Aaron Smith

It is often understood that supernova (SN) feedback in galaxies is responsible for regulating star formation (SF) and generating gaseous outflows. However, a detailed look at the small-scale effects of SNe on the interstellar medium (ISM) in simulations shows that the macroscopic processes of SF suppression and outflow generation proceed in distinct channels. We demonstrate this finding in two independent simulations of isolated dwarf galaxies with very high (m_gas ~ Msun) numerical resolution, LYRA and RIGEL. Our findings suggest that the macroscopic effect of a given SN on the galaxy is best predicted by its local density. Outflows are primarily linked to SNe in diffuse regions expanding to their cooling radii on large (~kpc) scales, while dense SF regions are disrupted in a localized (~pc) manner. However, these separate feedback channels are only distinguishable at very high resolutions capable of following mass scales ^2 . When averaging on coarser scales, ISM densities are greatly mis-estimated, and variations between different SF and SNe-affected regions are severely washed out. It therefore cannot be self-consistently determined, from coarse-resolution information alone , (1) whether a SN tends to contribute to outflows or direct SF suppression, and (2) the rate of SF in a given region. In particular, commonly used parameters in coarse-resolution (subgrid) models, such as the SN cooling radius and SF density threshold, may require more detailed treatments informed by high-resolution studies.

Observations with the JWST in lensed fields have revealed that galaxies at cosmic dawn may concentrate their star formation in highly dense, compact, star clusters. The high columns and low metallicities encountered in their birth environments suggest that Ly α radiation pressure may be crucial to their formation and evolution. In this study, we address this question by postprocessing snapshots from radiation-hydrodynamic simulations of dense star-cluster-forming clouds (Σ _* ≳ 10 ³ M _⊙ pc ⁻² ) with a range of dust abundances ( Z _d = 0–0.1 Z _d,⊙ ) using the colt Monte Carlo code. We infer that Ly α is likely to have mild (∼10%) effects on the gas-to-star conversion efficiencies ( ϵ _* ≳ 60%) for Z _d ≳ 0.01 Z _d,⊙ , and even in dust-free environments, ϵ _* ≳ 25%—much higher than the ≲10% values typical of star-forming regions in the local Universe. This is because the densest filaments dominating stellar mass assembly ( n ≳ 10 ⁴ cm ⁻³ ) remain sub-Eddington ( f _Edd < 1). On the other hand, the bulk of the gas volume ( n ≲ 10 ³ cm ⁻³ ) has f _Edd > 1, with noticeable fractions having f _Edd ≳ 10, implying that Ly α can launch dynamically significant winds from these systems rapidly (≲4 Myr), with possible implications for ionizing photon escape and galactic outflows. The Ly α force multiplier M _F is highly sensitive to Z _d , with M _F ≲ 3 (≲500) for 0.1 Z _d,⊙ (dust-free) environments, respectively. Nevertheless, Ly α dominates over UV and IR radiation pressure at all values of Z _d ≲ 0.1 Z _d,⊙ , by factors of ∼3–500. Our results suggest that Ly α radiation pressure reinforces the emerging picture of locally efficient, bursty star formation accompanied by rapid outflows in galaxies at cosmic dawn.

Context . Emission lines from ionized gas are key diagnostics of star formation, metallicity, and ionization conditions in galaxies. Interpreting spatially resolved observations from integral field surveys (e.g., MaNGA , VLT/MUSE , JWST/NIRSpec ) and comparing them with hydrodynamical simulations requires three-dimensional photoionization models that handle realistic geometries, dust attenuation, and synthetic instrument output. Aims . We present a new photoionization module for the Monte Carlo radiative transfer code SKIRT that predicts emission-line luminosities of ionized gas in three dimensions. It combines precomputed Cloudy tables for gas temperature and opacity with a direct calculation of ion fractions and line emissivities. Methods . The module characterizes the local ionizing radiation field by log U and four spectral-shape ratios across the ionizing continuum (1–6 Ryd). Precomputed Cloudy tables map these to gas temperature and photoionization opacities, and they converge through SKIRT ’s existing iteration cycle. Emission-line luminosities are computed directly. An inline solver determines ion fractions from the converged radiation field and temperature and evaluates emissivities from recombination coefficients and collisional excitation rates. We validated the module against Cloudy on 60 spherical shell models and against COLT on a Milky Way-analog galaxy. Results . Across the one-dimensional benchmark grid, hydrogen recombination lines agree with Cloudy to within a few percent (H α median ratio 0.97), and the forbidden lines to within ∼5%, except [S II ] λ 6717 (median ratio 1.23), whose offset traces a temperature overestimate near the ionization front. In the three-dimensional galaxy comparison, integrated luminosities agree with COLT to within 18% for the hydrogen lines and 2% for [N II ], while [O III ] and [S II ] are systematically elevated by ∼70% and ∼80%. Pixel-by-pixel correlation coefficients reach r ≥ 0.92, with luminosity-weighted scatter of 0.14–0.31 dex, and broadly consistent BPT line ratios. Conclusions . The module enables self-consistent synthetic observations in which ionized-gas emission lines, dust attenuation, and dust reemission are computed in a single Monte Carlo radiative transfer simulation applicable to any hydrodynamical simulation.

