arXiv subjects

Enrico Garaldi

Publications and source records attributed to Enrico Garaldi.

At least 19 recordsLinked to original sources

arXiv2026

Probing Helium Reionization Through the $^{3}\mathrm{He}^{+}$ Hyperfine Transition Line

We investigate the hyperfine transition of $^{3}\mathrm{He}^{+}$ as a promising probe of the IGM during the final stages of helium reionization. Utilising the most recent helium reionization simulation, we generate three-dimensional maps of the 3.5cm ($8.67$,GHz) differential brightness temperature and analyze its evolution. Our `fiducial' results show that the volume-averaged brightness temperature declines rapidly from $\sim 10^{-2}\,μ\mathrm{K}$ (in the `optimistic' approach, i.e. $T_{\rm s} = T_{\rm gas}$, $\sim 1,μ\mathrm{K}$) at $z = 3$ to $\sim 8 \times 10^{-5},μ\mathrm{K}$ ($\sim 2.5 \times 10^{-3},μ\mathrm{K}$) by $z = 2.3$, tracing the He,\textsc{ii} to He,\textsc{iii} transition driven by quasars. The power spectrum of the 3.5cm signal exhibits a scale-dependent evolution, peaking on small scales and declining as reionization progresses. We explore the cross-correlation of the 3.5cm transition line with the distribution of AGNs, which shows a transition from positive to negative correlation as ionized regions grow. Although current observational upper limits lie several orders of magnitude above theoretical predictions, future radio arrays such as \texttt{SKA-mid} offer promising prospects. Overall, this study highlights the $^{3}\mathrm{He}^{+}$ hyperfine transition as a sensitive tracer of the thermal and ionization history of the IGM during helium reionization. The predicted signal is also subject to additional uncertainty from spin temperature modelling, and the true amplitude is expected to lie between the limiting cases explored in this work.

astro-ph.CO↗

arXiv2026

Gas distributions inside and around haloes in the alternative dark matter simulations AIDA-TNG

The nature of Dark Matter (DM) is one of the greatest mysteries of modern astrophysics. While the standard Cold DM (CDM) model successfully explains observations on most astrophysical scales, DM particles have not yet been detected, leaving room for a plethora of different models. In order to identify their observable signatures, we use the AIDA-TNG cosmological simulation suite to predict the distributions of gas and neutral hydrogen (HI) in the CDM, Self-Interacting DM (SIDM), velocity-dependent SIDM (vSIDM), and Warm DM (WDM) models. We find that the DM models investigated have very limited impact on the median gas and HI profiles of haloes. Nevertheless, for the most massive haloes ($M_{\rm vir}\sim10^{14}\,\mathrm{M}_\odot$), we find that DM self-interactions can reduce the central potential, thereby lowering gravitational heating and the central gas temperature. We find that, in all models, the halo-to-halo variation in the HI profiles is explained by AGN feedback, and that the specific characteristics of the DM model are largely subdominant. Nevertheless, we detect some systematic difference in the case of SIDM, with more HI surviving close to the centre with respect to other models. We provide fitting functions for the gas and HI profiles. We investigate the galaxy-Ly$α$ cross-correlation function (\galacc) for different halo masses, redshifts and observational strategies. We find that at $z=0$ vSIDM can be distinguished from CDM in haloes with $10^{12}\lesssim M_{\rm vir}\lesssim10^{13}\,{\rm M}_\odot$, while SIDM1 can be distinguished from CDM in haloes with $M_{\rm vir}\gtrsim10^{13}\,{\rm M}_\odot$. We estimate that statistically-robust detection requires sampling $\sim160$ haloes with $\sim20$ sightlines each, a task that can be achieved with current and future facilities like WEAVE, 4MOST, PFS, ELT and WST.

astro-ph.GA↗

arXiv2026

The THESAN-ZOOM project: clumpiness of high-redshift galaxies and its connection to bursty star formation

