metallicity in three systems at z ∼ 6 − 8, namely A2744-YD4, BDF-3299, and COSMOS24108, with JWST NIRSpec IFU low-resolution (R ∼ 100) spectroscopic observations. These are among the highest-z sources in which metallicity gradients have been probed so far. Each of these systems hosts several spatial components in the process of merging within a few kiloparsecs, identified from the rest-frame UV and optical stellar continuum and ionised gas emission line maps. The sources have heterogeneous properties, with stellar masses log(M*/M⊙) ∼7.6–9.3, star formation rates (SFRs) ∼1–15 M⊙ yr−1, and gas-phase metallicities 12+log(O/H) ∼7.7–8.3, which exhibit a large scatter within each system. Their properties are generally consistent with those of the highest-redshift samples to date (z ∼ 3 − 10), though the sources in A2744-YD4 and COSMOS24108 are at the high end of the mass-metallicity relation (MZR) defined by the z ∼ 3 − 10 sources. Moreover, the targets in this work follow the predicted slope of the MZR at z ∼ 6 − 8 from most cosmological simulations. The gas-phase metallicity gradients are consistent with being flat in the main sources of each system. Flat metallicity gradients are thought to arise from gas mixing processes on galaxy scales, such as mergers or galactic outflows and supernova winds driven by intense stellar feedback, which wash out any gradient formed in the galaxy. The existence of flat gradients at z ∼ 6 − 8 sets also important constraints on future cosmological simulations and chemical evolution models, whose predictions on the cosmic evolution of metallicity gradients often differ significantly, especially at high redshift, but are mostly limited to z ≲ 3 so far. [ABSTRACT FROM AUTHOR]

Context. Theoretical models struggle to reproduce dynamically cold disks with significant rotation-to-dispersion support (Vrot/σ) observed in star-forming galaxies in the early Universe at redshift z > 4. Aims. We aim to explore the possible emergence of dynamically cold disks in cosmological simulations and to understand whether different kinematic tracers can help reconcile the tension between theory and observations. Methods. We used 3218 galaxies from the SERRA suite of zoom-in simulations, with 8 ≤ log(M⋆/M⊙) ≤ 10.3 and star formation rates SFR ≤ 128 M⊙/yr, within the 4 ≤ z ≤ 9 range. We generated hyperspectral data cubes for 2 × 3218 synthetic observations of Hα and [CII]. Results. We find that the choice of kinematic tracer strongly influences gas velocity dispersion (σ) estimates. In Hα ([C II]) synthetic observations, we observe a strong (mild) correlation between σ and M⋆. This difference mostly arises for M⋆ > 109 M⊙ galaxies, for which σHα > 2σ[CII] for a significant fraction of the sample. Regardless of the tracer, our predictions suggest the existence of massive (M⋆ > 1010 M⊙) galaxies with Vrot/σ > 10 at z > 4, maintaining cold disks for > 10 orbital periods (∼ 200 Myr). Furthermore, we find no significant redshift dependence for the Vrot/σ ratio in our sample. Conclusions. Our simulations predict the existence of dynamically cold disks in the early Universe. However, different tracers are sensitive to different kinematic properties. While [C II] effectively traces the thin gaseous disk of galaxies, Hα includes the contribution from ionized gas beyond the disk region, characterized by prevalent vertical or radial motions that may be associated with outflows. We show that Hα halos could be a signature of these galactic outflows. This result emphasizes the importance of combining ALMA and JWST/NIRspec studies of high-z galaxies. [ABSTRACT FROM AUTHOR]

