arXiv subjects

Andrea Pallottini

Publications and source records attributed to Andrea Pallottini.

At least 19 recordsLinked to original sources

arXiv2026

REBELS-25: multi-phase morphology and kinematics at z = 7.31

We present a spatially resolved, multi-wavelength study of the massive, star-forming galaxy REBELS-25 at a redshift of $z=7.31$. We combine new high-resolution ALMA [O III]88$μ$m observations with resolution-matched [C II], dust continuum, and JWST/NIRSpec IFU spectroscopy, providing a $\lesssim1$ kpc view of the morphology, interstellar medium (ISM) conditions, and multi-tracer gas kinematics of one of the most mature galaxies known in the reionisation era. We find differing morphologies from the rest-frame UV to far-infrared (FIR) emission, with the UV and optical emission appearing clumpy and irregular, whereas the FIR emission is well-described by near-exponential disc profiles, with [C II] being the most extended. Comparing the resolved UV and FIR emission, we find that obscured star formation contributes $\sim55$-$98$% of the total star formation rate across the galaxy, demonstrating that dust obscuration strongly shapes the observed UV and optical morphology. Resolved ionisation-line diagnostics show no significant variation across the source at $\sim1$ kpc resolution, consistent with broadly similar ISM conditions among the identified regions. Kinematic modelling reveals that both the warm ionised gas, traced by [O III]88$μ$m, and the colder neutral gas, traced by [C II], share the same large-scale rotating structure and are dynamically cold, with ratios of ordered-to-random motion, $V/σ$, of $\sim11$ and 4.5, respectively, although we find evidence for non-circular motions that are not well-described by a simple rotating disc. Overall, these results further support a picture in which REBELS-25 hosts a dusty, chemically enriched, and dynamically cold ISM already in place at $z=7.31$.

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arXiv2026

A first systematic study of [OIII] 88$μ$m at $z>8$: two luminous oxygen lines and a powerful ionized outflow in the first 600 million years

We present deep ALMA Band 7 observations of the [OIII] $88μ$m line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification $μ= 1.4-3.8$), JWST-selected galaxies at $z = 8.5 - 10.3$, with observed magnitudes $-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5$. [OIII] $88μ$m is confidently detected in UNCOVER-10646 at $z=8.5080 \pm 0.0011$ ($15σ$) and DHZ1 at $z=9.3113 \pm 0.0006$ ($6σ$), with both being intrinsically luminous systems [$L_\text{[OIII]} = (1.1 - 1.6) \times 10^9\,L_\odot$] that follow the local [OIII]-SFR relation. [OIII] $88μ$m remains undetected in the two $z>10$ targets, including in the $z=10.07$ X-ray AGN UHZ1, where we obtain a deep limit of $L_\text{[OIII]} < 6 \times 10^7\,L_\odot$. Dust emission is not detected in any individual source nor in a stack ($<3σ$). The high S/N [OIII] $88μ$m detection in UNCOVER-10646 uniquely reveals an additional broad component ($\mathrm{FWHM} = 1366_{-329}^{+473}\,\mathrm{km/s}$; $Δ\mathrm{BIC}\approx20$) indicative of an ionized outflow. We infer a high outflow rate of $\dot{M}_\mathrm{out} = 128_{-46}^{+80}\,M_\odot\,\mathrm{yr}^{-1}$, corresponding to a mass loading factor $η= \dot{M}_\mathrm{out}/\mathrm{SFR} = 2.9_{-1.0}^{+1.8}$ that matches or exceeds theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm and spatially resolve the outflow, this first systematic study at $z>8$ highlights the unique diagnostic power of [OIII] $88μ$m in characterizing galaxies in the early Universe.