Recent JWST observations have revealed diverse high-redshift galaxy morphologies, including a population with irregular and clumpy structures. The physical origin of these structures, and the extent to which observational biases shape their appearance, remain uncertain. We present a power-spectrum-based method for quantifying galaxy clumpiness across spatial scales, using the radiation-hydrodynamic simulation suite THESAN-ZOOM, which employs a state-of-the-art galaxy formation model that resolves the multiphase interstellar medium (ISM). Although the total stellar mass distributions in THESAN-ZOOM galaxies are usually smooth, clumpy structures appear in the H$α$, far-ultraviolet (FUV), and optical light distributions. Tracers sensitive to shorter-timescale star formation exhibit more pronounced small-scale structure ($\sim10^{2}$--$10^{3}{\rm pc}$). The corresponding projected light spectra follow $P(k)\propto k^{-1}$ to $k^{-2}$, with progressively shallower slopes for tracers sensitive to more recent star formation, reflecting enhanced small-scale power and greater spatial intermittency in young stellar populations. This behaviour is consistent with a highly compressible, shock-dominated ISM in which stellar feedback and outflows reorganise dense gas into filamentary and clumpy structures. We also find that galaxy clumpiness depends on the treatment of stellar feedback. Weaker early stellar feedback enhances small-scale power in both the mass and light distributions. Clumpiness also varies strongly over the bursty star formation cycle, implying that observed samples may be biased towards galaxies caught in phases of elevated star formation. Galaxy clumpiness, therefore, could provide a complementary probe of the bursty star formation in the early Universe.

astro-ph.GA↗

arXiv2026

Dynamic Zoom Simulations of structure formation beyond standard cosmology

(Abridged) A thorough interpretation of the current and upcoming generation of cosmological observations requires unprecedented large-scale, high-resolution simulations spanning multiple cosmological models and parameters. The realization of these computationally demanding simulations poses a crucial technical challenge. We present beyond - $Λ$CDM implementations of the Dynamic Zoom Simulations (DZS) method, a performance-enhancing technique tailored for large-scale simulations that produce lightcone-like outputs. This approach dynamically decreases the resolution of a simulation in the regions that are not in causal connection with the observer, saving computational resources without directly affecting the physical properties within the lightcone. We implemented the DZS algorithm in two state-of-the-art codes supporting non-standard cosmologies, namely modified $f(R)$ gravity in Arepo and dark sector interactions in Gadget4. We analyzed result accuracy and performance gains across resolution, simulation volume and model by comparing runs performed with and without the DZS algorithm. Our DZS reproduce the lightcone halo mass function, sky-projected massmaps, and matter and weak lensing convergence power spectra with an accuracy of $\simeq$ 0.1% or higher in most cases. In terms of performance, DZS runs in our test simulations can save up to $\sim$ 50% runtime compared to the non-DZS counterparts. A scaling to larger simulated volumes suggests that performance gains could improve by an additional $\sim$ 20% at the resolution levels of current state-of-the-art simulations. The validation of the DZS algorithm in non-standard models demonstrates that this technique can enable cost effective, large-scale ($\gtrsim$ 1 cGpc/h) simulations with state-of-the-art resolution, providing the computational framework needed to constrain and help the interpretation of forthcoming data.

astro-ph.CO↗

arXiv2026

Magnetogenesis by galactic processes: impact on circumgalactic and intergalactic fields

We investigate the origin and evolution of cosmic magnetic fields using a suite of large-volume cosmological magnetohydrodynamic simulations (L$_\mathrm{box}=25$ Mpc/h) run with the moving-mesh code AREPO. Atop the IllustrisTNG galaxy formation model, we implement additional recipes for magnetogenesis in which magnetic energy is injected during supernovae (SNe) and supermassive black hole (SMBH) feedback events, and compare these to simulations initialized with uniform primordial seed fields. Halo magnetic field strengths at $z=0$ are largely similar across seeding models and are primarily amplified and sustained by small-scale and halo-scale dynamo action. Nevertheless, we find differences in magnetic field topology, with SMBH-driven models exhibiting systematically smaller coherence lengths than primordial-only and SNe-only runs. We find that feedback-driven injection accelerates the onset of dynamo growth, leading to more rapid convergence of magnetic field strengths with numerical resolution, particularly in low-mass halos. In the intergalactic medium (IGM), SNe-only injection underproduces magnetic fields relative to inferred lower limits from $γ$-ray cascade constraints at both $z=0$ and $z \sim 3$, whereas our specific SMBH-based injection prescription satisfies present-day constraints but remains in mild tension at high redshifts. Reconciling these specific high-$z$ constraints therefore likely requires either modified feedback prescriptions or an additional primordial seeding component.

