Primordial Galaxies Research Articles

COLDSIM predictions of [C II] emission in primordial galaxies

COLDSIM predictions of [C II] emission in primordial galaxies

Diverse Oxygen Abundance in Early Galaxies Unveiled by Auroral Line Analysis with JWST

We present deep JWST NIRSpec observations in the sight line of MACS J1149.5+2223, a massive cluster of galaxies at z = 0.54. We report the spectroscopic redshift of 28 sources at 3 < z < 9.1, including nine sources with the detection of the OIIIλ4363 auroral line. Combining these with 16 OIIIλ4363 -detected sources from publicly available JWST data, our sample consists of 25 galaxies with robust gas-phase metallicity measurements via the direct method. We observe a positive correlation between stellar mass and metallicity, with an ∼0.5 dex offset down below the local relation. Interestingly, we find a larger-than-expected scatter of ∼0.3 dex around the relation, which cannot be explained by redshift evolution among our sample or other third parameters. The scatter increases at higher redshift, and we tentatively attribute this to the enrichment process having higher stochasticity, due to shallower potential wells, more intense feedback processes, and a higher galaxy merger rate. Despite reaching a considerably low-mass regime ( logM*/M⊙∼7.3 ), our samples have metallicity of log(O/H) +12 ≳ 7, i.e., comparable to the most metal-poor galaxies in the local Universe. The search for primordial galaxies may be accomplished by extending toward a lower mass and/or by investigating inhomogeneities at smaller spatial scales. Lastly, we investigate potential systematics caused by the limitation of JWST’s Micro-Shutter Assembly observations. Caution is warranted when the target exceeds the slit size, as this situation could allow an overestimation of global metallicity, especially under the presence of a strong negative metallicity gradient.

Evidence of Pop III stars’ chemical signature in neutral gas at z ∼ 6

Aims. This study explores the metal enrichment signatures attributed to the first generation of stars (Pop III) in the Universe, focusing on the E-XQR-30 sample – a collection of 42 high signal-to-noise ratio spectra of quasi-stellar objects (QSOs) with emission redshifts ranging from 5.8 to 6.6. We aim to identify traces of Pop III metal enrichment by analyzing neutral gas in the interstellar medium of primordial galaxies and their satellite clumps, detected in absorption. Methods. To chase the chemical signature of Pop III stars, we studied metal absorption systems in the E-XQR-30 sample, selected through the detection of the neutral oxygen absorption line at 1302 Å. The O I line is a reliable tracer of neutral hydrogen and allowed us to overcome the challenges posed by the Lyman-α forest’s increasing saturation at redshifts above ∼5 to identify damped Lyman-α systems (DLAs). We detected and analyzed 29 O I systems at z ≥ 5.4, differentiating between proximate DLAs (PDLAs) and intervening DLAs. Voigt function fits were applied to obtain ionic column densities, and relative chemical abundances were determined for 28 systems. These were then compared with the predictions of theoretical models. Results. Our findings expand the study of O I systems at z ≥ 5.4 fourfold. No systematic differences were observed in the average chemical abundances between PDLAs and intervening DLAs. The chemical abundances in our sample align with literature systems at z &gt; 4.5, suggesting a similar enrichment pattern for this class of absorption systems. A comparison between these DLA-analogs at 4.5 &lt; z &lt; 6.5 with a sample of very metal-poor DLAs at 2 &lt; z &lt; 4.5 shows in general similar average values for the relative abundances, with the exception of [C/O], [Si/Fe] and [Si/O] which are significantly larger for the high-z sample. Furthermore, the dispersion of the measurements significantly increases in the high-redshift bin. This increase is predicted by the theoretical models and indicates a potential retention of Pop III signatures in the probed gas. Conclusions. This work represents a significant advancement in the study of the chemical properties of highly neutral gas at z ≥ 5.4, shedding light on its potential association with the metal enrichment from Pop III stars. Future advancements in observational capabilities, specifically high-resolution spectrographs, are crucial for refining measurements and addressing current limitations in the study of these distant absorption systems.

