Abstract The reionisation of the intergalactic medium (IGM) was driven by the first stars, galaxies, and accreting black holes. However, the relative importance of these sources and the efficiency by which ionising photons escape into the IGM remain poorly understood. Most reionisation modelling frameworks assume idealised, isotropic emissions. We investigate this assumption by examining a suite of simulations incorporating directed, anisotropic photon emissions. We find that such anisotropic emissions of ionising photons yield a different reionisation geometry compared to the standard, isotropic, case. During the early stages of reionisation (when less than 30percnt of the Universe is ionised), simulations with narrow photon leakage channels produce smaller ionised bubbles on average. However, these bubbles grow to similar sizes during the middle stages of reionisation. This anisotropy not only produces a distinctive evolution of the size distribution of the ionised regions, but also imprints a feature onto the spherically averaged power spectra of the 21-cm signal throughout reionisation. We observe a suppression in power by about 10-40percnt at scales corresponding to wavenumbers k = 0.1 − 1 h Mpc−1, corresponding to the range in which current radio interferometers are most likely to measure the power spectrum. The simulation with the narrowest channel of ionisation emission shows the strongest suppression. However, this anisotropic emission process does not introduce any measurable anisotropy in the 21-cm signal.

We present a search for z ≃ 7 Lyman-break galaxies using the 1.72 deg 2 near-infrared (NIR) UltraVISTA survey in the COSMOS field, reaching 5 σ depths in Y of 26.2. We incorporated deep Euclid optical and Euclid + Spitzer NIR imaging for a full spectral energy distribution (SED) fitting analysis. We found 289 candidate galaxies at 6.5 ≤ z ≤ 7.5 covering −22.6 ≤ M UV ≤ −20.2, faint enough to overlap with Hubble Space Telescope studies. We conducted a separate selection by including complementary Euclid performance verification imaging (reaching 5 σ depths of 26.3), yielding 140 galaxies in 0.65 deg 2 , with 38 sources unique to this sample. We computed the rest-frame UV luminosity function (UV LF) from our samples, extending below the knee ( M ∗ = 21.14 +0.28 −0.25 ). We find that the shape of the UV LF is consistent with both a Schechter function and a double power law (DPL) at the magnitudes probed by this sample, with a DPL preferred at M UV < −22.5 when bright-end results are included. The UltraVISTA + Euclid sample provides a clean measurement of the LF due to the overlapping NIR filters identifying molecular absorption features in the SEDs of ultra-cool dwarf interlopers, and additional faint galaxies were recovered. A comparison with JWST LFs at z > 7 suggests a gentle evolution in the bright-end slope, although this is limited by a lack of robust bright-end measurements at z > 9. We forecast that in the Euclid Deep Fields, the removal of contaminant ultra-cool dwarfs as point sources will be possible at J E < 24.5. Finally, we present a high-equivalent-width Lyman- α emitter candidate identified by combining HSC, VISTA, and Euclid broadband photometry, highlighting the synergistic power these instruments will have in the Euclid Auxiliary Fields for identifying extreme sources in the epoch of reionisation.

Constraining the Epoch of Reionization (EoR) with physically motivated simulations is hampered by the high cost of conventional parameter inference. We present an efficient emulator-based framework that dramatically reduces this bottleneck for the photon-conserving semi-numerical code SCRIPT. Our approach combines (i) a reliable coarse-resolution MCMC to locate the high-likelihood region (exploiting the large-scale convergence of SCRIPT) with (ii) an adaptive, targeted sampling strategy to build a compact high-resolution training set for an artificial neural network based emulator of the model likelihood. With only ≈ 103 high-resolution simulations, the trained emulators achieve excellent predictive accuracy (R 2 ≈ 0.97–0.99) and, when embedded within an MCMC framework, reproduce posterior distributions from full high-resolution runs. Compared to conventional MCMC, our pipeline reduces the number of expensive simulations by a factor of ∼ 100 and lowers total CPU cost by up to a factor of ∼ 70, while retaining statistical fidelity. This computational speedup makes inference in much higher-dimensional models tractable (e.g., those needed to incorporate JWST and upcoming 21 cm datasets) and provides a general strategy for building efficient emulators for next generation of EoR constraints.