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arXiv2026

Direct detection of cool molecular gas in a star-forming galaxy at $z=7.31$

We investigate the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive, star-forming galaxy at $z=7.31$. Deep VLA Q-band and ALMA Band 3 observations reveal CO(3-2) and CO(7-6) emission (both at $\sim3.5σ$), and provide an upper limit on [C I](2-1). From the CMB-corrected CO(3-2) flux-representing the highest-redshift detection of a low-$J$ CO transition to date-we derive a molecular gas mass of $M_{\rm mol}=(1.0\pm0.4)\times10^{11}\,(α_{\rm CO}/(3\,$M$_{\odot}$(K$\,$\kms$\,$pc$^2)^{-1}))\,$M$_{\odot}$, directly confirming the presence of a very massive gas reservoir only $\simeq700\,$Myr after the Big Bang. This implies an extreme gas fraction of $f_{\rm gas}\simeq0.95$, a gas-to-dust ratio of $δ_{\rm GDR}\simeq6\times10^2$, and a depletion timescale of $τ_{\rm dep}\simeq1.2\,$Gyr, broadly consistent with extrapolated scaling relations for main-sequence galaxies at lower redshift. Using the radiative transfer code TUNER, we self-consistently model CO and dust continuum emission in the context of the significant CMB background, constraining ISM properties and recovering $M_{\rm mol}= (1.8^{+1.0}_{-0.9})\times10^{11}\,$M$_{\odot}$, independent of assumptions about $r_{31}$ and $α_{\rm CO}$. We further discuss the use of alternative molecular gas tracers at early epochs. Combining CO and [C II] measurements, we infer an empirical [C II]-to-H$_2$ conversion factor of $α_{\rm [C II]}=(60\pm25)\,$M$_{\odot}$/L$_{\odot}$, suggesting [C II] remains a viable molecular gas tracer in the Epoch of Reionization. These results demonstrate the detectability of low-$J$ CO emission even at $z>7$, paving the way for next-generation facilities, and provide critical insights into the rapid mass assembly of galaxies during the first billion years of cosmic history.

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arXiv2026

The ALPINE-CRISTAL-JWST Survey: The Fast Metal Enrichment of Massive Galaxies at z~5

We present the stellar mass-metallicity relation (MZR) and mass-metallicity-star formation relation ("fundamental metallicity relation"; FMR) of 18 massive (log(M/Msun) = 9.5 - 11) main-sequence galaxies at z~5 from the ALPINE-CRISTAL-JWST sample. This sample complements recent studies by JWST at up to two orders of magnitude lower stellar masses. The metallicities are derived using strong optical lines, and verified by temperature-based oxygen abundance measurements for five galaxies for which faint auroral lines are detected. We find that the metal abundance evolves, on average, from 40% to 60% solar between z~5 and cosmic noon (z~2) at the massive end of the MZR, suggesting already significant metal enrichment at early times. The FMR at z=5 exhibits a 5x larger scatter (preferentially to lower metallicities) compared the local FMR relation. This scatter can be explained by a bursty star formation and the direct build-up of metals in early galaxies as well as differences in age and outflow efficiencies. Capitalizing on all available samples, we find that the observed MZR and FMR over three orders of stellar mass is generally in good agreement with results from cosmological simulation, although some underestimate the metal enrichment at low stellar masses. This may be due to too efficient metal-rich outflows. We show that the ALPINE-CRISTAL-JWST galaxies likely joined the current FMR at z~10 and will evolve into massive (log(M/Msun)~11.4) galaxies with super-solar metallicities by z=0.