Context. Observations at UV and optical wavelengths have revealed that galaxies at z ∼ 1 − 4 host star-forming regions, dubbed "clumps", which are believed to form due to the fragmentation of gravitationally unstable, gas-rich disks. However, the detection of the parent molecular clouds that give birth to such clumps is still possible only in a minority of galaxies, mostly at z ∼ 1. Aims. We investigated the [C II] and dust morphology of a z ∼ 3.4 lensed galaxy hosting four clumps detected in the UV continuum. We aimed to observe the [C II] emission of individual clumps that, unlike the UV, is not affected by dust extinction, to probe their nature and cold gas content. Methods. We conducted ALMA observations probing scales down to ∼300 pc and detected three [C II] clumps. One (dubbed "NE") coincides with the brightest UV clump, while the other two ("SW" and "C") are not detected in the UV continuum. We do not detect the dust continuum. Results. We converted the [C II] luminosity of individual clumps into molecular gas mass and found Mmol ∼ 108 M⊙. By complementing it with the star formation rate (SFR) estimate from the UV continuum, we estimated the gas depletion time (tdep) of clumps and investigated their location in the Schmidt–Kennicutt plane. While the NE clump has a very short tdep = 0.16 Gyr, which is comparable with high-redshift starbursts, the SW and C clumps instead have longer tdep > 0.65 Gyr and are likely probing the initial phases of star formation. The lack of dust continuum detection is consistent with the blue UV continuum slope estimated for this galaxy (β ∼ −2.5) and it indicates that dust inhomogeneities do not significantly affect the detection of UV clumps in this target. Conclusions. We pushed the observation of the cold gas content of individual clumps up to z ∼ 3.4 and showed that the [C II] line emission is a promising tracer of molecular clouds at high redshift, allowing the detection of clumps with a large range of depletion times. [ABSTRACT FROM AUTHOR]

Recent attempts to detect [O  iii ] 88 |$\, \mu$| m emission from super-early (z > 10) galaxy candidates observed by JWST have been unsuccessful. Non-detections can be either due to wrong photometric redshifts or to the faintness of the line in such early systems. By using zoom-in simulations, we show that if redshifts of these galaxies are confirmed, they are faint and mostly fall below the local metal-poor |$\rm [O\, {\small III}]-SFR$| relation as a result of their low ionization parameter, U ion ≲ 10−3. Such low U ion values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample (⁠|$\rm {log}[L_{\rm {[O\, {\small III}]}}/\mathrm{L}_\odot ] = 8.4$|⁠ , U ion ≈ 0.1) could be detected by ALMA in only 2.8 h. [ABSTRACT FROM AUTHOR]

ALMA observations have revealed that many high-redshift galaxies are surrounded by extended (10–15 kpc) [C  ii] -emitting haloes that are not predicted by even the most advanced zoom-in simulations. Using a semi-analytical model, in a previous work we suggested that such haloes are produced by starburst-driven, catastrophically cooling outflows. Here, we further improve the model and compare its predictions with data from seven star-forming (⁠|$10\lesssim \rm SFR/{\rm M}_{\odot }\, yr^{-1}\lesssim 100$|⁠) galaxies at z  = 4–6, observed in the ALPINE survey. We find that (a) detected [C  ii]  haloes are a natural by-product of starburst-driven outflows; (b) the outflow mass loading factors are in the range 4 ≲ η ≲ 7, with higher η values for lower mass, lower star formation rate systems, and scale with stellar mass as |$\eta \propto M_*^{-0.43}$|⁠ , consistently with the momentum-driven hypothesis. Our model suggests that outflows are widespread phenomena in high- z galaxies. However, in low-mass systems the halo extended [C  ii]  emission is likely too faint to be detected with the current levels of sensitivity. [ABSTRACT FROM AUTHOR]