Abstract Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx), a NASA Explorer satellite launched on 2025 March 11, is carrying out the first all-sky near-infrared spectral survey. The satellite observes in 102 spectral bands from 0.75 to 5.0 μ m with a resolving power ranging from λ /Δ λ = 35–130 in 6 . ″ 2 pixels. The observatory obtains a 5 σ depth of 19.5–19.9 AB mag for 0.75 < λ < 3.8 μ m with λ /Δ λ ∼ 40 and 17.8–18.8 AB mag for 3.8 < λ < 5.0 μ m with λ /Δ λ ∼ 120 after mapping the full sky four times over two years. Scientifically, SPHEREx will produce a large galaxy redshift survey over the full sky to constrain the amplitude of inflationary non-Gaussianity. The observations will produce two deep spectral maps near the ecliptic poles that use intensity mapping to probe the evolution of galaxies over cosmic history. By mapping the depth of infrared absorption features over the Galactic plane, SPHEREx will comprehensively survey the abundance and composition of water and other biogenic ice species in the interstellar medium. The project will release initial data rapidly in the form of spectral images, and specialized data products over the life of the mission as the surveys proceed. The science team will also produce spectral catalogs of planet-bearing and low-mass stars, solar system objects, and galaxy clusters three years after launch. We describe the design of the instrument and spacecraft, which flow from the core science requirements. Finally, we present an initial evaluation of the satellite’s in-flight performance and key characteristics.

Abstract The “near–far” approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ( z ≳ 6), beyond the current James Webb Space Telescope detection limits ( M UV ≳ −15). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to ∼50% of the total stellar-mass budget of the proto-MW/LG at z = 6−9. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of ∼2–3 and reduces the halo-to-halo variation in the slope by ∼20%–40%. This enables robust constraints down to at least the resolution limit of the simulations, near M ⋆ ∼ 10 5 M ⊙ or M UV ∼ −10 at z ≳ 6. We also show that “fossil-record” reconstructions—which assume each present-day system descends from a single reionization-era progenitor—are sensitive to the stellar-mass/UV-magnitude thresholds, which introduces bias in the inferred slopes at the low-mass/faint end. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with M UV ≲ −15 to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction (∼50%) of streams with M ⋆ ≳ 10 6 M ⊙ at z = 0 should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

Abstract The Ly α absorption profile in star-forming galaxies serves as a powerful tracer of the extended, dense neutral hydrogen in their surroundings during the Epoch of Reionization (EoR). We report a unique galaxy, A2744-z7DLA, at z ≈ 6.87, gravitationally lensed by the foreground galaxy cluster A2744, which exhibits both moderate Ly α emission and damped Ly α absorption, suggesting the presence of a dense neutral hydrogen environment. Our analysis suggests that the UV continuum turnover near Ly α is more likely shaped by a damped Ly α system rather than nebular continuum from the two-photon process. We analyze the physical properties of A2744-z7DLA with spectroscopic and photometric data from the JWST and the Hubble Space Telescope. The galaxy shows a compact morphology ( r e ∼ 0.3 kpc) and a broadened H α emission line, suggesting possible active galactic nucleus (AGN) activity. The broad component of H α has an FWHM of 2721 ± 200 km s −1 , corresponding to a black hole mass of M BH = 2.9 0 − 1.28 + 2.35 × 1 0 7 M ⊙ and a black hole to stellar mass ratio of log ( M BH / M ⊙ ) = − 1.3 1 − 0.36 + 0.37 . The Balmer decrement (H α /H β ) yields a dust attenuation of A V ≈ 1.15 ± 0.23. Furthermore, we perform spectral energy distribution fitting using both stellar and AGN models. The results show that the UV and optical wavelengths are dominated by star-forming regions, while the AGN component contributes primarily at longer wavelengths. This work provides new insights into the interplay between star formation, neutral gas, and potential AGN activity in galaxies during the EoR.