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arXiv2026

Clustering constraints on super-early galaxy formation scenarios

The unexpectedly high abundance of bright, blue, super-early galaxies ($z\gtrsim10$) has challenged most pre-JWST models of early galaxy formation and motivated a wide range of proposed explanations. We systematically investigate whether galaxy clustering can discriminate among representative scenarios that reproduce the observed UV luminosity function. Using the Shin-Uchuu dark-matter-only simulation, we populate $z \approx 11$ halos with galaxies according to solutions based on i) attenuation-free, ii) feedback-free bursts, iii) bursty star formation, and iv) primordial black hole models. For each model, we compute the two-point correlation function and predict the galaxy bias for flux-limited samples at different thresholds in the $-20 < {\rm M_{UV}} < -16$ magnitude range. We find that all models predict similar bias values ($b \approx 7$) for faint galaxies (${\rm M_{UV}}\approx-16$), but diverge at ${\rm M_{UV}}\lesssim-18$, as the underlying halo-mass to ${\rm M_{UV}}$ relations differ significantly. In particular, the primordial black hole scenario predicts an almost luminosity-independent bias, whereas the other models generally predict increasing bias with luminosity, reaching $b \approx 14$ for ${\rm M_{UV}} \approx -19$. Current observational estimates of the bias cannot yet rule out any of the models at a significant statistical confidence. More precise measurements from future JWST programs, together with improved theoretical predictions, will be required to break the present degeneracies. Ideally, constraints from a complete sample of galaxies with ${\rm M_{UV}} < -18$ would probe the knee of the $b({\rm M_{UV}})$ function, taking advantage of the difference in model predictions and strengthening our analysis. Although requiring further refinement, galaxy clustering is confirmed to be a promising probe of the physical origin of the JWST high-redshift luminosity function.

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arXiv2026

Intense and extended CIII] emission suggests a strong outflow in JADES-GS-z14-0

JWST has revealed an overabundance of very bright, blue galaxies at z>10, raising fundamental questions about how star formation and feedback operate at Cosmic Dawn. We present new JWST/NIRSpec MSA PRISM/CLEAR spectroscopy of JADES-GS-z14-0 (z=14.18) obtained with the JADES and OASIS programmes. While the rest-frame UV continuum flux level and shape are consistent between the two datasets, the OASIS spectrum shows a 10$σ$ detection of the CIII]$λ\lambda1907,1909$ emission line, with a luminosity three times higher than that measured in the JADES data. This difference is naturally explained by the offset in shutter placement between OASIS and JADES, implying that the CIII] emission is spatially displaced by $\sim400$ pc from the stellar continuum. The non-detection of CIII] in NIRCam medium-band imaging indicates that the emitting region is extended on scales $\gtrsim165$ pc, with a surface brightness below the detection threshold. Interpreting this diffuse, carbon-enriched gas as the result of ongoing or past outflows, we infer a mass outflow rate of $\dot{M}_{\rm out}\sim160~{\rm M_\odot\,yr^{-1}}$. We compare it with the star-formation rate (SFR) and derive a mass-loading factor of $η= \dot{M}_{\rm out}/{\rm SFR} = 4-15$, suggesting highly efficient feedback at very early times. Finally, we show that, if outflows are one of the mechanisms regulating star formation in JADES-GS-z14-0, the instantaneous star-formation efficiency in massive haloes is constrained to $ε_\star\lesssim0.08$. These results support a scenario in which outflows play a crucial role during the earliest phases of galaxy formation. Comparing our results with the current theoretical galaxy formation model, we conclude that a combination of moderate star-formation efficiency and reduced dust attenuation can account for the emergence of luminous galaxies at the highest redshifts.

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arXiv2026

The ALPINE-CRISTAL-JWST Survey: Chemical Abundance Comparison Between the ISM and CGM of Main-Sequence Galaxies at z=4-6

Gaseous halos around galaxies play an important role in galaxy evolution. The exchange of metals from the interstellar medium (ISM) to the circumgalactic medium (CGM) are caused by the formation, feedback, and/or merging history of galaxies. We study the variation in chemical composition between the ISM ($\lesssim3\,$kpc) and CGM ($\sim5-10\,$kpc) for a sample of $M_{\star}>10^{9.5}\,M_{\odot}$ main-sequence galaxies at $4<z<6$ with both JWST and ALMA observations. Using JWST/NIRSpec integral field spectroscopy, we derive the optical line ratios from the ISM and the CGM for our sample focusing on the typical optical lines used for metallicity studies. Our comparison shows that the ISM and the CGM have similar chemical abundances. This indicates that the CGM of these typical $4<z<6$ galaxies is enriched to the level of their ISM in the early universe. Using statistical tests, we find that some of the line ratios show marginal differences between the ISM and CGM. Combined with \texttt{Cloudy} modeling, our results suggest that a difference in ionization level (higher for the ISM) is the dominant reason for the observed ratio difference of oxygen line ratios. There is also indication of a deficit in the nitrogen abundance with respect to oxygen in the CGM, which suggests a delay in redistribution of secondary nitrogen. Finally, an enhanced $F_{[\rm CII]\rm 158μm}/F_{\rm Hα}$ ratio is observed in the CGM, suggesting that feedback and/or mergers play a key role in metal mixing.