astro-ph.GA↗

arXiv2026

The Thesan-Zoom project: bursty star formation is incompatible with prolonged dust survival

Cosmic dust is a key regulator of galaxy evolution, but its build-up and survival in the first billion years remain poorly constrained. We present a systematic analysis of dust in the thesan-zoom suite of radiation-hydrodynamical zoom-in simulations, which self-consistently model dust formation, growth, destruction, and its coupling to radiative transfer in galaxies at $z \geq 3$, a multi-phase ISM and bursty star formation histories. The simulated galaxies reproduce the observed trends of dust-to-gas and dust-to-metal ratios with gas metallicity, while showing a dust deficit at high specific star-formation rates. They also broadly match observed dust temperatures and UV-IR spatial offsets. We find that dust and its properties are strongly time-variable and tightly linked to bursty star formation, with short-lived IR-bright phases (median duration of $20.3^{+2.3}_{-2.4}$ Myr) and longer dust-poor phases, naturally producing a correlation between dust temperature and distance from the star-forming main sequence. The predicted attenuation at $1500$ Å is low compared to observations, even when including unresolved dust through post processing, indicating that a mechanism able to shield dust from strong feedback events is necessary to reconcile our galaxy formation model with observations. In our model, bursty star formation prevents the survival of large dust reservoirs ($M_{dust} / M_{star} \geq 10^{-3}$) over a significant fraction of cosmic time. This implies that bursty star formation can produce the observed overabundance of UV-bright galaxies at $z \geq 10$ only if it rapidly settles down by $z \sim 8$ (where large dust reservoirs are detected). It is also possible that our models lack physical ingredients or emergent phenomena that aid the survival of dust. Future observations of high-redshift dust will be key to diagnose the physical mechanism at play in the first galaxies.

astro-ph.GA↗

arXiv2026

Globular cluster abundance patterns inherited from giant molecular clouds

Globular clusters exhibit large star-to-star variations and anticorrelations in their light element abundances that are commonly interpreted in terms of in-cluster self-enrichment, in which ejecta from early-forming cluster stars pollute the gas from which later stars form over millions of years. Yet proposed self-enrichment scenarios suffer from a severe mass-budget problem or invoke exotic stellar populations. Using cosmological radiation-hydrodynamic simulations with a standard chemical enrichment model, we identify a population of giant molecular clouds whose internal abundance patterns reproduce several key globular cluster signatures: large light-element abundance spreads and nitrogen-oxygen anticorrelations at nearly constant iron abundance. These clouds form at the restart of star-formation activity after an earlier starburst, where previously ejected oxygen-rich gas collides with nitrogen-rich galactic gas, and are sites of dense star-cluster formation. In this picture, the chemical abundance patterns of globular clusters need not require extended in-cluster star formation, but can be inherited at birth from chemically structured interstellar gas shaped by the baryon cycle. Globular clusters therefore provide a fossil record of chemical enrichment and gas flows in high-redshift galaxies.

astro-ph.GA↗

arXiv2026

Development of TIFUUN: Terahertz Integral Field Units with Universal Nanotechnology

TIFUUN (THz Integral Field Units with Universal Nanotechnology) is an ultra-wideband mm-submm wave imaging spectrometer that capitalizes on the highly scalable integrated superconducting spectrometer technology. TIFUUN has two slots for integral field units (IFUs), which can jointly be optimized as open-hardware for each astronomical observation in terms of spatial and spectral coverage. These IFUs can have observation frequencies in the range of 90--360 GHz, with spectral resolution up to $R\equiv F/ΔF \le 1,000$, with up to $\sim$18,000 kinetic inductance detectors (shared by the two IFUs with a flexible ratio). The ultra-wide 4:1 (2 octave) bandwidth optics fits in a remarkably compact volume, by means of thin silicon lenses and a high chief ray angle design. The first pair of IFUs are being developed for the SUBLIME (Study of the Universe By Line Intensity Mapping Experiments) experiment that aims to map CII emission at redshift $\sim$6 to trace the cosmic large-scale structure and the buildup of galaxies during reionization, using TIFUUN on the ASTE 10-m telescope. The scalability, flexibility and compactness makes TIFUUN a highly compatible and portable system suited also for upcoming telescope facilities in the vicinity, such as FYST and AtLAST/LST.

