arXiv subjects

A. Ferrara

Publications and source records attributed to A. Ferrara.

At least 19 recordsLinked to original sources

arXiv2026

The gradual decline of Ly$\alpha$ visibility in the CANDELS fields: evidence for the combined effects of galaxy evolution and reionization

We investigate the evolution of Ly$\alpha$ visibility and the physical properties of Ly$\alpha$ emitters (LAEs) across the five CANDELS fields using publicly available JWST/NIRSpec PRISM spectroscopy. Our catalog comprises 3446 spectroscopically confirmed sources at 4 $\leq $ z < 14.2, including 3361 star-forming galaxies (SFGs), of which 539 are robust (S/N > 3) LAEs. We measure the fraction of LAEs with EW$_0$ > 25A (X$_{\mathrm{Ly\alpha}}$) and trace its redshift evolution in two UV luminosity bins, namely -20.25 < M$_{\mathrm{UV}}$ < -18.75 and -21.75 < M$_{\mathrm{UV}}$ < -20.25. Within the fainter-UV range, X$_{\mathrm{Ly\alpha}}$ increases from z = 5 to z = 6 at 3$\sigma$ significance and subsequently declines toward higher redshifts with a significant monotonic trend at z > 6. We also investigate the physical properties of both LAEs and the full SFG population. We find that the stellar mass, UV slope $\beta$, stellar reddening, SFR, metallicity, sSFR, and burstiness of LAEs remain approximately stable with redshift. The only exception is the mass-weighted age which decreases with increased redshift. Conversely, the properties of the full SFG population evolve significantly, progressively approaching the region of galaxy-property space occupied by LAEs as redshift increases. This suggests that galaxy evolution may enhance the intrinsic production and escape of Ly$\alpha$ photons toward earlier epochs. We argue that this effect should be accounted when inferring the evolution of the neutral hydrogen content of the IGM from the observed visibility of Ly$\alpha$ emission. To this end, we employ a physically motivated framework based on the Attenuation-Free Model, jointly accounting for galaxy evolution and IGM attenuation. Our observations favor reionization histories that proceed gradually over scenarios characterized by a rapid increase in the cosmic neutral hydrogen fraction.

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arXiv2026

On the detection of Population III galaxies: Emission Line Diagnostics for Hybrid Stellar Populations

Identifying Population III (Pop III) stars, the first generation of metal-free stars in the early Universe, remains a central challenge in astrophysics. High-ionization emission lines, such as HeII are commonly used as tracers of Pop III signatures. However, realistic galaxies may host hybrid stellar populations, including both Pop III and metal-enriched Population II (Pop II) stars, complicating the interpretation of observed spectra. We aim to investigate how hybrid Pop III/Pop II stellar populations affect emission line diagnostics and assess the detectability of Pop III stars across different galactic environments and redshifts. We select galaxies with varying Pop III-to-total mass ratios from the IllustrisTNG cosmological simulations. Using self-consistent photoionisation models, we compute integrated spectra by adopting local physical conditions from the simulations to study the resulting emission line diagnostics. We find that emission line diagnostics are strongly dependent on the relative Pop III contribution. Current diagnostics can identify galaxies dominated totally by Pop III stars but fail for systems where Pop II stars contribute significantly, introducing degeneracies in interpreting observed spectra. Our results highlight the limitations of existing emission line diagnostics in hybrid systems and emphasize the need for additional methods that account for mixed stellar populations to reliably detect Pop III stars during and after the epoch of reionization.

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arXiv2026

Multiphase images of a powerful supernova-driven wind in the early Universe

Galactic winds are considered a likely driver of rapid quenching in early massive galaxies, but until now there has been no direct evidence that such systems drive winds powerful enough to meaningfully suppress their star-formation. We present resolved cold gas and ionized gas observations of a powerful supernova-driven wind in a massive galaxy 1.1 billion years after the Big Bang (at $z$=5.3). The outflow, likely triggered by ongoing merger activity, is removing gas at twice the rate of star-formation and could plausibly eject all the cold gas from the galaxy within 100 Myr. Our results suggest that powerful merger-driven outflows may be a key mechanism to produce abundant massive quiescent galaxies in the early Universe when a large fraction of massive galaxies are interacting. The mass and energetics of this distant outflow are consistent with nearby starburst-driven superwinds, suggesting that the efficiency of stellar feedback has remained relatively constant over the last 12 billion years of cosmic history.