Illuminating the Dark Side of Cosmic Star Formation. II. A Second Date with RS-NIRdark Galaxies in COSMOS

About 12 billion years ago, the Universe was first experiencing light again after the dark ages, and galaxies filled the environment with stars, metals, and dust. How efficient was this process? How fast did these primordial galaxies form stars and dust? We can answer these questions by tracing the star formation rate density (SFRD) back to its widely unknown high-redshift tail, traditionally observed in the near-infrared (NIR), optical, and UV bands. Thus, objects with a large amount of dust were missing. We aim to fill this knowledge gap by studying radio-selected NIR-dark (RS-NIRdark) sources, i.e., sources not having a counterpart at UV-to-NIR wavelengths. We widen the sample of Talia et al. from 197 to 272 objects in the Cosmic Evolution Survey (COSMOS) field, including also photometrically contaminated sources, which were previously excluded. Another important step forward consists in the visual inspection of each source in the bands from u* to MIPS 24 μm. According to their “environment” in the different bands, we are able to highlight different cases of study and calibrate an appropriate photometric procedure for the objects affected by confusion issues. We estimate that the contribution of RS-NIRdark sources to the cosmic SFRD at 3 < z < 5 is ∼10%–25% of that based on UV-selected galaxies.

Nuclear Activity in the Low-metallicity Dwarf Galaxy SDSS J0944-0038 : A Glimpse into the Primordial Universe

Local low-metallicity dwarf galaxies are relics of the early universe and are thought to hold clues into the origins of supermassive black holes. While recent studies are uncovering a growing population of active galactic nuclei (AGNs) in dwarf galaxies, the vast majority reside in galaxies with solar or supersolar metallicities and stellar masses comparable to that of the LMC. Using Multi-Unit Spectroscopic Explorer (MUSE) and Very Large Telescope observations, we report the detection of [Fe x] λ6374 coronal line emission and a broad Hα line in the nucleus of SDSS J094401.87−003832.1, a nearby (z = 0.0049) metal-poor dwarf galaxy almost 500 times less massive than the LMC. Unlike the emission from the lower-ionization nebular lines, the [Fe x] λ6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of ≈100 pc, similar to that seen in well-known AGNs. The [Fe x] luminosity is ≈1037 erg s−1, within the range seen in previously identified AGNs in the dwarf-galaxy population. The [Fe x] emission has persisted over the roughly 19 yr time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The FWHM of the broad component of the Hα line is 446 ± 17 km s−1 and its luminosity is ≈1.5 × 1038 erg s−1, corresponding to a black hole mass of ≈ 3150 M ⊙, in line with its stellar mass if virial mass relations and black hole–galaxy scaling relations apply in this mass regime. These observations, together with previously reported multiwavelength observations, can most plausibly be explained by the presence of an accreting intermediate-mass black hole in a primordial galaxy analog.

A millimetre-wave superconducting hyper-spectral device

Abstract Millimetre-wave observations represent an important tool for cosmology studies. The line intensity mapping technique has been proposed to map in three dimensions the specific intensity due to line (e.g. [C ii] and CO) emission, e.g. from the primordial galaxies, as a function of redshift. Hyper-spectral integrated devices have the potential to replace the current Fourier transform, or the planned Fabry–Perot-based instruments operating at millimetre and sub-millimetre wavelengths. The aim is to perform hyper-spectral mapping, with a spectral resolution R = λ/Δλ = 100–1000, over large, i.e. thousands of beams, instantaneous patches of the sky. The innovative integrated device that we have developed allows avoiding moving parts, complicated and/or dispersive optics, or tunable filters to be operated at cryogenic temperatures. The prototype hyper-spectral focal plane is sensitive in the 75- to 90-GHz range and contains 19 horns for 16 spectral-imaging channels, each selecting a frequency band of about 0.1 GHz. For each channel a conical horn antenna, coupled to a planar superconducting resonant absorber made of thin aluminium, collects the radiation. A capacitively coupled titanium–aluminium bi-layer Lumped Element Kinetic Inductance Detector is then in charge of dissipating and sensing the supercurrent established in the resonant absorber. The prototype is fabricated with only two photolithography steps over a commercial mono-crystalline sapphire substrate. It exhibits a spectral resolution R = λ/Δλ ≈ 800. The optical noise equivalent power of the best channels is in the observational relevant $4\cdot 10^{-17} W/\sqrt{Hz}$ range. The average sensitivity of all the channels is around $1\cdot 10^{-16} W/\sqrt{Hz}$. The device, as expected from three-dimensional simulations, is polarization-sensitive, paving the way to spectro-polarimetry measurements over very large instantaneous field of views.