astro-ph.IM↗

arXiv2026

From THESAN-ZOOM to JWST: Predicting ionizing photon escape and the rise of UV-bright reionization sources

Understanding the sources and evolution of cosmic reionization remains a central challenge in astrophysics, with the escape of ionizing Lyman-continuum (LyC) photons from early galaxies representing a major uncertainty. In this work, we use more than 35,000 galaxy realisations from the THESAN-ZOOM cosmological radiation-hydrodynamic simulations to identify indirect diagnostics of the LyC photon escape fraction ($f_\mathrm{esc}$) and the LyC photon escape rate ($\dot{N}_\mathrm{ion,esc}$) across the redshift range $z=3-16$. We train random forest regression models using these diagnostics to predict both quantities. We present four models: two trained with the full set of simulation-derived indicators to predict $f_\mathrm{esc}$ and $\dot{N}_\mathrm{ion,esc}$, and two restricted to observables accessible to JWST photometric surveys. We find the 10-to-100$\,$Myr star-formation rate ratio ($\mathrm{SFR}_{10} / \mathrm{SFR}_{100}$) and the gas-to-stellar mass ratio ($M_\mathrm{gas} / M_*$) to be the strongest diagnostics of $f_\mathrm{esc}$, suggesting a strong relationship between ionizing photon escape and gas clearing through bursty star formation. In contrast, rest-frame UV ($1500 \, Å$) absolute magnitude ($M_\mathrm{UV}$) dominates $\dot{N}_\mathrm{ion,esc}$ prediction. Motivated by the strong predictive power of $M_\mathrm{UV}$, we combine observed UV luminosity functions with derived $\dot{N}_\mathrm{ion,esc} - M_\mathrm{UV}$ relations to construct histories of reionization. These are consistent with observational constraints, avoiding the recently reported crisis in the ionizing photon budget. Our analysis suggests that the bulk of reionization occurred rapidly after $z \approx 8$, driven by UV-bright galaxies, with the $M_\mathrm{UV} < -17$ populations providing the dominant contribution.

astro-ph.GA↗

arXiv2026

Probing Direct Contributions of Galaxies and AGN to Cosmic Reionization in a Quasar Field J0226+0302 with JWST NIRCam and NIRSpec

We present JWST Cycle 2 NIRCam and NIRSpec observations in a quasar field J0226+0302 at z=6.5412 to probe the direct connections between the intergalactic medium (IGM), galaxies, and AGN during reionization. This field was previously observed by the JWST ASPIRE program and eight [OIII]-emitting galaxies were detected at 5.3<z<6.4 with a single NIRCam pointing. Using new NIRCam and NIRSpec observations, we identify 65 additional line-emitting galaxies at 5.3<z<6.4. The IGM-galaxy cross-correlation function shows a ~2 sigma excess IGM transmission at ~10-40 cMpc from galaxies when compared with the average IGM transmission, suggesting a significant contribution from regions traced by star-forming galaxies to the local ionizing background during reionization. The IGM-galaxy cross-correlation function is consistent with THESAN simulations with an IGM neutral fraction of 5%-7% and an average ionizing photon escape fraction f_esc of 6% from galaxies. Among 49 line-emitting galaxies observed by NIRSpec, we identify four AGN through detection of broad H-alpha emission lines with an AGN fraction of (8+/-4)%. By measuring the IGM effective optical depth around the AGN and the IGM-AGN cross-correlation function, we find that the IGM transmission is higher within 5 cMpc/h of the AGN than around the majority of [OIII] emitters. We interpret the excess IGM transmission as resulting from the local radiation enhancement by the AGN, and estimate f_esc of 50%-100% of the AGN from the IGM-AGN cross-correlation function. Future JWST NIRSpec observations in quasar fields will yield a more constraining IGM-AGN cross-correlation function, providing further insights into the roles of galaxies and AGN in reionization.