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arXiv2026

Amaryllis: a digital twin of the earliest galaxies in the Universe

Synergies between JWST and ALMA are unveiling a population of bright, super-early ($z>10$) galaxies, including systems like GS-z14-0 ($z=14.2$) and GHZ2 ($z=12.3$) with extreme FIR line ratios ([OIII] 88$\,$um / [CII] 158$\,$um $>3$) that challenge galaxy formation models. To address this, we identify a synthetic analog of these sources, "Amaryllis", within the SERRA zoom-in simulations, and track its evolution from $z=16$ to $z=7$. During this period, Amaryllis grows from $\log(M_\star/M_{\odot}) \sim 7.4$ to $10.3$, linking super-early progenitors to the massive galaxy population at the end of reionization. At $z \sim 11.3$, Amaryllis closely matches the observed properties of GS-z14-0, including $M_\star$, SFR, and the luminosity of FIR ([OIII] 88$\,$um) and UV (e.g. CIII]$\,1908$) lines. We find that high [OIII]/[CII] ratios appear during short, merger-driven starburst episodes, when low metallicity ($Z \sim 0.1\,Z_{\odot}$) and high ionization conditions ($U_{\mathrm{ion}} \sim 0.3$) push the ISM far from equilibrium. These extreme FIR line ratios are thus transient and linked to major mergers that ignite strong ionized gas outflows. Strikingly, despite this dynamical activity, Amaryllis develops a rotation-supported gaseous disk ($V/σ\sim 4$-6) by $z \sim 11$, while stars remain dispersion-dominated. This coexistence of ordered gas rotation and merger-driven disturbances occurs within a massive yet typical $Λ$CDM halo, enabling disk formation even at cosmic dawn.

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arXiv2025

High-Redshift Galactic Outflows: Orientation Effects, Kinematics, and Metallicity in TNG50 and SERRA

Context: Recently, JWST/NIRSpec observations have provided the first detections of warm ionised outflows in low-mass galaxies at high redshifts (z>3), revealing an occurrence rate of 25-40% depending on the intensity of the emission lines. This fraction is lower than predicted by simulations, which suggest that fast outflowing gas should be a common feature of all star-forming galaxies in the early Universe. Aims: In order to better understand the discrepancies between simulations and observations, we identify and characterize outflows in high-redshift galaxies using the TNG50 cosmological and SERRA zoom-in simulations. Our study examines how outflow detectability depends on the line of sight, explores the properties of the fast gas, and investigates its relationship with key galactic properties. Methods: We analyse approximately 60000 galaxies from TNG50 and 3000 galaxies from SERRA over the redshift ranges z=3-5 and z=4-5, respectively, spanning stellar masses of Mstar=10^7.5-10^11Msun. Outflows in the immediate vicinity of each galaxy are identified using a Gaussian mixture model algorithm that uses the gas velocity, star-formation-rate, and location as input parameters. We subsequently compare the simulated outflows to those observed in the JWST/JADES NIRSpec survey. Results: Outflow masses in both TNG50 and SERRA broadly reproduce the JWST/JADES measurements within roughly 0.5dex, though simulations tend to predict slightly higher values, suggesting that optical emission lines capture only a fraction of the multiphase outflow. However, simulated outflow velocities are typically an order of magnitude lower than those inferred from observations. TNG50 indicates a clear orientation dependence as outflows in face-on galaxies are approximately 15% more likely to be detected than in edge-on systems, with this difference increasing to nearly 40% for more massive, disc-shaped galaxies.