Abstract The overionized regions surrounding high-redshift quasars, known as proximity zones, provide a window into the interaction between supermassive black holes (SMBHs) and the intergalactic medium (IGM) during the epoch of reionization (EoR). We present new homogeneous measurements of proximity zone sizes (Rp) for a sample of 59 quasars spanning redshifts 5.77 ≤ z ≤ 7.54 (median z = 6.59). For 15 of these sources, we measure Rp for the first time. The quasars in our catalog have absolute magnitudes at rest-frame 1450 Åin the range −29.13 ≤ M1450 ≤ −25.20 (median M1450 ≃ −26.49), providing one of the most extensive data sets for exploring Rp at these epochs. The distribution of Rp values shows a large scatter at fixed redshift and luminosity, likely reflecting variations in quasar lifetimes (tQ), IGM density fluctuations, and IGM neutral fraction. We fit a bivariate power-law model to a large sample of 100 objects to study the dependence of Rp with both M1450 and z: we find that the evolution of Rp with luminosity is in agreement with the models ($R_{\mathrm{p}} \propto 10^{-0.4 M_{1450}/2.87}$), while the evolution of Rp with z is steeper than previous works (Rp∝(1 + z)−2.44). We identify 13 quasars with small proximity zone size, defined using the residuals of our fit. In all cases, except for J2211−6320, we rule out the presence of associated dense absorbers that prematurely truncate Rp, and suggest a short tQ (≲ 104 yr) as a possible explanation for their small proximity zone sizes.

Abstract We present Keck/MOSFIRE H -band spectroscopic measurements covering the [C iii ] λ 1907, C iii ] λ 1909 doublet for a sample of eight z ∼ 7 spectroscopically confirmed star-forming galaxies drawn from the Reionization Era Bright Emission Line Survey (REBELS). This REBELS–MOSFIRE sample is notable for its bright median UV luminosity ( M UV = −22.5 AB) and large median stellar mass ( log M * / M ⊙ = 9.2 ). Although three sources show tentative evidence of a C iii ] detection, we obtain no confident detections for any of the eight REBELS–MOSFIRE sources. The median [C iii ] λ 1907+C iii ] λ 1909 3 σ upper limit in equivalent width (EW) for the REBELS–MOSFIRE sample is 6.5 Å, and a stack of their H -band MOSFIRE spectra yields a nondetection with an associated 3 σ upper limit of 2.6 Å. These upper limits fall significantly below the C iii ] EW measured in a composite spectrum of representative z ∼ 7 star-forming galaxies, as well as those measured for notable early star-forming galaxies such as GN-z11, GHZ2, GS-z12, and RXCJ2248-ID. The lack of strong C iii ] emission can be understood within the context of the stellar populations of the REBELS galaxies, as well as the ionization conditions and gas-phase metallicity implied by rest-frame optical spectroscopic properties ([O iii ]+H β EWs, and [O iii ] λ 5007/[O ii ] λ 3727 and [Ne iii ] λ 3869/[O ii ] λ 3727 line ratios). The REBELS–MOSFIRE sample represents the higher-mass, higher-metallicity, lower-excitation tail of the z ∼ 7 galaxy population, whose ionizing properties must be fully characterized to constrain the role of star-forming galaxies during cosmic reionization.

Abstract We investigate the association between galaxies and neutral O i absorption systems at z ∼ 6, which trace metal-enriched gas during the epoch of reionization. We identify 40 galaxies across six quasar fields, residing in 15 overdensities within 300 kpc of the background sight lines. Five O i absorption systems are associated with five of these overdensities, yielding a covering fraction of 0.2 7 − 0.10 + 0.13 within 300 kpc. The absorption occurs beyond typical virial radii, indicating that the gas traces extended overdensity environments rather than individual galaxy halos, unlike the z ∼ 0 circumgalactic medium (CGM), which is largely bound to halos. These galaxy-associated absorbers account for ∼35% of all O i systems seen in blind quasar surveys, implying the remainder arise in lower-mass galaxies below our detection threshold or in dense neutral intergalactic medium pockets. The CGM around these galaxies contains ≳ 2 × 10 6 M ⊙ of oxygen, comparable to the interstellar medium oxygen mass of the galaxies themselves, suggesting that the surrounding environment holds as much metal mass as the galaxies. All five galaxy-associated systems show significantly higher log ( N CII / N OI ) ratios than absorbers lacking galaxy associations. Furthermore, relative abundance ratios ([Si/O], [C/O]) reveal that four of the five exhibit enrichment patterns consistent with Population III nucleosynthesis at the outskirts of galaxy overdensities. These rare systems offer a unique window into the role of first-generation stars in shaping the early metal enrichment of galaxies and their environments.