astro-ph.GA↗

arXiv2026

The THESAN project: Lyman-alpha emitters as probes of ionized bubble sizes

We use the THESAN radiation-hydrodynamics simulations to investigate how Lyman-$α$ emitters (LAEs) trace ionized bubble sizes during the Epoch of Reionization. We generate realistic LAE catalogs by combining accurate intrinsic Ly$α$ production and intergalactic transmission with an empirical model for dust absorption and gas outflows. By calibrating to observationally-constrained Ly$α$ luminosity functions, we reproduce the rapid decline in Ly$α$ visibility toward higher redshifts while revealing mild tensions in LAE fractions near the end of reionization. Before the midpoint of reionization, galaxies within larger line-of-sight bubbles ($\gtrsim 10$ cMpc) have higher observed Ly$α$ luminosity and equivalent width (EW), demonstrating that the evolving LAE fraction provides a practical statistical tracer for bubble size. These correlations weaken as percolation progresses and the IGM becomes increasingly ionized. In LAE selected samples with $L_{\text{Ly}α} > 10^{41.5}\ \text{erg s}^{-1}$, Ly$α$ properties correlate with bubble size more strongly than UV magnitude, especially at $z \gtrsim 7$. This simulation-based framework maps LAE selections to bubble-size statistics, clarifies biases in more idealized models, and will supply public catalogs to interpret current and forthcoming JWST and narrow-band LAE surveys in terms of the evolving topology of reionization.

astro-ph.GA↗

arXiv2026

Metallicity Gradients in Modern Cosmological Simulations II: The Role of Bursty Versus Smooth Feedback at High-Redshift

The distribution of gas-phase metals within galaxies encodes the impact of stellar feedback on galactic evolution. At high-redshift, when galaxies are rapidly assembling, feedback-driven outflows and turbulence can strongly reshape radial metallicity gradients. In this work, we use the FIRE-2, SPICE, Thesan and Thesan Zoom cosmological simulations -- spanning a range of stellar feedback from bursty (time-variable) to smooth (steady) -- to investigate how these feedback modes shape gas-phase metallicity gradients at $3 10^{9}~{\rm M_\odot}$. These results demonstrate that bursty stellar feedback provides sufficient turbulence to prevent strong negative gradients from forming, while smooth stellar feedback does not generically allow for efficient radial redistribution of metals thereby keeping gradients steep. Finally, we compare with recent observations, finding that the majority -- but, notably, not all -- of the observed gradients may favor a bursty stellar feedback scenario. In all, these results highlight the utility of high-resolution observations of gas-phase metallicity at high-redshift as a key discriminator of these qualitatively different feedback types.

astro-ph.GA↗

arXiv2026

The THESAN-ZOOM project: Mystery N/O more -- uncovering the origin of peculiar chemical abundances and a not-so-fundamental metallicity relation at $3<z<12$

We present an analysis of metallicities and chemical abundances at $3 -0.6$) without the need for exotic yields in our chemical network. Instead, bursty star formation naturally generates order-of-magnitude excursions in N/O on $\lesssim$100 Myr timescales due to temporally differential galactic winds; after a starburst, stellar feedback expels gas, leaving a large population of asymptotic-giant-branch stars to dominate the enrichment of the relatively low-mass interstellar medium. NRGs lie below the main sequence and typically exhibit $\mathrm{EW}[H$β$]\lesssim40$ Å, in apparent tension with observed high-EW NRGs. This tension is reconciled if observed NRGs are in the initial stages of a subsequent starburst, illuminating previously enriched gas, which is supported by the finding of high SFR surface density nitrogen-rich giant molecular clouds.

astro-ph.GA↗

arXiv2026

The THESAN-ZOOM project: Burst, quench, repeat -- unveiling the evolution of high-redshift galaxies along the star-forming main sequence