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arXiv2026

No Blue without Red: Evolutionary Properties of Super-Early Galaxies

The discovery of numerous luminous, super-early galaxies at $z>10$ by JWST has revealed a striking diversity in their ultraviolet (UV) properties, ranging from extremely blue, dust-poor systems to a smaller population of significantly reddened sources. We investigate the physical origin of this diversity within the framework of the Attenuation-Free Model (AFM), in which radiation-driven outflows redistribute dust to large galactic radii, reducing the effective attenuation. Applying the model to a sample of 32 spectroscopically confirmed super-early galaxies, we derive their key physical properties, including halo mass, star formation efficiency, metallicity, and outflow extent. We find that these systems reside in massive halos ($\log M/M_\odot \sim 10.7$) and exhibit moderate ($0.01 \lesssim \epsilon_* \lesssim 0.05$) star formation efficiencies, while frequently reaching super-Eddington conditions that trigger powerful outflows. Within this framework, we propose an evolutionary sequence in which galaxies transition from a dust-obscured ``Red Monster'' phase to a UV-bright ``Blue Monster'' phase as outflows clear their central regions. The recently confirmed red galaxy EGS-z11-R0 at $z=11.45$ is naturally interpreted as a system observed during this obscured phase. Compact ($r_e \lesssim 150$ pc) sources are instead difficult to reconcile within AFM; we speculate that their emission is dominated by an AGN. Our results provide a unified interpretation of super-early galaxy properties and highlight the key role of radiation-driven outflows in shaping galaxy evolution at cosmic dawn. Future observations with JWST and ALMA will be essential to test these predictions and further constrain the nature of the earliest galaxies.

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arXiv2025

Resolving dust and Ly{\alpha} emission in a lensed galaxy at the epoch of reionization with JWST/CANUCS

Lyman $\alpha$ emission is highly sensitive to dust and neutral hydrogen and is therefore expected to be strongly suppressed in dusty or gas-rich galaxies during the epoch of reionization (EoR). Nevertheless, numerous moderately dusty Ly$\alpha$ emitters (LAEs) are observed at this epoch, suggesting that complex interstellar medium (ISM) geometries and feedback-driven outflows may facilitate Ly$\alpha$ escape. We investigate the dust, gas, and stellar properties of the gravitationally lensed LAE HCM 6A at $z=6.5676$ to characterize its multiphase ISM and the physical conditions regulating Ly$\alpha$ escape. We combine JWST/NIRISS slitless spectroscopy, HST+JWST/NIRCam imaging, and JWST/NIRSpec slit spectra from the CANUCS program. Using a customized BAGPIPES SED-fitting framework with a flexible attenuation law, we derive stellar, nebular, and dust properties on integrated ($\sim$kpc), slit-level ($\sim$0.1 kpc), and pixel-level ($\sim$25 pc) scales, enabled by strong lensing ($\mu \approx 8.3$-$9.1$). A Ly$\alpha$ map from SLEUTH traces the spatial distribution of Ly$\alpha$ emission. We measure an unlensed stellar mass of $\log M_\ast = 8.3$-$8.4$ and an intrinsic UV magnitude of $M_{\rm UV} = -19.8 \pm 0.1$. The older region (S1) is moderately dusty with consistent stellar and nebular attenuation indicators, implying a uniform ISM geometry, while the youngest region (S3) shows strong discrepancies among dust tracers, indicating a feedback-shaped multiphase ISM. Ly$\alpha$ emission arises primarily from S3, where a dust-cleared central clump enables efficient Ly$\alpha$ escape. We find Calzetti-like attenuation curves with a UV bump that strengthens with stellar age and decreasing $A_V$. Our observations provide a uniquely detailed, spatially resolved view of a moderately dusty LAE during the EoR, demonstrating how feedback and multiphase ISM structure govern Ly$\alpha$ escape.