Recent baryonic acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) are mildly discrepant (2.2σ) with the cosmic microwave background (CMB) when interpreted within ΛCDM. When analyzing these data with extended cosmologies this inconsistency manifests as a ≃3σ preference for subminimal neutrino mass or evolving dark energy. It is known that the preference for subminimal neutrino mass from the suppression of structure growth could be alleviated by increasing the optical depth to reionization τ. We show that, because the CMB-inferred τ is negatively correlated with the matter fraction, a larger optical depth resolves a similar preference from geometric constraints. Optical depths large enough to resolve the neutrino mass tension (τ∼0.09) reduce the preference for evolving dark energy from ≃3σ to ≃1.5σ and increase the CMB-inferred values of n_{s} and H_{0} to 0.968±0.004 and 67.94±0.44 km/s/Mpc, respectively. Conversely, within ΛCDM the combination of DESI BAO, high-ℓ CMB, and CMB lensing yields τ=0.090±0.012, which is in ≃3-5σ tension with Planck low-ℓ polarization data when taken at face value. Essentially all current CMB analyses-including recent results from WMAP + ACT and SPT-adopt the Planck measurement of τ; thus a systematic in large-scale Planck polarization would serve as a "single-point failure" for most modern cosmological analyses that include CMB data. While there is no evidence for systematics in the large-scale Planck data, τ remains the least well-constrained ΛCDM parameter and is far from its cosmic variance limit. This strengthens the case for future large-scale CMB experiments as well as direct probes of the epoch of reionization.

The escape fraction of Lyman continuum photons (f_ ̊m esc (̊m LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction ( of ionizing photons in the epoch of reionization (EoR) is impossible, due to the opacity of the intergalactic medium (IGM). However, a high leaves clear imprints in the spectrum of a galaxy, due to reduced nebular line and continuum emission, which also leads to bluer UV continuum slopes (̆vbeta). In this work, we exploited the large archive of deep (JWST) NIRSpec spectra from the DAWN JWST archive to analyze over 1400 galaxies at 5<z_ <10 and constrain their James Webb Space Telescope spec based on spectral-energy-distribution fitting enhanced with a picket-fence model. We identify 71 high-confidence sources with significant based on Bayes-factor analysis strongly favoring 0 over 0 solutions. We compare the characteristics of this high-escape subset against both the parent sample and established diagnostics including ̆vbeta slope, O32, and SFR surface density (Σ_̊m SFR). For the overall sample, we find that most sources have a low escape fraction (<1$%$); however, a small subset of sources seems to emit a large number of their ionizing photons into the IGM, such that the average is found to be ∼10%, as needed for galaxies to drive reionization. Although uncertainties remain regarding recent burstiness and the intrinsic stellar ionizing-photon output at low metallicities, our results demonstrate the unique capability of JWST/NIRSpec to identify individual LyC leakers, measure average and thus constrain the drivers of cosmic reionization.

Abstract Characterizing the mechanisms and galaxy properties conducive to the escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization and identify the sources that powered it. Using Hubble Space Telescope data, the Ly α and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multiwavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission f esc LyC . We find strong anticorrelations between f esc LyC and size ( r 20 , r 50 , and r 80 ) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, a filter tracing UV continuum and Ly α . We find that ≥48% of LaCOS galaxies, and ≥41% of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The f esc LyC properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) f esc LyC tends to be larger in galaxies with a small number of compact, centrally located, UV-emitting star-forming regions, (ii) mergers at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at z ∼ 0.3, and (iii) mergers can facilitate the escape of LyC photons from galaxies.

Abstract The massive and bright galaxies observed by the James Webb Space Telescope (JWST) at high redshifts ( z > 6) have challenged our understanding of the Universe. This may require revisiting the physics of galaxy formation and evolution, or modifying the ΛCDM cosmological model to explain these observations, or both. We show that high-resolution cosmic microwave background (CMB) experiments such as the Simons Observatory (or CMB-S4) can measure smoking-gun signatures jointly in weak lensing and kinematic Sunyaev–Zeldovich (kSZ) power spectra, which can shed light on both these scenarios. An increase in the matter power spectrum at small scales will enhance the number density of dark matter halos at high redshifts, thereby increasing the galaxy formation rate. This will cause enhanced weak lensing signal from these redshifts and also lead to an enhanced patchy-kSZ signal from the Epoch of Reionization. However, if only galaxy astrophysics is modified, without any modification in the matter power spectrum, then the patchy-kSZ signal gets altered, while the weak lensing signal remains nearly unaltered. We show that we can measure the modified astrophysical and cosmological scenarios at a statistical significance of 10.4 σ (and 29.8 σ ) from the Simons Observatory (and CMB-S4), which will enable a conclusive understanding on what physical process is driving the high-redshift observations of JWST.