We introduce serra , a suite of zoom-in high-resolution (⁠|$1.2\times 10^4 \, {\rm M}_{\odot }$|⁠ , |$\simeq 25\, {\rm {pc}}$| at z  = 7.7) cosmological simulations including non-equilibrium chemistry and on-the-fly radiative transfer. The outputs are post-processed to derive galaxy ultraviolet (UV) + far-infrared (FIR) continuum and emission line properties. Results are compared with available multiwavelength data to constrain the physical properties [e.g. star formation rates (SFRs), stellar/gas/dust mass, metallicity] of high-redshift 6 ≲ z ≲ 15 galaxies. This flagship paper focuses on the z  = 7.7 sub-sample, including 202 galaxies with stellar mass |$10^7 \, {\rm M}_{\odot }\lesssim M_\star \lesssim 5\times 10^{10}\, {\rm M}_{\odot }$|⁠ , and specific star formation rate ranging from |${\rm sSFR} \sim 100\, {\rm Gyr}^{-1}$| in young, low-mass galaxies to |$\sim 10\, {\rm Gyr}^{-1}$| for older, massive ones. At this redshift, serra galaxies are typically bursty, i.e. they are located above the Schmidt–Kennicutt relation by a factor |$\kappa _s = 3.03^{+4.9}_{-1.8}$|⁠ , consistent with recent findings for [O  iii ] and [C  ii ] emitters at high z. They also show relatively large InfraRed eXcess (IRX = L FIR/ L UV) values as a result of their compact/clumpy morphology effectively blocking the stellar UV luminosity. Note that this conclusion might be affected by insufficient spatial resolution at the molecular cloud level. We confirm that early galaxies lie on the standard [C  ii ] |$\!-\!\rm SFR$| relation; their observed L [OIII]/ L [CII] ≃ 1–10 ratios can be reproduced by a part of the serra galaxies without the need of a top-heavy initial mass function and/or anomalous C/O abundances. [O  i ] line intensities are similar to local ones, making ALMA high- z detections challenging but feasible (⁠|$\sim 6\, \rm h$| for an SFR of |$50\, \, {\rm M}_{\odot }\, {\rm yr}^{-1}$|⁠). [ABSTRACT FROM AUTHOR]

We investigate the spatially resolved morphology of galaxies in the early Universe. We consider a typical redshift z  = 6 Lyman break galaxy, 'Althæa', from the SERRA hydrodynamical simulations. We create mock rest-frame ultraviolet (UV), optical, and far-infrared observations, and perform a two-dimensional morphological analysis to deblend the galaxy disc from substructures (merging satellites or star-forming regions). We find that the [C  ii ]158 μm emitting region has an effective radius 1.5–2.5 times larger than the optical one, consistent with recent observations. This [C  ii ] halo in our simulated galaxy arises as the joint effect of stellar outflows and carbon photoionization by the galaxy UV field, rather than from the emission of unresolved nearby satellites. At the typical angular resolution of current observations (≳ 0.15 arcsec) only merging satellites can be detected; detection of star-forming regions requires resolutions of ≲ 0.05 arcsec. The [C  ii ]-detected satellite has a 2.5-kpc projected distance from the galaxy disc, whereas the star-forming regions are embedded in the disc itself (distance ≲ 1 kpc). This suggests that multicomponent systems reported in the literature, which have separations ≳ 2 kpc, are merging satellites, rather than galactic substructures. Finally, the star-forming regions found in our mock maps follow the local L [C  ii ]–SFRUV relation of galaxy discs, although sampling the low-luminosity, low-SFR tail of the distribution. We show that future James Webb Space Telescope observations, bridging UV and [C  ii ] data sets, will be exceptionally suited to characterize galaxy substructures, thanks to their exquisite spatial resolution and sensitivity to both low-metallicity and dust-obscured regions that are bright at infrared wavelengths. [ABSTRACT FROM AUTHOR]

ALMA observations have revealed that [C  ii ] 158  μ m line emission in high- z galaxies is ≈2–3 × more extended than the UV continuum emission. Here we explore whether surface brightness dimming (SBD) of the [C  ii ] line is responsible for the reported [C  ii ] deficit, and the large |$L_{\rm [O\, \small {III}]}/L_{\rm [C\, \small {II}]}$| luminosity ratio measured in early galaxies. We first analyse archival ALMA images of nine z > 6 galaxies observed in both [C  ii ] and [O  iii ]. After performing several uv -tapering experiments to optimize the identification of extended line emission, we detect [C  ii ] emission in the whole sample, with an extent systematically larger than the [O  iii ] emission. Next, we use interferometric simulations to study the effect of SBD on the line luminosity estimate. About 40 per cent of the extended [C  ii ] component might be missed at an angular resolution of 0.8 arcsec, implying that |$L_{\rm [C\, \small {II}]}$| is underestimated by a factor ≈2 in data at low (<7) signal-to-noise ratio. By combining these results, we conclude that |$L_{\rm [C\, \small {II}]}$| of z > 6 galaxies lies, on average, slightly below the local |$L_{\rm [C\, \small {II}]}-\mathrm{ SFR}$| relation (Δ z  =  6–9 = −0.07 ± 0.3), but within the intrinsic dispersion of the relation. SBD correction also yields |$L_{\rm [O\, \small {III}]}/L_{\rm [C\, \small {II}]}\lt 10$|⁠ , i.e. more in line with current hydrodynamical simulations. [ABSTRACT FROM AUTHOR]