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arXiv2025

REBELS-IFU: Dust Build-up in Massive Galaxies at Redshift 7

In recent years, observations with the JWST have started to map out the rapid metal enrichment of the early Universe, while (sub)millimeter observations have simultaneously begun to reveal the ubiquity of dust beyond $z\gtrsim6$. However, the pathways that led to the assembly of early dust reservoirs remain poorly quantified, and require pushing our understanding of key scaling relations between dust, gas and metals into the early Universe. We investigate the dust build-up in twelve $6.5 \lesssim z \lesssim 7.7$ galaxies drawn from the REBELS survey that benefit from (i) JWST/NIRSpec strong-line metallicity measurements, (ii) ALMA [CII]-based redshifts and gas masses, and (iii) dust masses from single- or multi-band ALMA continuum observations. Combining these measurements, we investigate the dust-to-gas (DtG), dust-to-metal (DtM), and dust-to-stellar mass (DtS) ratios of our sample as a function of metallicity. While our analysis is limited by systematic uncertainties related to the [CII]-to-H$_2$ conversion factor and dust temperature, we explore a wide range of possible values, and carefully assess their impact on our results. Under a fiducial set of assumptions, we find an average $\log(\mathrm{DtG}) = -3.02 \pm 0.23$, only slightly below that of local metal-rich galaxies. On the other hand, at fixed metallicity our average $\log(\mathrm{DtS}) = -2.15 \pm 0.42$ is significantly larger than that of low-redshift galaxies. Finally, through a comparison to various theoretical models of high-redshift dust production, we find that assembling the dust reservoirs in massive galaxies at $z\approx7$ likely requires the combination of rapid supernova enrichment and efficient ISM dust growth.

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arXiv2025

A warm ultra-luminous infrared galaxy just 600 million years after the Big Bang

We present an Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 continuum detection ($3.3 σ$) of MACS0416_Y1 that confirms the suspected warm dust (91$^{+62}_{-35}$ K) of this Lyman-Break Galaxy (LBG) at $z = 8.3$ with $\log_{10} M_{\ast}/$M$_{\odot} = 9.0 \pm 0.1$. A modified black-body fit to the ALMA Bands 3 through 9 data of MACS0416_Y1 finds an intrinsic infrared luminosity of 1.0$^{+1.8}_{-0.6} \times{} 10^{12}\ \mathrm{L_{\odot}}$, placing this UV-selected LBG in the regime of Ultra Luminous Infrared Galaxies (ULIRGs). Its luminous but modest dust reservoir (1.4$^{+1.3}_{-0.5} \times{} 10^{6}\ \mathrm{M_{\odot}}$) is co-spatial to regions with a UV-continuum slope $β_{\rm UV} \approx -1.5$ as seen by James Webb Space Telescope (JWST) imaging. Although this implies some dust obscuration, the JWST photometry implies less obscured star formation than seen in the complete characterization by ALMA, implying some spatial separation of dust and stars on scales below 200 pc, i.e., smaller than those probed by JWST and ALMA. This source is an extreme example of dust-obscured star formation contributing strongly to the cosmic build-up of stellar mass, which can only be revealed through direct and comprehensive observations in the (sub)mm regime.