Inferring the HII region escape fraction of ionizing photons from infrared emission lines in metal-poor star-forming dwarf galaxies

Local metal-poor galaxies stand as ideal laboratories for probing the properties of the interstellar medium (ISM) in chemically unevolved conditions. Detailed studies of this primitive ISM can help gain insights into the physics of the first primordial galaxies that may be responsible for the reionization. Quantifying the ISM porosity to ionizing photons in nearby galaxies may improve our understanding of the mechanisms leading to Lyman continuum photon leakage from galaxies. The wealth of infrared (IR) tracers available in local galaxies and arising from different ISM phases allows us to constrain complex models in order to estimate physical quantities.

Astroparticle Constraints from Cosmic Reionization and Primordial Galaxy Formation

We derived astroparticle constraints in different dark matter scenarios that are alternatives to cold dark matter (CDM): thermal relic warm dark matter, WDM; fuzzy dark matter, ψDM; self-interacting dark matter, SIDM; sterile neutrino dark matter, νDM. Our framework is based on updated determinations of the high-redshift UV luminosity functions for primordial galaxies to redshift z∼10, on redshift-dependent halo mass functions in the above DM scenarios from numerical simulations, and on robust constraints on the reionization history of the Universe from recent astrophysical and cosmological datasets. First, we built an empirical model of cosmic reionization characterized by two parameters, namely the escape fraction fesc of ionizing photons from primordial galaxies, and the limiting UV magnitude MUVlim down to which the extrapolated UV luminosity functions steeply increased. Second, we performed standard abundance matching of the UV luminosity function and the halo mass function, obtaining a relationship between UV luminosity and the halo mass, whose shape depends on an astroparticle quantity X specific to each DM scenario (e.g., WDM particle mass); we exploited such a relationship to introduce (in the analysis) a constraint from primordial galaxy formation, in terms of the threshold halo mass above which primordial galaxies can efficiently form stars. Third, we performed Bayesian inference on the three parameters fesc, MUVlim, and X via a standard MCMC technique, and compared the outcomes of different DM scenarios on the reionization history. We also investigated the robustness of our findings against educated variations of still uncertain astrophysical quantities. Finally, we highlight the relevance of our astroparticle estimates in predicting the behavior of the high-redshift UV luminosity function at faint, yet unexplored magnitudes, which may be tested with the advent of the James Webb Space Telescope.

The Effects of Radiative Feedback and Supernova-induced Turbulence on Early Galaxies

The recently launched James Webb Space Telescope promises unparalleled advances in our understanding of the first stars and galaxies, but realizing this potential requires cosmological simulations that capture the key physical processes that affected these objects. Here, we show that radiative transfer and subgrid turbulent mixing are two such processes. By comparing simulations with and without radiative transfer but with exactly the same physical parameters and subgrid turbulent mixing model, we show that tracking radiative transfer suppresses the Population III star formation density by a factor ≈4. In both simulations, ≳90% of Population III stars are found in the unresolved pristine regions tracked by our subgrid model, which does a better job at modeling the regions surrounding proto-galaxy cores where metals from supernovae take tens of megayears to mix thoroughly. At the same time, radiative transfer suppresses Population III star formation, via the development of ionized bubbles that slow gas accretion in these regions, and it results in compact high-redshift galaxies that are surrounded by isolated low-mass satellites. Thus, turbulent mixing and radiative transfer are both essential processes that must be included to accurately model the morphology, composition, and growth of primordial galaxies.