Characterizing the evolution of the star-forming main sequence (SFMS) at high redshift is crucial to contextualize the observed extreme properties of galaxies in the early Universe. We present an analysis of the SFMS and its scatter in the THESAN-ZOOM simulations, where we find a redshift evolution of the SFMS normalization scaling as $\propto (1+z)^{2.64\pm0.03}$, significantly stronger than is typically inferred from observations. We can reproduce the flatter observed evolution by filtering out weakly star-forming galaxies, implying that current observational fits are biased due to a missing population of lulling galaxies or overestimated star-formation rates. We also explore star-formation variability using the scatter of galaxies around the SFMS ($σ_{\mathrm{MS}}$). At the population level, the scatter around the SFMS increases with cosmic time, driven by the increased importance of long-term environmental effects in regulating star formation at later times. To study short-term star-formation variability, or ''burstiness'', we isolate the scatter on timescales shorter than 50 Myr. The short-term scatter is larger at higher redshift, indicating that star formation is indeed more bursty in the early Universe. We identify two starburst modes: (i) externally driven, where rapid large-scale inflows trigger and fuel prolonged, extreme star formation episodes, and (ii) internally driven, where cyclical ejection and re-accretion of the interstellar medium in low-mass galaxies drive bursts, even under relatively steady large-scale inflow. Both modes occur at all redshifts, but the increased burstiness of galaxies at higher redshift is due to the increasing prevalence of the more extreme external mode of star formation.

astro-ph.GA↗

arXiv2026

The THESAN-ZOOM project: central starbursts and inside-out quenching govern galaxy sizes in the early Universe

We explore the evolution of galaxy sizes at high redshift ($3 < z < 13$) using the high-resolution THESAN-ZOOM radiation-hydrodynamics simulations, focusing on the mass range of $10^6\,\mathrm{M}_{\odot} < \mathrm{M}_{\ast} < 10^{10}\,\mathrm{M}_{\odot}$. Our analysis reveals that galaxy size growth is tightly coupled to bursty star formation. Galaxies above the star-forming main sequence tend to form stars in a central starburst, which decreases their radial size. These galaxies quench inside-out, causing spatially extended star formation and increasing their radial size, leading to oscillatory behavior around the size-mass relation. Notably, we find a positive intrinsic size-mass relation at high redshift, consistent with observations but in tension with large-volume simulations. We attribute this discrepancy to the bursty star formation captured by our multi-phase interstellar medium framework, but missing from simulations using the effective equation-of-state approach with hydrodynamically decoupled feedback. We also find that the normalization of the size-mass relation follows a double power law as a function of redshift, with a break at $z\approx6$, because the majority of galaxies at $z > 6$ show rising star-formation histories, and therefore are in a compaction phase. We demonstrate that H$α$ emission is systematically extended relative to the UV continuum by a median factor of 1.7, consistent with recent JWST studies. However, in contrast to previous interpretations that link extended H$α$ sizes to inside-out growth, we find that Lyman-continuum (LyC) emission is spatially disconnected from H$α$. Instead, a simple Strömgren sphere argument reproduces observed trends, suggesting that extreme LyC production during central starbursts is the primary driver of extended nebular emission.

astro-ph.GA↗

arXiv2026

Overmassive black holes in the early Universe can be explained by gas-rich, dark matter-dominated galaxies

JWST has revealed the apparent evolution of the black hole (BH)-stellar mass ($M_\mathrm{BH}$-$M_\rm{\ast}$) relation in the early Universe, while remaining consistent with the BH-dynamical mass ($M_\mathrm{BH}$-$M_\mathrm{dyn}$) relation. We predict BH masses for $z>3$ galaxies in the high-resolution THESAN-ZOOM simulations by assuming the $M_\mathrm{BH}$-$M_\mathrm{dyn}$ relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed $M_\mathrm{BH}$ distribution, including overmassive BHs relative to the local $M_\mathrm{BH}$-$M_\mathrm{\ast}$ relation. We find that $M_\mathrm{BH}/M_\mathrm{\ast}$ declines with $M_\mathrm{\ast}$, evolving from $\sim$0.1 at $M_\mathrm{\ast}=10^6\,\mathrm{M_\odot}$ to $\sim$0.01 at $M_\mathrm{\ast}=10^{10.5}\,\mathrm{M_\odot}$. This trend reflects the dark matter ($f_\mathrm{DM}$) and gas fractions ($f_\mathrm{gas}$), which decrease with $M_\mathrm{\ast}$ but show little redshift evolution down to $z=3$, resulting in small $M_\mathrm{\ast}/M_\mathrm{dyn}$ ratios and thus overmassive BHs in low-mass galaxies. We use $\texttt{Prospector}$-derived stellar masses and star-formation rates to infer $f_\mathrm{gas}$ across 48,022 galaxies in JADES at $3<z<9$, finding excellent agreement with our simulation. Our results demonstrate that overmassive BHs would naturally result from a fundamental $M_\mathrm{BH}$-$M_\mathrm{dyn}$ relation and be typical of the gas-rich, dark matter-dominated nature of low-mass, high-redshift galaxies. Such overmassive BHs may strongly influence early galaxy formation, and we caution that our approach does not include the self-consistent BH-galaxy co-evolution required for a complete understanding.