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arXiv2025

The ALPINE-CRISTAL-JWST Survey: NIRSpec IFU Data Processing and Spatially-resolved Views of Chemical Enrichment in Normal Galaxies at z=4-6

We present a statistical study of spatially resolved chemical enrichment in 18 main-sequence galaxies at $z=4$--6, observed with \jwst/NIRSpec IFU as part of the ALPINE-CRISTAL-\jwst\ survey. Performing an optimized reduction and calibration procedure, including local background subtraction, light-leakage masking, stripe removal, and astrometry refinement, we achieve robust emission-line mapping on kiloparsec scales. Although line-ratio distributions vary across galaxies in our sample, we generally find mild central enhancements in [O\,\textsc{iii}]/H$β$, [O\,\textsc{ii}]/[O\,\textsc{iii}], [S\,\textsc{ii}]$_{6732}$/[S\,\textsc{ii}]$_{6718}$, H$α$/H$β$, and $L_{\rm Hα}/L_{\rm UV}$, consistent with elevated electron density, dust obscuration, and bursty star formation accompanied by reduced metallicity and ionization parameter. These features point to inside-out growth fueled by recent inflows of pristine gas. Nevertheless, the median metallicity gradient is nearly flat over a few kpc scale, $Δ\log({\rm O/H}) = 0.02 \pm 0.01$ dex kpc$^{-1}$, implying efficient chemical mixing through inflows, outflows, and mergers. From pixel-by-pixel stellar and emission-line characterizations, we further investigate the resolved Fundamental Metallicity Relation (rFMR). Metallicity is described by a fundamental plane with stellar mass and SFR surface densities, but with a stronger dependence on $Σ_{\rm SFR}$ than seen in local galaxies. Our results indicate that the regulatory processes linking star formation, gas flows, and metal enrichment were already vigorous $\sim$1 Gyr after the Big Bang, producing the nearly flat metallicity gradient and a stronger coupling between star formation and metallicity than observed in evolved systems in the local universe.

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arXiv2025

REBELS-IFU: Linking damped Lyman-$α$ absorption to [CII] emission and dust content in the EoR

Neutral gas in galaxies during the Epoch of Reionisation regulates star formation, dust growth, and the escape of ionising photons, making it a key ingredient in understanding both galaxy assembly and reionisation. Yet, direct constraints on the HI content of galaxies at z>6 have been scarce. With JWST, Ly$α$ damping wings in galaxy spectra can now provide a direct probe of this neutral component. We analyse JWST/NIRSpec prism spectra of 12 UV-luminous galaxies from the REBELS-IFU program at z~6.5-7.7, deriving HI column densities by modelling Ly$α$ damping wings. Significant damped Ly$α$ absorption is detected in eight galaxies, with $N_{\mathrm{HI}}\gtrsim10^{21}$ cm$^{-2}$. We use the column densities and sizes derived for these sources to estimate their HI mass and compare with $L_{\mathrm{[CII]}}$-$M_{\mathrm{HI}}$ calibrations. The resulting HI masses show a tentative correlation with those inferred from [CII], although the [CII]-based estimates are systematically larger, suggesting that the HI reservoirs may extend beyond the [CII]-emitting gas. We also combine the DLA-based measurements with FIR-derived dust-to-gas ratios, dust attenuation, and gas-phase metallicities. No correlation is found between DLA-based and FIR-based dust-to-gas ratios, but combining the REBELS-IFU sample with literature samples at lower metallicities reveals a strong correlation between $A_{\mathrm{V}}/N_{\mathrm{HI}}$ and metallicity. These findings suggest that by $z\sim7$ massive galaxies can already host substantial, enriched reservoirs of neutral gas and dust, consistent with $A_{\mathrm{V}}$/$N_{\mathrm{HI}}$-metallicity trends at lower redshift. At the highest redshifts ($z>8$), however, we see tentative evidence for systematically lower $A_{\mathrm{V}}$/$N_{\mathrm{HI}}$ at fixed metallicity, which may point to pristine gas accretion or more efficient dust destruction/expulsion.