EMPRESS. IV. Extremely Metal-poor Galaxies Including Very Low-mass Primordial Systems with M * = 104–105 M ⊙ and 2%–3% (O/H): High (Fe/O) Suggestive of Metal Enrichment by Hypernovae/Pair-instability Supernovae

Abstract We present Keck/LRIS follow-up spectroscopy for 13 photometric candidates of extremely metal-poor galaxies (EMPGs) selected by a machine-learning technique applied to the deep (∼26 AB mag) optical and wide-area (∼500 deg2) Subaru imaging data in the EMPRESS survey. Nine out of the 13 candidates are EMPGs with an oxygen abundance (O/H) less than ∼10% solar value (O/H)⊙, and four sources are contaminants of moderately metal-rich galaxies or no emission-line objects. Notably, two out of the nine EMPGs have extremely low stellar masses and oxygen abundances of 5 × 104–7 × 105 M ⊙ and 2%–3% (O/H)⊙, respectively. With a sample of five EMPGs with (Fe/O) measurements, two (three) of which are taken from this study (the literature), we confirm that two EMPGs with the lowest (O/H) ratios of ∼2% (O/H)⊙ show high (Fe/O) ratios of ∼0.1, close to the solar abundance ratio. Comparing galaxy chemical enrichment models, we find that the two EMPGs cannot be explained by a scenario of metal-poor gas accretion/episodic star formation history due to their low (N/O) ratios. We conclude that the two EMPGs can be reproduced by the inclusion of bright hypernovae and/or hypothetical pair-instability supernovae (SNe) preferentially produced in a metal-poor environment. This conclusion implies that primordial galaxies at z ∼ 10 could have a high abundance of Fe that did not originate from Type Ia SNe with delays and that Fe may not serve as a cosmic clock for primordial galaxies.

Extremely Metal-Poor Asymptotic Giant Branch Stars

Little is known about the first stars, but hints on this stellar population can be derived from the peculiar chemical composition of the most metal-poor objects in the Milky Way and in resolved stellar populations of nearby galaxies. In this paper, we review the evolution and nucleosynthesis of metal-poor and extremely metal-poor (EMP) stars with low and intermediate masses. In particular, new models of 6 M⊙ with three different levels of metallicity, namely Z=10−4, 10−6 and 10−10, are presented. In addition, we illustrate the results obtained for a 2 M⊙, Z=10−5 model. All these models have been computed by means of the latest version of the FuNS code. We adopted a fully coupled scheme of solutions for the complete set of differential equations describing the evolution of the physical structure and the chemical abundances, as modified by nuclear processes and convective mixing. The scarcity of CNO in the material from which these stars formed significantly affects their evolution, their final fate and their contribution to the chemical pollution of the ISM in primordial galaxies. We show the potential of these models for the interpretation of the composition of EMP stars, with particular emphasis on CEMP stars.

Subaru/FOCAS IFU revealed the metallicity gradient of a local extremely metal-poor galaxy

Abstract We present the first measurement of the metallicity gradient in extremely metal-poor galaxies (EMPGs). With Subaru/Faint Object Camera And Spectrograph (FOCAS) Integral Field Unit (IFU), we have observed a nearby, low-mass EMPG, HSC J1631+4426, whose oxygen abundance and stellar mass are known to be 12 + log (O/H) = 6.9 and log10(M*/M⊙) = 5.8, respectively. The measured metallicity gradient is −0.36 ± 0.04 dex kpc−1 corresponding to −0.049 ± 0.006 dex$\:R_\mathrm{e}^{-1}$ for the continuum effective radius of Re = 0.14 kpc. Our observation has successfully demonstrated that three-dimensional spectroscopy with 8 m-class telescopes is powerful enough to reveal the metallicity distribution in local EMPGs, providing precious information on the baryon cycle in local analogs of primordial galaxies in the early Universe.