We study the structure of spatially resolved, line-of-sight velocity dispersion for galaxies in the Epoch of Reionization (EoR) traced by [C  |$\scriptstyle \rm II$| ]  |$158\, \mu \rm {m}$|  line emission. Our laboratory is a simulated prototypical Lyman-break galaxy, 'Freesia, part of the serra suite'. The analysis encompasses the redshift range 6 < z < 8, when Freesia is in a very active assembling phase. We build velocity dispersion maps for three dynamically distinct evolutionary stages (Spiral Disc at z  = 7.4, Merger at z  = 8.0, and Disturbed Disc at z  = 6.5) using [C  |$\scriptstyle \rm II$| ] hyperspectral data cubes. We find that, at a high spatial resolution of 0.005 arcsec (≃30 pc), the luminosity-weighted average velocity dispersion is |$\sigma _{\rm {CII}}\simeq 23{\text {--}}38\, {\rm km\, s^{-1}}$| with the highest value belonging to the highly structured Disturbed Disc stage. Low-resolution observations tend to overestimate σC  ii values due to beam smearing effects that depend on the specific galaxy structure. For an angular resolution of 0.02 arcsec (0.1 arcsec), the average velocity dispersion is |$16{\!-\!}34{{\ \rm per\ cent}}$| (⁠|$52{\!-\!}115{{\ \rm per\ cent}}$|⁠) larger than the actual one. The [C  |$\scriptstyle \rm II$| ] emitting gas in Freesia has a Toomre parameter |$\mathcal {Q}\simeq 0.2$| and rotational-to-dispersion ratio of v c/σ ≃ 7 similar to that observed in z  = 2−3 galaxies. The primary energy source for the velocity dispersion is due to gravitational processes, such as merging/accretion events; energy input from stellar feedback is generally sub-dominant (⁠|$\lt 10{{\ \rm per\ cent}}$|⁠). Finally, we find that the resolved σC  ii −ΣSFR relation is relatively flat for |$0.02\lt {\Sigma }_{\rm SFR}/{\rm M}_{\odot }\rm {yr}^{-1} {\rm kpc}^{-2} \lt 30$|⁠ , with the majority of data lying on the derived analytical relation |$\sigma \propto \Sigma _{\rm SFR}^{5/7}$|⁠. At high SFR, the increased contribution from stellar feedback steepens the relation, and σC  ii rises slightly. [ABSTRACT FROM AUTHOR]

A tight relation between the [C  ii ] 158  |$\mu$| m line luminosity and star formation rate is measured in local galaxies. At high redshift (z > 5), though, a much larger scatter is observed, with a considerable (15–20 per cent) fraction of the outliers being [C  ii ]-deficient. Moreover, the [C  ii ] surface brightness (⁠|$\Sigma_{\rm [C\, \small {II}]}$|⁠) of these sources is systematically lower than expected from the local relation. To clarify the origin of such [C  ii ]-deficiency, we have developed an analytical model that fits local [C  ii ] data and has been validated against radiative transfer simulations performed with cloudy. The model predicts an overall increase of |$\Sigma_{\rm [C\, \small {II}]}$|  with ΣSFR. However, for ΣSFR  |${\gtrsim} 1 \, \mathrm{M}_\odot \,{\rm yr}^{-1}\,{\rm kpc}^{-2}$|⁠ , |$\Sigma_{\rm [C\, \small {II}]}$|  saturates. We conclude that underluminous [C  ii ] systems can result from a combination of three factors: (a) large upward deviations from the Kennicutt–Schmidt relation (κs ≫ 1), parametrized by the 'burstiness' parameter κs; (b) low metallicity; (c) low gas density, at least for the most extreme sources (e.g. CR7). Observations of [C  ii ] emission alone cannot break the degeneracy among the above three parameters; this requires additional information coming from other emission lines (e.g. [O  iii ]88  |$\mu$| m, C  iii ]1909 Å, CO lines). Simple formulae are given to interpret available data for low- and high- z galaxies. [ABSTRACT FROM AUTHOR]