Abstract Spatially resolved multiwavelength analysis is essential to study galaxy formation and evolution. A UV-bright galaxy COS-2987030247 at z = 6.81 is one of the Rosetta Stones in the epoch of reionization for which JWST NIRSpec Integral Field Spectroscopy, NIRCam imaging, and Atacama Large Millimeter/submillimeter Array data are available thanks to the RIOJA program. We identified the rest-frame optical emission lines from the ionized hydrogen, oxygen, and neon gas. The [O iii ] 5008 Å line emission and the NIRCam images show a complex kinematical and morphological structure where two bright main and three faint clumps are identified in a 10 kpc extent. The system is not classified as a purely rotation-dominated disk. The multiple clumps are instead consistent with a merger-related origin, including either distinct galaxies in interaction or star-forming clumps formed through tidal gas compression during a merger. The spatially resolved emission line fluxes show that dust attenuation, metal enrichment, and ionization parameter are preferentially enhanced in the star formation peaks. Our spectral energy distribution fitting suggests that the main clumps are in a moderately dust-attenuated star-forming phase ( A V = 0.2–0.3 and SFR(H α ) ∼10 M ⊙ yr −1 ) with almost zero escape fraction of ionizing photons. In contrast, the subclumps are dust-free and lying on or below the main sequence of star-forming galaxies. These subclumps may work as a perturber that triggers the clumpy starburst in the surrounding gas through the merger event.

ABSTRACT To achieve the sensitivity required to detect signals from neutral hydrogen from the Cosmic Dawn and Epoch of Reionization it is critical to have a well-calibrated instrument which has a stable calibration over the course of the observation. Previous Bayesian calibration methods do not explicitly use the time information available and make assumptions on the impedance matching of the reference sources. Here, we present a new calibration method based on noise wave parameters which fits a calibration solution over time and frequency to the data, interpolating the solutions to the times at which the antenna is being measured. To test this method, we simulate a data set using measurements of the REACH receiver, modelling a low-noise amplifier which is drifting over time. Fitting a polynomial surface in frequency and time to the simulated data demonstrates that we can remove the drift in the calibrated solution over time but leaves a chromatic residual. We further show that we can remove assumptions on the reflection coefficients of the reference noise source and the cold load, reducing degeneracies in the parameter fits. Applying this new calibration equation and surface fitting method to the simulated data removes the chromatic residual in the calibrated spectrum and recovers the parameters to within 0.06 per cent of the truth and a 97 per cent reduction in the RMSE of the spectrum of the validation source compared with previous calibration methods. For two parameters, we report up to six times smaller fit error after the degeneracies are removed from the time-based calibration.

We present an improved matched filter method for detecting large ionized regions in 21 cm observations of the Epoch of Reionization. In addition to detection, the method constrains the properties of these regions, offering insights into the underlying source populations. Extending a previously developed Bayesian framework, we replace the spherical filter with an eight-parameter spheroidal filter, enabling a more flexible characterization of ionized bubbles. This enhancement significantly improves both detectability and recovery of bubble orientations. For a representative reionization scenario with a mean ionization fraction of 0.4 at z = 7, we find that a 10σ detection of the largest ionized region can be achieved with ∼ 1 h of observation using the SKA-low AA4 and AA★ layouts. Our method can help identify regions in the observed field that host large ionized bubbles, making them prime targets for deeper follow-up observations.

We report on Chandra X-ray observations of four narrow-line quasar candidates at z ∼ 6, selected from the Subaru high-z exploration of low-luminosity quasars (SHELLQs) project, based on the Subaru Hyper Suprime-Cam survey. These objects are characterised by narrow (FWHM ≤ 310 km s −1 ), luminous (> 10 44 erg s −1 ) Ly α , and faint UV continuum ( M 1450 = −22 to −21), prompting us to examine whether they are obscured luminous active galactic nuclei (AGNs) at the epoch of reionization. However, none of these objects were detected by Chandra, giving an upper limit to their rest-frame 2−10 keV luminosity ( L X ) of 2 × 10 44 erg s −1 (2 σ ), assuming a spectral slope of Γ = 2. Subsequent rest-frame optical spectroscopy of these objects by the James Webb Space Telescope, presented in a companion paper, shows weak broad Balmer emission at the base of narrow cores. With the scaling relation for low-redshift AGNs, the observed strong [O III ] λ 5007 flux of these sources would predict L X to be around 10 45 erg s −1 , which is well above the Chandra upper limits. These optical spectra and X-ray quietness are reminiscent of JWST-selected broad-line AGNs. We attribute the weak broad Balmer emission to the broad-line regions hidden partially by optically thick obscuring matter that also hides the optical and X-ray continuum emission from the accretion disc. Compton-thick obscuration, which would strongly suppress X-ray emission, could be due to a dense interstellar medium that is often present in galaxies at high redshifts. Alternatively, the same effect could be obtained from an inflated disc at the innermost radii in a supercritical accretion flow, when the disc is viewed at inclined angles.