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arXiv2025

The Early Maturity of High-Redshift Galaxies: Insights from sSFR, M/L and SFHs at z~7-14

The James Webb Space Telescope (JWST) has revealed an unexpected excess of UV-bright galaxies at z>10, unaccounted for by extrapolations from pre-JWST observations and theoretical models. Understanding the physical properties and star formation histories (SFHs) of high-redshift systems is key to distinguishing between the proposed scenarios. We identified and analysed a sample of 2420 robust candidates at z~7-14 drawn from the ASTRODEEP-JWST dataset over ~0.2 deg^2, and modelled their properties with non-parametric SFHs to derive the specific star formation rate (sSFR) and stellar population properties. We find that the median sSFR and M/L remain roughly constant across the probed redshift range. We show that this result is robust against potential systematics unless a hidden population of dust-enshrouded starbursts, undetectable in current data, exists at these redshifts. In any case, the absence of observed high-sSFR systems at the highest redshifts suggests that any dust-free starburst phase must be short-lived. The observed sSFR evolution is in tension with most theoretical models, making it a key quantity for discriminating among competing scenarios. The sample shows a wide range of physical conditions and galaxy classes, including systems with low sSFRs and high mass-to-light ratios (M/L) up to z~10, indicative of already-evolved galaxies only a few hundred Myr after the Big Bang, and different degrees of dust attenuation. We finally reconstructed the assembly histories of two sub-samples, namely the highest-M/L galaxies at z~7-8, which appear to have formed the bulk of their stars at least 500 Myr before observation, implying progenitors observable as UV-bright sources at z>20, and the full sample of z>11 galaxies, which formed through stochastic SFHs, remaining UV-faint for most of their early evolution, before undergoing recent (~50 Myr old) episodes of major growth.

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arXiv2025

How much gas and dust is in the $z=5.7$ Lyman Break Galaxy HZ10? An ALMA Band 10 to 4 and JWST/NIRSpec study of its interstellar medium

A complete overview of the stellar, gas and dust contents of galaxies is key to understanding their assembly at early times. However, an estimation of molecular and atomic gas reservoirs at high redshift relies on various indirect tracers, while robust dust mass measurements require multi-band far-infrared continuum observations. We take census of the full baryonic content of the main-sequence star-forming galaxy HZ10 at $z=5.65$, a unique case study where all necessary tracers are available. We present new ALMA Band 10 ($\lambda_\mathrm{rest}=50\mu$m) and Band 4 ($300\mu$m) observations towards HZ10, which combined with previously taken ALMA Band 6 through 9 data ($70-200\mu$m) constrains its dust properties. We complete the baryonic picture using archival high-resolution [CII] observations that provide both a dynamical mass and molecular and atomic gas mass estimates, a JVLA CO(2-1)-based molecular gas mass, and JWST metallicity and stellar mass measurements. We detect continuum emission from HZ10 in Bands 10 and 4 at the $3.4-4.0\sigma$ level, and measure a dust temperature of $T_\mathrm{dust} = 37_{-5}^{+6}$K and dust mass $\log(M_\mathrm{dust}/M_\odot) = 8.0 \pm 0.1$. Leveraging the dynamical constraints, we infer its total gas budget, and find that commonly used [CII]-to-H$_2$ and [CII]-to-HI conversions overpredict the gas mass relative to the dynamical mass. For this reason, we derive a [CII]-to-total ISM mass (atomic + molecular) conversion factor, which for HZ10 corresponds to $\alpha_\mathrm{[CII]}^\mathrm{ISM} = 39^{+50}_{-25}M_\odot L_\odot^{-1}$. We also find that HZ10 falls below the local scaling relation between dust-to-gas ratio and metallicity, suggesting inefficient ISM dust growth. These results demonstrate a powerful synergy between ALMA and JWST in disentangling the baryonic components of early galaxies, paving the way for future studies of larger samples.