Extremely weak CO emission in IZw 18

Local metal-poor galaxies are ideal analogues of primordial galaxies with the interstellar medium (ISM) barely being enriched with metals. However, it is unclear whether carbon monoxide remains a good tracer and coolant of molecular gas at low metallicity. Based on the observation with the upgraded Northern Extended Millimeter Array, we report a marginal detection of CO J = 2–1 emission in IZw18, pushing the detection limit down to L′CO(2-1) = 3.99 × 103 K km s−1 pc−2, which is at least 40 times lower than previous studies. As one of the most metal-poor galaxies, IZw18 shows extremely low CO content despite its vigorous star formation activity. Such low CO content relative to its infrared luminosity, star formation rate, and [C II] luminosity, compared with other galaxies, indicates a significant change in the ISM properties at a few percent of the Solar metallicity. In particular, the high [C II] luminosity relative to CO implies a larger molecular reservoir than the CO emitter in IZw18. We also obtain an upper limit of the 1.3 mm continuum, which excludes a sub-millimetre excess in IZw18.

Heavy-elements heritage of the falling sky

Context. A fundamental element of galaxy formation is the accretion of mass through mergers of satellites or gas. Recent dynamical analyses based on Gaia data have revealed major accretion events in the history of the Milky Way. Nevertheless, our understanding of the primordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondingly oldest accreted stars remains challenging. Aims. Galactic archaeology needs a new accretion diagnostic to understand primordial stellar populations. Contrary to α-elements, neutron-capture elements present unexplained large abundance spreads for low-metallicity stars, which could result from a mixture of formation sites. Methods. We analysed the abundances of yttrium, europium, magnesium, and iron in Milky Way satellite galaxies, field halo stars, and globular clusters. The chemical information was complemented by orbital parameters based on Gaia data. In particular, we considered the average inclination of the orbits. Results. The [Y/Eu] abundance behaviour with respect to the [Mg/Fe] turnovers for satellite galaxies of various masses reveals that higher-luminosity systems, for which the [Mg/Fe] abundance declines at higher metallicities, present enhanced [Y/Eu] abundances, particularly in the [Fe/H] regime between −2.25 dex and −1.25 dex. In addition, the analysis has uncovered a chemo-dynamical correlation for both globular clusters and field stars of the Galactic halo, accounting for about half of the [Y/Eu] abundance spread. In particular, [Y/Eu] under-abundances typical of protracted chemical evolutions are preferentially observed in polar-like orbits, pointing to a possible anisotropy in the accretion processes. Conclusions. Our results strongly suggest that the observed [Y/Eu] abundance spread in the Milky Way halo could result from a mixture of systems with different masses. They also highlight that both nature and nurture are relevant to the formation of the Milky Way since its primordial epochs, thereby opening new pathways for chemical diagnostics of the build-up of our Galaxy.

UV regulated star formation in high-redshift galaxies

ABSTRACT The first galaxies forming a few hundred million years after the big bang are the key drivers of cosmic evolution and ideal laboratories to study theories of galaxy formation. We here study the role of UV radiation in suppressing star formation in primordial galaxies by destroying molecular hydrogen, the main coolant in primordial gas, and provide estimates of cold dense gas at the onset of star formation. To accomplish this goal, we perform three-dimensional cosmological simulations of minihaloes in different environments forming at z ∼ 25 by varying strength of background UV flux below the Lyman limit between 0.01–1000 in units of $\rm J_{21}=10^{-21}\, erg \,cm^{-2} \,s^{-1} \,Hz^{-1} \,sr^{-1}$. Particularly, we include photodetachment of $\rm H^-$, the self-shielding of $\rm H_2$, which both were neglected in previous studies and use updated reaction rates. Our results show that depending on the background level $\rm H_2$ formation is suppressed, delaying gravitational collapse until haloes reach the atomic cooling limit. We find that the formation of cold dense molecular gas and subsequently star formation gets delayed by 100–230 Myr depending on the level of the background radiation and the growth history of the dark matter haloes. The fraction of dense self-shielded gas is a strong function of the background flux and exponentially declines with the strength of incident UV flux above $\rm J_{21} \ge 1$. We find that taking into account $\rm H_2$ self-shielding is crucial for accurately estimating the amount of cold dense gas available for star formation.