Modern machine learning techniques can unlock the vast cosmological information encoded in forthcoming Square Kilometre Array (SKA) observations. We show that tomographic 21 cm data from the reionisation era can yield stringent tests of inflationary models — here illustrated with Starobinsky R + R 2 inflation. Using a simulation-based inference (SBI) framework, we compare neural summaries (convolutional network and vision transformer) with a traditional power spectrum summary and perform a fully joint SBI analysis combining 21 cm data with data of the cosmic microwave background (CMB). Forecasts based on realistic mock observations indicate that SKA alone will achieve constraints competitive with Planck, and that the combined SKA + CMB dataset will tighten bounds on both inflationary and ΛCDM parameters considerably while improving precision on key astrophysical quantities.

Abstract Observations by the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) are combined with counterpart Spitzer spectral map data to study the aromatic, aliphatic, and polycyclic aromatic hydrocarbon (PAH) size evolution across the northwest photodissociation region (PDR) of the Iris Nebula (NGC 7023). The 3.3–3.4 μ m complex (I 3.3 ) and 11.2 μ m (I 11.2 ) PAH band strength are determined through direct integration. In addition, the former is decomposed into a 3.3 ( I ′ 3.3 ) and 3.4 μ m ( I ′ 3.4 ) subfeature by fitting SPHEREx bandpass-integrated photometry using a modeled, highly sampled, multicomponent spectrum. I 3.3 , I 11.2 , I ′ 3.3 , and I ′ 3.4 all peak at the PDR. The NASA Ames PAH IR Spectroscopic Database is used to obtain the average number of carbon atoms ( N C ¯ ) and small PAH fraction ( f small ) by fitting the isolated PAH component of the Spitzer segment: 70 ≲ N C ¯ ≲ 76 and 0.24 ≲ f small ≲ 0.36, respectively. I ′ 3.4 / I ′ 3.3 , I 11.2 /I 3.3 , N C ¯ , and f small all show a demarcation that matches the large-scale morphology of the region. For I ′ 3.3 and I ′ 3.4 this is reflected by two distinct trends when plotted against each other, one associated with the dense medium and the other with the diffuse medium: [ N H , ali / N H , aro ] dense = 0.42 ± 0.01 and [ N H , ali / N H , aro ] diffuse = 0.10 ± 0.01, respectively. N C ¯ and f small are tentatively correlated with I 11.2 /I 3.3 ( R = 0.54 ± 0.05 and −0.45 ± 0.05, respectively). A wider variety of large(r) extended interstellar medium objects is required to tighten the correlations, turn them into quantitative calibrators for PAH size, and pin down the discrepancy of correlations with I ′ 3.3 involved.

Abstract It is widely believed that the ultraviolet background produced during the epoch of reionization conspires against the formation of low-mass galaxies. Indeed, this mechanism is often invoked as part of the solution to the so-called “missing satellites problem.” In this paper we employ FIREbox , a large-volume cosmological simulation based on the Feedback In Realistic Environments physics model, to characterize the mechanisms governing galaxy ignition in the postreionization era. By carefully matching recently ignited halos (with stellar ages below 100 Myr at the time of selection) to halos that failed to form any stars, we conclude that the presence of cold dense gas and halo concentration helps incite the process of galaxy formation. Concretely, we find that 100% of recently ignited halos experience cold dense gas enhancements relative to their matched failed counterparts. Likewise, approximately 83% display enhancements in both cold dense gas and Navarro–Frenk–White concentration ( c NFW ), while the remaining ∼17% exhibit enhanced cold dense gas content and suppressed c NFW values. Lastly, our simulation suggests that galaxy ignition can occur as late as z = 2, potentially allowing us to observationally catch this process “in the act” in the foreseeable future.