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arXiv2025

REBELS-IFU: Steeply rising star formation histories and the importance of dust obscuration in massive $z \simeq7$ galaxies revealed by multi-wavelength observations

Reliable star formation rate (SFR) measurements are essential for understanding early galaxy evolution, yet derived values rely on several assumptions. To address this problem, we investigate the SFRs of 12 massive ($9~<~\log(M_{\star}/{\rm M}_{\odot})~<~10$) Lyman-break galaxies at $z=6.5-7.7$, drawn from the Atacama Large Millimeter/submillimeter Array (ALMA) Reionization Era Bright Emission Line Survey (REBELS) program. The multi-wavelength data, including JWST NIRSpec IFU spectroscopy and ALMA observations, make this a unique sample for investigating SFR tracers at this epoch. We compare SFRs derived from the rest-UV, H$\alpha$, and far-infrared emission, and from spectral energy distribution (SED) fits. We apply robust dust attenuation corrections, which are crucial since between $50-80$ per cent of the star formation is obscured, and find a stellar-to-nebular attenuation ratio of $f=0.50\pm0.08$, consistent with local star-forming galaxies. The majority of the derived total SFRs (medians $25-120$ ${\rm M}_{\odot}$ yr$^{-1}$) place the REBELS galaxies systematically above $z=7$ literature star-forming main-sequence relations, and our best-fit star formation histories (SFHs) rise more steeply than lower-mass galaxies at the same redshift. We show that these rising SFHs mean commonly used luminosity-to-SFR conversion factors, derived assuming a constant SFH over given timescales, overestimate the SFRs averaged over these timescales for our galaxies. We provide updated luminosity-to-SFR calibrations for $z\simeq7$ galaxies with rising SFHs, showing that commonly assumed rest-UV conversion factors overestimate the $100$ Myr average SFR by a factor of $\simeq3$. Finally, we investigate burstiness indicators in the REBELS-IFU galaxies, finding that the rising SFHs imply that the H$\alpha$-to-UV luminosity ratio is an unreliable probe of bursty star formation.

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arXiv2025

Lyman-$\alpha$ radiation pressure regulates star formation efficiency

Order-unity star formation efficiencies (SFE) in early galaxies may explain the overabundance of bright galaxies observed by JWST at high redshift. Here we show that Lyman-$\alpha$ (Ly$\alpha$) radiation pressure limits the gas mass converted into stars, particularly in primordial environments. We develop a shell model including Ly$\alpha$ feedback, and validate it with one-dimensional hydrodynamical simulations. To account for Ly$\alpha$ resonant scattering, we adopt the most recent force multiplier fits, including the effect of Ly$\alpha$ photon destruction by dust grains. We find that, independently of their gas surface density $\Sigma_g$, clouds are disrupted on a timescale shorter than a free-fall time, and even before supernova explosions if $\Sigma_g \gtrsim 10^3\,M_{\odot}\ \rm pc^{-2}$. At $\log(Z/Z_{\odot}) = -2$, relevant for high-redshift galaxies, the SFE is $0.01 \lesssim \hat{\epsilon}_{*} \lesssim 0.66$ for $10^3 \lesssim\Sigma_g [M_{\odot}\ \rm pc^{-2}] \lesssim 10^5$. The SFE is even lower for decreasing metallicity. Near-unity SFEs are possible only for extreme surface densities, $\Sigma_{g} \gtrsim 10^5\;M_{\odot}\ \rm pc^{-2}$, and near-solar metallicities. We conclude that Ly$\alpha$ radiation pressure severely limits a possible extremely efficient, feedback-free phase of star formation in dense, metal-poor clouds.