Active Galactic Nuclei in Dusty Starbursts at z = 2: Feedback Still to Kick in

Abstract We investigate a sample of 152 dusty sources at 1.5 &lt; z &lt; 2.5 to understand the connection of enhanced star formation rate (SFR) and black hole accretion rate. The sources are Herschel-selected, having stellar masses M * &gt; 1010 M ⊙ and SFR (∼100–1000 M ⊙ yr−1) elevated (&gt;4×) above the star-forming “main sequence,” classifying them as starbursts (SBs). Through a multiwavelength fitting approach (including a dusty torus component), we divided the sample into active SBs (dominated by an active galactic nucleus (AGN) emission, SBs-AGN, ∼23% of the sample) and purely star-forming SBs (SBs-SFR). We visually inspected their Hubble Space Telescope/ultraviolet (UV) rest frame maps: SBs-SFR are generally irregular and composite systems; ∼50% of SBs-AGN are instead dominated by regular compact morphologies. We then found archival Atacama Large Millimeter/submillimeter Array continuum counterparts for 33 galaxies (12 SBs-AGN and 21 SBs-SFR). For these sources we computed dust masses, and, with standard assumptions, we also guessed total molecular gas masses. SBs turn into gas-rich systems (f gas = M gas/(M gas + M *) ≃ 20%–70%), and the gas fractions of the two SB classes are very similar (f gas = 43% ± 4% and f gas = 42% ± 2%). Our results show that SBs are consistent with a mixture of: (1) highly star-forming merging systems (dominating the SBs-SFR) and (2) primordial galaxies, rapidly growing their M * together with their black hole (mainly the more compact SBs-AGN). Feedback effects have not yet reduced their f gas. Indeed, SBs at z = 2, with relatively low bolometric AGN luminosities in the range 1044 &lt; L bol(AGN) &lt; 1046 erg s−1 (compared to bright optical and X-ray quasars), are still relatively far from the epoch when the AGN feedback will quench the SFR in the host and will substantially depress the gas fractions.

On the Prevalence of Supermassive Black Holes over Cosmic Time

Abstract We investigate the abundance of supermassive black hole (SMBH) seeds in primordial galaxy halos. We explore the assumption that dark matter halos outgrowing a critical halo mass M c have some probability p of having spawned an SMBH seed. Current observations of local, intermediate-mass galaxies constrain these parameters: for M c = 1011 M ⊙, all halos must be seeded, but when adopting smaller M c masses the seeding can be much less efficient. The constraints simultaneously put lower limits on the number density of black holes in the local and high-redshift universe. Reproducing z ∼ 6 quasar space densities depends on their typical halo mass, which can be constrained by counting nearby Lyman Break Galaxies and Lyman-alpha emitters. For both observables, our simulations demonstrate that single-field predictions are too diverse to make definitive statements, in agreement with mixed claims in the literature. If quasars are not limited to the most massive host halos, they may represent a tiny fraction (≈10−5) of the SMBH population. Finally, we produce a wide range of predictions for gravitational events from SMBH mergers. We define a new diagnostic diagram for the Laser Interferometer Space Antenna to measure both SMBH space density and the typical delay between halo merger and black hole merger. While previous works have explored specific scenarios, our results hold independent of the seed mechanism, seed mass, obscuration, fueling methods and duty cycle.