We study the kinematical properties of galaxies in the Epoch of Reionization via the [C  ii ]158 μm line emission. The line profile provides information on the kinematics as well as structural properties such as the presence of a disc and satellites. To understand how these properties are encoded in the line profile, first we develop analytical models from which we identify disc inclination and gas turbulent motions as the key parameters affecting the line profile. To gain further insights, we use 'Althæa', a highly resolved (⁠|$30\, \rm pc$|⁠) simulated prototypical Lyman-break galaxy, in the redshift range z  = 6–7, when the galaxy is in a very active assembling phase. Based on morphology, we select three main dynamical stages: (I) merger, (II) spiral disc, and (III) disturbed disc. We identify spectral signatures of merger events, spiral arms, and extra-planar flows in (I), (II), and (III), respectively. We derive a generalized dynamical mass versus [C  ii ]-line FWHM relation. If precise information on the galaxy inclination is (not) available, the returned mass estimate is accurate within a factor 2 (4). A Tully–Fisher relation is found for the observed high- z galaxies, i.e. L [C  ii ] ∝ (FWHM)1.80 ± 0.35 for which we provide a simple, physically based interpretation. Finally, we perform mock ALMA simulations to check the detectability of [C  ii ]. When seen face-on, Althæa is always detected at >5σ; in the edge-on case it remains undetected because the larger intrinsic FWHM pushes the line peak flux below detection limit. This suggests that some of the reported non-detections might be due to inclination effects. [ABSTRACT FROM AUTHOR]

We study the formation and evolution of a sample of Lyman break galaxies in the epoch of reionization by using high-resolution (∼10 pc), cosmological zoom-in simulations part of the serra suite. In serra , we follow the interstellar medium thermochemical non-equilibrium evolution and perform on-the-fly radiative transfer of the interstellar radiation field (ISRF). The simulation outputs are post-processed to compute the emission of far infrared lines ([C  ii ], [N  ii ], and [O  iii ]). At z  = 8, the most massive galaxy, 'Freesia', has an age |$t_\star \simeq 409\, \rm Myr$|⁠ , stellar mass M ⋆ ≃ 4.2 × 109M⊙, and a star formation rate (SFR), |${\rm SFR}\simeq 11.5\, {\rm M}_{\odot }\, {\rm yr}^{-1}$|⁠ , due to a recent burst. Freesia has two stellar components (A and B) separated by |${\simeq } 2.5\, {\rm kpc}$|⁠ ; other 11 galaxies are found within 56.9 ± 21.6 kpc. The mean ISRF in the Habing band is |$G = 7.9\, G_0$| and is spatially uniform; in contrast, the ionization parameter is |$U = 2^{+20}_{-2} \times 10^{-3}$|⁠ , and has a patchy distribution peaked at the location of star-forming sites. The resulting ionizing escape fraction from Freesia is |$f_{\rm esc}\simeq 2{{\ \rm per\ cent}}$|⁠. While [C  ii ] emission is extended (radius 1.54 kpc), [O  iii ] is concentrated in Freesia-A (0.85 kpc), where the ratio |$\Sigma _{\rm [O\,{\small III}]}/\Sigma _{\rm [C\,{\small II}]}\simeq 10$|⁠. As many high- z galaxies, Freesia lies below the local [C  ii ]–SFR relation. We show that this is the general consequence of a starburst phase (pushing the galaxy above the Kennicutt–Schmidt relation) that disrupts/photodissociates the emitting molecular clouds around star-forming sites. Metallicity has a sub-dominant impact on the amplitude of [C  ii ]–SFR deviations. [ABSTRACT FROM AUTHOR]