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arXiv2025

Extreme equivalent width-selected low-mass starbursts at $z=4-9$: insights into their role in cosmic reionization

We investigate the properties of extreme emission line galaxies (EELGs) at $z=4-9$ and their role in reionization. Compact, low-mass galaxies with intense optical emission lines are linked to elevated specific star formation rates (sSFRs) and recent bursts of star formation. Feedback in these systems may enable the leakage of ionizing radiation into the intergalactic medium. Using JWST/NIRSpec spectroscopy from the CAPERS, CEERS, and RUBIES surveys, we compile 160 NIRCam-selected EELGs in the EGS field. These galaxies show extreme rest-frame equivalent widths (EWs), with a median EW([O III]+H${\beta}$)=1616\r{A} and EW(H${\alpha}$)=763\r{A}. They are low-mass (median log(M$_{\star}$/M$_{\odot}$)=8.26) with high sSFRs (median 43 Gyr$^{-1}$), above the $z\sim6$ main sequence. UV slopes are diverse, with a mean $\beta=-2.0$, and only 7% have extremely blue continua ($\beta<-2.6$). Emission-line diagnostics suggest stellar populations as the primary ionizing source, although an AGN fraction of 14% is cannot be entirely ruled out. These galaxies are efficient ionizing photon producers, with mean log($\xi_{\rm ion}$ [Hz erg$^{-1}$])=25.37, exceeding typical values at similar redshifts. Escape fractions, however, are heterogeneous: 16% of EELGs at $z<7$ show escape fractions $>$5% for both Ly${\alpha}$ and LyC photons, while 82% lack detectable Ly${\alpha}$ emission. The median inferred LyC escape fraction is modest (5%) but enhanced in compact super-Eddington systems with sSFR >25 Gyr$^{-1}$. These results indicate that EELGs contribute approximately 16-40% of the total ionizing emissivity required to sustain hydrogen reionization. EELGs are extremely compact, with a median effective radius of 0.49 kpc, and exhibit a recent star-formation burst. Our analysis indicates that sSFR and star-formation rate surface density are the primary drivers of their extreme emission line strengths.

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arXiv2025

The ALPINE-CRISTAL-JWST Survey: JWST/IFU Optical Observations for 18 Main-Sequence Galaxies at z=4-6

To fully characterize the formation and evolution of galaxies, we need to observe their stars, gas, and dust on resolved spatial scales. We present the ALPINE-CRISTAL-JWST survey, which combines kpc-resolved imaging and spectroscopy from HST, JWST, and ALMA for 18 representative main-sequence galaxies at z=4-6 and log(M/$M_\odot$) > 9.5 to study their star formation, chemical properties, and extended gas reservoirs. The co-spatial measurements resolving the ionized gas, molecular gas, stars, and dust on 1-2 kpc scales make this a unique benchmark sample for the study of galaxy formation and evolution at $z\sim5$, connecting the Epoch of Reionization with the cosmic noon. In this paper, we outline the survey goals and sample selection, and present a summary of the available data for the 18 galaxies. In addition, we measure spatially integrated quantities (such as global gas metallicity), test different star formation rate indicators, and quantify the presence of H$\alpha$ halos. Our targeted galaxies are relatively metal rich (10-70% solar), complementary to JWST samples at lower stellar mass, and there is broad agreement between different star formation indicators. One galaxy has the signature of an active galactic nuclei (AGN) based on its emission line ratios. Six show broad H$\alpha$~emission suggesting type 1 AGN candidates. We conclude with an outlook on the exciting science that will be pursued with this unique sample in forthcoming papers.