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arXiv2025

Lessons Learned from Studying H$α$ Galaxy Kinematics with Mock JWST/NIRSpec IFU Observations at $z > 6$

Galaxies with a disk morphology have been established at $z > 9$ with the James Webb Space Telescope (JWST). However, confirming their disky nature requires studying their gas kinematics, which can be challenging when relying solely on the warm gas observed by JWST. Unlike the cold gas traced by the Atacama Large Millimetre/Submillimetre Array (ALMA), warm gas is sensitive to outflows, complicating the interpretation of the disk dynamics. This elicits the question of how to compare information obtained from varied tracers, as well as how to physically interpret the low angular and spectral resolution observations generally available at high redshift. We address these challenges through comparative kinematic analysis of idealised and realistic NIRSpec/IFU mock observations derived from two galaxies in the SERRA suite of cosmological zoom-in simulations. With these synthetic data, we determine the robustness of dynamical information recovered from typical IFU observations, and test widely-used criteria for identifying disks and gaseous outflows at high redshift. We find that at the typical NIRSpec/IFU spectral and angular resolution ($\sim$ 0.05"/pixel), non-circular motions due to inflows or outflows can mimic the smooth velocity gradient indicative of a disk, and bias measured velocity dispersion upwards by a factor of $2-3\times$. As a result, the level of rotational support may be underestimated in the NIRSpec/IFU observations. However, the recovered dynamical mass appears to be relatively robust despite biases in $v_\text{rot}$ and $σ$.

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arXiv2025

Beyond the first galaxies primordial black holes shine

The presence of nine candidate galaxies at $z=17$ and $z=25$ discovered by the James Webb Space Telescope in relatively small sky areas, if confirmed, is virtually impossible to reconcile with the predictions of the current galaxy formation model. We show here that the implied UV luminosity density can be produced by a population of primordial black holes (PBHs) of mass $M_{\rm PBH} = 10^{4-5} \, M_{\odot}$ residing in low-mass halos ($M_h \approx 10^{7} \, M_{\odot}$), and accreting at a moderate fraction of the Eddington luminosity, $λ_E \simeq 0.36$. These sources precede the first significant episodes of cosmic star formation. At later times, as star formation is ignited, PBH emission becomes comparable to, or subdominant with respect to, the galactic emission. This PBH+galaxy scenario reconciles the evolution of the UV luminosity function (LF) from $z=25$ to $z=11$. If ultra-early sources are powered purely by accretion, this strongly disfavours seed production mechanisms requiring the presence of stars (massive stars, Pop III stars, or clusters), or their UV radiation (direct collapse BHs), leaving PBHs as the only alternative solution available so far. Alternative explanations, such as isolated, large clusters ($\approx 10^7 \,M_{\odot}$) of massive ($m_\star =10^3 M_{\odot}$) Pop III stars are marginally viable, but require extreme and unlikely conditions that can be probed via UV and far-infrared (FIR) emission lines or gravitational waves.

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arXiv2025

ALMA Observations of [OI]145um and [NII]205um Emission lines from Star-Forming Galaxies at $z\sim7$

We present results of new observations of [OI]145um and [NII]205um emission lines from four star-forming galaxies at redshifts between $z=6.58$ and $7.68$ that have previous detections of \Ciium\ and dust continua. Using ALMA, we successfully detect [OI]145um emission from all targets at $>4\,σ$ significance. However, [NII]205um emission is undetected in all galaxies (SNR $<3.5\,σ$) except for a tentative detection from A1689-zD1. From the observed high [CII]/[NII] emission line ratios ($\gtrsim20 - 80$), we find that most of the [CII]158um emission arise from neutral gas regions ($3\,σ$ lower limits of $\gtrsim 74 - 96\%$). From [OI]145um, [CII]158um lines, and infrared luminosities, we estimate the neutral gas densities of $n_{\rm H}=10^{3.5}$ - $10^6\,{\rm cm^{-3}}$ and the far-ultraviolet (FUV) radiation strengths of $G_0\sim10^{2.5}$-$10^{3}$. While the neutral gas densities are similar to those of high-redshift starburst galaxies, the FUV strengths are lower compared to both local and high-redshift starbursts. Finally, we estimate atomic hydrogen masses using [OI]145um emission lines and the oxygen abundances measured from recent JWST observations. We find gas mass ratios of $f_{\rm gas}\sim0.3$ - $0.8$, which are similar to earlier studies using [CII]158um. Starting from this pilot observation, future large [OI]145um emission line surveys will provide us with currently little-known neutral gas properties of star-forming galaxies in the early Universe.