A case study of hurdle and generalized additive models in astronomy: the escape of ionizing radiation

The dark ages of the Universe end with the formation of the first generation of stars residing in primeval galaxies. These objects were the first to produce ultraviolet ionizing photons in a period when the cosmic gas changed from a neutral state to an ionized one, known as Epoch of Reionization (EoR). A pivotal aspect to comprehend the EoR is to probe the intertwined relationship between the fraction of ionizing photons capable to escape dark haloes, also known as the escape fraction ($f_{esc}$), and the physical properties of the galaxy. This work develops a sound statistical model suitable to account for such non-linear relationships and the non-Gaussian nature of $f_{esc}$. This model simultaneously estimates the probability that a given primordial galaxy starts the ionizing photon production and estimates the mean level of the $f_{esc}$ once it is triggered. The model was employed in the First Billion Years simulation suite, from which we show that the baryonic fraction and the rate of ionizing photons appear to have a larger impact on $f_{esc}$ than previously thought. A naive univariate analysis of the same problem would suggest smaller effects for these properties and a much larger impact for the specific star formation rate, which is lessened after accounting for other galaxy properties and non-linearities in the statistical model.

Origins of ultradiffuse galaxies in the Coma cluster – II. Constraints from their stellar populations

In this second paper of the series we study, with new Keck/DEIMOS spectra, the stellar populations of seven spectroscopically confirmed ultradiffuse galaxies (UDGs) in the Coma cluster. We find intermediate to old ages (∼7 Gyr), low metallicities ([Z/H] ∼ −0.7 dex) and mostly supersolar abundance patterns ([Mg/Fe] ∼ 0.13 dex). These properties are similar to those of low-luminosity (dwarf) galaxies inhabiting the same area in the cluster and are mostly consistent with being the continuity of the stellar mass scaling relations of more massive galaxies. These UDGs’ star formation histories imply a relatively recent infall into the Coma cluster, consistent with the theoretical predictions for a dwarf-like origin. However, considering the scatter in the resulting properties and including other UDGs in Coma, together with the results from the velocity phase-space study of the Paper I in this series, a mixed-bag of origins is needed to explain the nature of all UDGs. Our results thus reinforce a scenario in which many UDGs are field dwarfs that become quenched through their later infall onto cluster environments, whereas some UDGs could be genuine primordial galaxies that failed to develop due to an early quenching phase. The unknown proportion of dwarf-like to primordial-like UDGs leaves the enigma of the nature of UDGs still open.

Kiloparsec-scale gaseous clumps and star formation at z = 5–7

We investigate the morphology of the [CII] emission in a sample of "normal" star-forming galaxies at $5<z<7.2$ in relation to their UV (rest-frame) counterpart. We use new ALMA observations of galaxies at $z\sim6-7$ as well as a careful re-analysis of archival ALMA data. In total 29 galaxies were analysed, 21 of which are detected in [CII]. For several of the latter the [CII] emission breaks into multiple components. Only a fraction of these [CII] components, if any, is associated with the primary UV systems, while the bulk of the [CII] emission is associated either with fainter UV components, or not associated with any UV counterpart at the current limits. By taking into account the presence of all these components, we find that the L$_{\rm [CII]}$-SFR relation at early epochs is fully consistent with the local relation, but it has a dispersion of 0.48$\pm$0.07 dex, which is about two times larger than observed locally. We also find that the deviation from the local L$_{\rm [CII]}$-SFR relation has a weak anti-correlation with the EW(Ly$\alpha$). The morphological analysis also reveals that [CII] emission is generally much more extended than the UV emission. As a consequence, these primordial galaxies are characterised by a [CII] surface brightness generally much lower than expected from the local ${\rm \Sigma_{[CII}-\Sigma_{SFR}}$ relation. These properties are likely a consequence of a combination of different effects, namely: gas metallicity, [CII] emission from obscured star-forming regions, strong variations of the ionisation parameter, and circumgalactic gas in accretion or ejected by these primeval galaxies.