Several Lyman-alpha (Lyα) emitters deep into the reionization era exhibit double-peaked Lyα emission profiles, raising the question of how the intergalactic medium can transmit photons blueward of the Lyα resonance at such high redshifts. To investigate this, we compute Lyα transmission along sightlines originating from galaxies in the Cosmic Dawn III simulation and identify cases that closely reproduce the observed double-peaked emission. In these cases, the sightlines intersect highly underdense voids located a few comoving megaparsecs from the source galaxy. These voids allow photons emitted blueward of Lyα to redshift through resonance without scattering while traversing them. The low opacity arises because the neutral hydrogen density scales with the square of the underlying gas density under ionization equilibrium, making sufficiently underdense regions with ≲ 30 % of cosmic mean density highly transmissive. Such voids naturally occur in the fluctuating cosmic density field, even in the vicinity of galaxies, and can also be associated with transmissive spikes in the Lyα forest. We find that the global probability of observing double-peaked emission is ∼0.5 % during reionization at an 80 % global ionization fraction, while no cases are found at 60 % ionization. We also find that this probability depends sensitively on the local ionizing background intensity, increasing by a factor of ∼103 for a tenfold increase in intensity. Recent accretion episodes of supermassive black holes can further increase this probability. These results suggest that double-peaked Lyα emission in high-z galaxies can serve as a sensitive probe of the ionizing background during the late stages of cosmic reionization.

The physical processes that led to the formation of billion-solar-mass black holes within the first 700 million years of cosmic time, a period known as cosmic dawn, remain a puzzle ¹ . Several theoretical scenarios have been proposed to seed and rapidly grow black holes ^2–4 , but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that exhibits singular properties: among the largest hydrogen Balmer breaks reported at any redshift, broad multi-peaked Hβ emission, and Balmer line absorption in several transitions. We model this source as an enshrouded black hole in which the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free envelope around a supermassive black hole ^5,6 . This source may provide evidence of an early black hole embedded in dense gas—a theoretical configuration proposed to rapidly grow black holes by super-Eddington accretion ^7,8 . Radiation from the black hole seems to dominate almost all observed light, leaving limited room for contribution from its host galaxy. If the source merged with its brighter neighbour, it would resemble the recently discovered ‘little red dots’ with perplexing spectral energy distributions ^9–11 . The redness of the black hole is due to gas, not dust ^12,13 , and scattering, not kinematics, gives rise to the complex line shapes and luminosities—black hole masses of these sources may therefore be overestimated by orders of magnitude.

The central engines of Little Red Dots (LRDs) may be “black hole stars” (BH*s), early stages of black hole growth characterized by dense gas envelopes. So far, the most direct evidence for BH*s comes from a handful of sources where the host galaxy is completely outshone as suggested by their remarkably steep Balmer breaks. Here we present a novel scheme to disentangle BH*s from their host galaxies assuming that the [OIII]5008Å line arises exclusively from the host. Using a sample of 98 LRDs ( z ≈ 2 − 9 ) with high quality NIRSpec/PRISM spectra, we demonstrate that the host-subtracted median stack displays a Balmer break > 2 × stronger than massive quiescent galaxies, with the rest-optical continuum resembling a blackbody-like SED ( K, erg s, au). We measure a steep Balmer decrement (H α /H β > 10 ) and numerous density-sensitive features (e.g., FeII, HeI, OI). These are hallmark signatures of dense gas envelopes, providing population-level evidence that BH*s indeed power LRDs. In the median LRD, BH*s account for ∼ 20 % of the UV emission, ∼ 50 % at the Balmer break, and ∼ 90 % at wavelengths longer than H α with the remainder arising from the host. BH*s preferentially reside in low-mass galaxies () undergoing recent starbursts, as evidenced by extreme emission line EWs (e.g., [OIII]5008Å ≈ 1100 Å, CIII] ≈ 12 Å), thereby favoring BH* origins linked to star-formation. We show V-shaped LRD selections are biased to high BH*/host fractions ( ≳ 60 % at 5500Å) – less dominant BH*s may be powering JWST’s blue broad-line AGN. We find BH*s are so commonplace and transient (duty cycle ∼ 1 % , lifetime ∼ 10 Myrs) that every massive black hole may have once shone as a BH*.