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arXiv2025

Strong nebular HeII emission induced by He$^+$ ionizing photons escaping through the clumpy winds of massive stars

The origin of nebular HeII-emission in both local and high-redshift galaxies remains an unsolved problem. Various theories have been proposed to explain it, including HeII-ionization by high mass X-ray binaries, ultra-luminous X-ray sources, or "stripped" He stars, shock ionization, and hidden AGNs. All these theories have shortcomings, however, leaving the cause of nebular HeII emission unclear. We investigate the hypothesis that the photons responsible for driving nebular HeII emissions are produced by the evolution of single massive stars and/or WR stars. We combine models of stellar evolution with population synthesis and nebular models to identify the most favorable scenarios for producing nebular HeII via this channel. We find that, if WR winds are clumpy enough to become close to optically thin, stellar populations with a wide range of metallicities and rotation rates can produce HeII ionizing photons at rates sufficient to explain the observed nebular $I(HeII)/I(\mathrm{H}β)$ ratio $\sim 0.004-0.07$ found in HeII-emitting galaxies. Metal-poor, rapidly rotating stellar populations ($[\mathrm{Fe}/\mathrm{H}]=-2.0$, $v/v_\mathrm{crit}=0.4$) also reach these levels of HeII production even for partially clumpy winds. These scenarios also yield HeII, H$β$, and "Blue-Bump" line equivalent widths comparable to those observed in HeII emitters. Only for laminar, non-clumpy winds, do we fail to find combinations of metallicity and stellar rotation rate that yield $I(HeII)/I(\mathrm{H}β)$ values as high as those observed in HeII-emitters. Contrary to previous findings, we conclude that single WR stars can be a strong source for nebular HeII emission if their winds are sufficiently clumpy allowing significant escape of hard ionizing photons.

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arXiv2025

Can primordial black holes explain the overabundance of bright super-early galaxies?

JWST is detecting an excess of high-redshift ($z\gtrsim 10$), bright galaxies challenging most theoretical predictions. To address this issue, we investigate the impact of Primordial Black Holes (PBHs) on the halo mass function and UV luminosity function (LF) of super-early galaxies. We explore two key effects: (i) the enhancement of massive halos abundance due to the compact nature and spatial distribution of PBHs, and (ii) the luminosity boost, characterized by the Eddington ratio $λ_E$, due to Active Galactic Nuclei (AGN) powered by matter accretion onto PBHs. We build an effective model, calibrated using data at lower redshifts ($z\approx 4-9$), to derive the evolution of the LF including the additional PBH contribution. Via Bayesian analysis, we find that: (a) Although a small fraction ($\log f_{\rm PBH} \approx -5.42$) of massive ($\log M_{\rm PBH} / {\rm M_{\odot}} \approx 8.37$), non-emitting ($λ_E=0$) PBHs can explain the galaxy excess via the halo abundance enhancement, this solution is excluded by CMB $μ$-distortion constraints on monochromatic PBHs. (b) If PBHs power an AGN emitting at super-Eddington luminosity ($λ_E \approx 10$), the observed LF can be reproduced by a PBH population with characteristic mass $\log M_{\rm PBH} / {\rm M_{\odot}} \approx 3.69$ constituting a tiny ($\log f_{\rm PBH} \approx -8.16$) fraction of the cosmic dark matter content. In the AGN scenario, about 75% of the observed galaxies with ${\rm M_{UV}}=-21$ at $z=11$ should host a PBH-powered AGN and typically reside in low mass halos, $M_h = 10^{8-9} {\rm M_{\odot}}$. These predictions can be tested with available and forthcoming JWST spectroscopic data. We note that our analysis considers a lognormal PBH mass function and compares its parameters with monochromatic limits on PBH abundance. Further work is required to relax such limitations.

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