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arXiv2025

Neutral hydrogen in and around galaxies during the Epoch of Reionization

JWST spectra revealing Lyman-$\alpha$ (Ly$\alpha$) absorption in $z\sim 5-14$ galaxies offer a unique probe of reionization's earliest stages. However, disentangling absorption by the increasingly neutral intergalactic medium (IGM) from that in galaxies' interstellar and circumgalactic medium (ISM, CGM) remains challenging due to the poorly constrained nature of neutral hydrogen (HI) in and around galaxies at these redshifts. We use the SERRA high-resolution cosmological zoom-in simulations to characterize the HI distribution and its evolution along sightlines to star-forming regions during reionization, to interpret the contribution of local HI (ISM+CGM) to Ly$\alpha$ absorption in $z>5$ spectra. We analyze $\sim 100$ $z=6-9.5$ galaxies, generating mock sightlines from each galaxy's star formation peak. We study the sightline distribution of HI column densities ($N_{\rm HI}$) and its variation with radius, halo mass and redshift. We find broad sightline variation in $N_{\rm HI}$ (0.5-1.5dex) due to complex ISM morphology driven by bursty star formation, with median $\log(N_{\rm HI}/{\rm cm}^{-2}) \simeq 21-22$. Dense ISM gas is the dominant origin of damped Ly$\alpha$ absorption (DLA) systems along sightlines towards star-forming regions, outweighing gas in the CGM, filaments and proximate absorbers. Median $N_{\rm HI}$ increases with halo mass, scaling roughly with the virial radius, as expected due to larger potentials and more extended CGM, but shows negligible redshift evolution at fixed halo mass. This suggests post-reionization $N_{\rm HI}$ distributions may provide useful priors to interpret IGM damping wings at higher redshifts. To interpret strong $z>5$ DLA candidates found by JWST, we investigate $N_{\rm HI}>10^{22}{\rm cm}^{-2}$ sightlines. These trace dense, metal-enriched ISM within <1 kpc of massive halos, a scenario testable with higher-resolution spectroscopy.

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arXiv2025

Is feedback-free star formation possible?

It has been suggested that, if the free-fall time of star-forming clouds is shorter than the lifetime ($\approx 3 $ Myr) of massive stars exploding as supernovae (SN), a large fraction of the cloud gas can be converted into stars during an allegedly `feedback-free' phase. Here, we show that radiation pressure from Ly$\alpha$ photons produced in the pre-SN phase can instead erase feedback-free conditions, and severely limit the star formation efficiency (SFE). We find that, for a constant star formation rate, all clouds with gas surface density $(37-1.7 \times 10^5)\ M_\odot\ \rm pc^{-2}$ have $\epsilon_* < 0.08$. Higher SFE values can only be reached if Ly$\alpha$-driven shells fragment and form stars. While advanced RHD simulations are required to establish the importance of this effect, adopting an optimistic guess, we find that the SFE increases with cloud surface density, rising from $\epsilon_*=0.023$ at $\Sigma_g = 37\ M_\odot\ \mathrm{pc^{-2}}$ to $\epsilon_*=0.27$ at $\Sigma_g = 1.7 \times 10^5\ M_\odot\ \mathrm{pc^{-2}}$. Given the optimistic assumptions adopted, these numbers should be regarded as upper limits. We conclude that Ly$\alpha$ radiation pressure strongly limits the SFE, even at solar metallicities, erasing the possibility that a feedback-free star formation mode with $\epsilon_* \gtrsim 0.4$ exists in the pre-SN phase. This conclusion remains valid even when other effects such as dust destruction of Ly$\alpha$ photons, presence of HII regions, velocity gradients, atomic recoil, and turbulence are considered.

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arXiv2025

The ALMA REBELS survey: [OIII]$_{88\mu \text{m}}$ line scans of UV-bright $z \gtrsim 7.6$ galaxies