astro-ph.GA↗

arXiv2026

Influence of the Spectral Energy Distribution of Reionization-Era Sources on the Lyman-$α$ Forest

Interpreting Lyman-$α$ forest properties during the epoch of reionization requires assumptions about the spectral energy distribution (SED) of ionizing sources. These are often simplified to blackbody or power-law spectra, potentially overlooking contributions from high-energy processes. In this work, we investigate how different SED models of reionization-era sources shape the thermal and ionization state of the intergalactic medium (IGM) and imprint on the Ly$α$ forest during the late stages of reionization. We perform $3D$ radiative transfer simulations with CRASH, post-processed on Sherwood-type hydrodynamical outputs, exploring both physically motivated SEDs including X-ray binaries, Bremsstrahlung from shock-heated interstellar medium, and binary stars and idealized blackbody and power-law spectra. While the large-scale morphology of ionized regions is broadly similar across all models, harder spectral components extend partially ionized zones, produce larger He III regions, and heat the surrounding IGM. By adopting simplified spectra there is the risk of underestimating the contribution of high-energy sources, which for most models we consider are found to alter the effective optical depth, the flux power, and the local transmissivity within the current $\sim 1 σ$ measurement uncertainties. The differences across models are most pronounced in the behavior of the proximity zone and in the power at intermediate scales, offering the most promising diagnostics to disentangle source populations. With upcoming high precision measurements from ELT and DESI, realistic SED modelling will be essential for robustly connecting Ly$α$ forest observations to the sources driving the end of reionization.

astro-ph.GA↗

arXiv2026

The THESAN-ZOOM project: The Hidden Neighbours of OI Absorbers during Reionization

Metal absorbers represent a powerful probe of galaxy feedback and reionization, as highlighted by both observational and theoretical results showing an increased abundance of low-ionised metal species at higher redshifts. The origin of such absorbers is currently largely unknown because of the low number of galaxy counterparts detected, suggesting that they might be surrounded by low-mass faint sources below the current detection threshold. We use the THESAN-ZOOM radiation hydrodynamic simulations to investigate the connection between properties of neutral oxygen (OI) absorbers and galaxies across redshift $z = 5 - 8$. We find that the virialised gas in haloes becomes progressively ionised with cosmic time, leading to a decrease of $\approx 0.2$ in the covering fraction of neutral oxygen, while the total oxygen covering fraction remains constant. Comparing the OI line density obtained from our covering fractions with the trend suggested by blind quasar observations, we determine that the observable absorbers ($N_{\rm OI} \gtrsim 10^{13}\,\text{cm}^{-2}$) are not confined to haloes: at $z \geq 5$ the majority ($\gtrsim 60\%$) arise beyond $R_{\rm{vir}}$, consistent with recent JWST results. Close to OI absorbers, low-mass galaxies ($M_\star \leq 10^8\,\rm{M}_\odot$) are more commonly found, while, depending on the simulated environment, we do not exclude the possibility of nearby more massive star-forming sources ($\geq 5\,\text{M}_\odot\,\text{yr}^{-1}$) similar to those suggested by the latest ALMA observations. These results establish OI absorbers as sensitive tracers of the evolving ionisation structure around faint galaxies to be probed by forthcoming deep spectroscopic surveys.

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