We present the [OIII]$_{88\mu \text{m}}$ spectral scan results from the ALMA Large Program REBELS (Reionization Era Bright Emission Line Survey). The generally high luminosity of [OIII]$_{88\mu \text{m}}$ and ALMA's Band 7 efficiency motivated its use for line scans of REBELS targets at $z>8$. Spectral scans of four sources covered 326.4-373.0 GHz ($z=8.10$-9.39), reaching [OIII]$_{88\mu \text{m}}$ luminosities of $\mathrm{\sim7.6\times10^8\ L_{\odot}}$ ($5\sigma$) for a FWHM of 400 km s$^{-1}$. No credible lines are detected for the four targets. For REBELS-04, the non-detection is unexpected given the $\geq92\%$ coverage of the redshift likelihood distribution and its estimated SFR of 40 $\text{M}_{\odot}\ \text{yr}^{-1}$. Possible explanations for the faint [OIII]$_{88\mu \text{m}}$ emission (assuming a FWHM of 100 km s$^{-1}$) include high ISM densities ($>n_{\text{crit}} \approx 510\ \text{cm}^{-3}$) and low ionization parameters ($\mathrm{log_{10}\ U_{ion}\lesssim -2.5}$). For REBELS-37, a subsequent detection of [CII]$_{158\mu \text{m}}$ ($z=7.643$) confirmed it lay outside our scan range. For REBELS-11 and REBELS-13, it remains unclear if the non-detection is due to the depth of the line scan or redshift coverage. REBELS-04 and REBELS-37 show significant ($\geq3.8\sigma$) dust continuum emission in Band 7. If the photometric redshift of REBELS-04 is accurate, i.e., $z_{\mathrm{phot}}=8.57^{+0.10}_{-0.09}$ or $z_{\mathrm{phot}}=8.43^{+0.10}_{-0.10}$ accounting for additional neutral hydrogen in the circumgalactic medium, REBELS-04 would constitute the most distant dust-detected galaxy identified with ALMA to date. Additional Band 6 dust observations of REBELS-37 constrain the shape of the far-IR SED, ruling out cold dust temperatures ($\lesssim28$ K) at $3\sigma$. Further insight into these galaxies will require spectroscopic redshifts and deeper multi-band dust observations.

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arXiv2025

Redshift evolution of Lyman continuum escape fraction after JWST

The LyC escape fraction from galaxies, $f_{\rm esc}$, is strongly boosted by galactic outflows. In the Attenuation-Free Model (AFM) accounting for the properties of $z>10$ galaxies, radiation-driven outflows develop once the galaxy specific star formation rate, ${\rm sSFR} \ge {\rm sSFR}^* \approx 25\ {\rm Gyr}^{-1}$. As the cosmic sSFR increases with redshift, so does $f_{\rm esc}(z)$, which, when globally averaged, grows from 0.007 to 0.6 in $0 < z < 20$. We successfully tested the model on specific data sub-samples. Our predictions are consistent with measurements of $f_{\rm esc}$ up to $z=9.5$, and provide a physical explanation for the observed decreasing trend of the mean UV galaxy spectral slope, $\beta$, towards high-$z$.

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arXiv2025

The ALMA-CRISTAL survey: Gas, dust, and stars in star-forming galaxies when the Universe was ~1 Gyr old I. Survey overview and case studies

We present the ALMA-CRISTAL survey, an ALMA Cycle 8 Large Program designed to investigate the physical properties of star-forming galaxies at $4 \lesssim z \lesssim 6$ through spatially resolved, multi-wavelength observations. This survey targets 19 star-forming main-sequence galaxies selected from the ALPINE survey, using ALMA Band 7 observations to study [CII] 158 $\mu$m line emission and dust continuum, complemented by JWST/NIRCam and HST imaging to map stellar and UV emission. The CRISTAL sample expanded to 39 after including newly detected galaxies in the CRISTAL fields, archival data, and pilot study targets. The resulting dataset provides a detailed view of gas, dust, and stellar structures on kiloparsec scales at the end of the era of reionization. The survey reveals diverse morphologies and kinematics, including rotating disks, merging systems, [CII] emission tails from potential interactions, and clumpy star formation. Notably, the [CII] emission in many cases extends beyond the stellar light seen in HST and JWST imaging. Scientific highlights include CRISTAL-10, exhibiting an extreme [CII] deficit similar to Arp 220; and CRISTAL-13, where feedback from young star-forming clumps likely causes an offset between the stellar clumps and the peaks of [CII] emission. CRISTAL galaxies exhibit global [CII]/FIR ratios that decrease with increasing FIR luminosity, similar to trends seen in local galaxies but shifted to higher luminosities, likely due to their higher molecular gas content. CRISTAL galaxies also span a previously unexplored range of global FIR surface brightness at high-redshift, showing that high-redshift galaxies can have elevated [CII]/FIR ratios. These elevated ratios are likely influenced by factors such as lower metallicity gas, the presence of significant extraplanar gas, and contributions from shock-excited gas.

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arXiv2025

Spatially resolved [CII]-gas conversion factor in early galaxies

Determining how efficiently gas collapses into stars at high-redshift is key to understanding galaxy evolution in the Epoch of Reionization (EoR). Globally, this process is quantified by the gas depletion time ($t_{dep}$); on resolved scales, by the slope and normalization of the Kennicutt-Schmidt (KS) relation. This work explores the global ($\alpha_{[CII]}$) and spatially resolved ($W_{[CII]}$) [CII]-to-gas conversion factors at high-$z$ and their role in inferring reliable gas masses, surface densities, and $t_{dep}$ in the EoR. We select galaxies at 4<z<9 from the SERRA cosmological zoom-in simulation, that features on-the-fly radiative transfer and resolves interstellar medium properties down to $\approx$30 pc. The [CII] emission modelling from photodissociation regions allow us to derive global $\alpha_{ [CII]}$, and maps of $W_{[CII]}$. We study their dependence on gas metallicity (Z), density (n), Mach number (M), and burstiness parameter ($k_s$), and provide best fit relations. The $\alpha_{[CII]}$ decreases with increasing $Z$ and galaxy compactness, while the resolved $W_{[CII]}$ shows two regimes: at $Z< 0.2 Z_\odot$, it anticorrelates with n and Z, but not with $k_s$; above this threshold, it also depends on $k_s$, with more bursty regions showing lower conversion factors. This implies $W_{[CII]}\propto \Sigma_{[CII]}^{-0.5}$, as dense, metal-rich, and bursty regions exhibit higher [CII] surface brightness. Applying a constant $\alpha_{[CII]}$ overestimates $\Sigma_{gas}$ in bright $\Sigma_{[CII]}$ patches, thus flattening the KS slope and overestimating $t_{dep}$ by a factor of $\approx$4.

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arXiv2025

Pushing JWST to the extremes: search and scrutiny of bright galaxy candidates at z$\simeq$15-30

We designed customized Lyman-break color selection techniques to identify galaxy candidates in the redshift ranges $15 \leq z \leq 20$ and $20 \leq z \leq 28$. The selection was performed on the ASTRODEEP-JWST multi-band catalogs of the CEERS, Abell-2744, JADES, NGDEEP, and PRIMER survey fields, covering a total area of $\sim0.2$ sq. deg. We identify five candidates at $15 \leq z \leq 20$, while no objects are found based on the $z\gtrsim20$ color selection criteria. Despite exhibiting a $>$1.5 mag break, all the objects display multimodal redshift probability distributions across different SED-fitting codes and methodologies. The alternative solutions correspond to poorly understood populations of low-mass quiescent or dusty galaxies at z$\sim$3-7. This conclusion is supported by the analysis of five F200W-dropout objects that we find to be interlopers on the basis of NIRSpec PRISM spectra: four dusty star-forming galaxies at z$\sim$2.2-6.6, and a passive galaxy at z=4.91 with log$(M_{\rm star}/{\rm M}_{\odot}) \lesssim$ 9. We measured the UV luminosity function under different assumptions on the contamination level within our sample. We find that if even a fraction of the candidates is indeed at $z\gtrsim15$, the resulting UV LF points to a very mild evolution compared to estimates at $z<15$, implying a significant tension with existing theoretical models. In particular, confirming our bright ($M_{\text{UV}}<-21$) candidates would require substantial revisions to the theoretical framework. In turn, if all these candidates will be confirmed to be interlopers, we conclude that future surveys may need ten times wider areas to select $M_{\text{UV}}\lesssim-20$ galaxies at $z>15$. Observations in the F150W and F200W filters at depths comparable to those in the NIRCam LW bands are also required to mitigate contamination from rare red objects at z$\lesssim$8.

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