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1. FlexRT — A fast and flexible cosmological radiative transfer code for reionization studies. Part I. Code validation

   https://doi.org/10.1088/1475-7516/2024/12/025  

   Cain, Christopher; D'Aloisio, Anson (December 2024, Journal of Cosmology and Astroparticle Physics)

   Abstract The wealth of high-quality observational data from the epoch of reionization that will become available in the next decade motivates further development of modeling techniques for their interpretation. Among the key challenges in modeling reionization are (1) its multi-scale nature, (2) the computational demands of solving the radiative transfer (RT) equation, and (3) the large size of reionization's parameter space. In this paper, we present and validate a new RT code designed to confront these challenges.FlexRT(Flexible Radiative Transfer) combines adaptive ray tracing with a highly flexible treatment of the intergalactic ionizing opacity. This gives the user control over how the intergalactic medium (IGM) is modeled, and provides a way to reduce the computational cost of aFlexRTsimulation by orders of magnitude while still accounting for small-scale IGM physics. Alternatively, the user may increase the angular and spatial resolution of the algorithm to run a more traditional reionization simulation.FlexRThas already been used in several contexts, including simulations of the Lyman-αforest of high-zquasars, the redshifted 21cm signal from reionization, as well as in higher resolution reionization simulations in smaller volumes. In this work, we motivate and describe the code, and validate it against a set of standard test problems from the Cosmological Radiative Transfer Comparison Project. We find thatFlexRTis in broad agreement with a number of existing RT codes in all of these tests. Lastly, we compareFlexRTto an existing adaptive ray tracing code to validateFlexRTin a cosmological reionization simulation. 

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2. RIGEL: Simulating dwarf galaxies at solar mass resolution with radiative transfer and feedback from individual massive stars

   https://doi.org/10.1051/0004-6361/202450699  

   Deng, Yunwei; Li, Hui; Liu, Boyuan; Kannan, Rahul; Smith, Aaron; Bryan, Greg L (November 2024, Astronomy & Astrophysics)

   Context.Feedback from stars in the form of radiation, stellar winds, and supernovae is crucial to regulating the star formation activity of galaxies. Dwarf galaxies are especially susceptible to these processes, making them an ideal test bed for studying the effects of stellar feedback in detail. Recent numerical models have aimed to resolve the interstellar medium (ISM) in dwarf galaxies with a very high resolution of several solar masses. However, when it comes to modeling the radiative feedback from stars, many models opt for simplified approaches instead of explicitly solving radiative transfer (RT) because of the computational complexity involved. Aims.We introduce the Realistic ISM modeling in Galaxy Evolution and Lifecycles (RIGEL) model, a novel framework to self-consistently model the effects of stellar feedback in the multiphase ISM of dwarf galaxies with explicit RT on a star-by-star basis. Methods.The RIGEL model integrates detailed implementations of feedback from individual massive stars into the state-of-the-art radiation-hydrodynamics code,AREPO-RT. It forms individual massive stars from the resolved multiphase ISM by sampling the initial mass function and tracks their evolution individually. The lifetimes, photon production rates, mass-loss rates, and wind velocities of these stars are determined by their initial masses and metallicities based on a library that incorporates a variety of stellar models. The RT equations are solved explicitly in seven spectral bins accounting for the infrared to He IIionizing bands, using a moment-base scheme with the M1 closure relation. The thermochemistry model tracks the nonequilibrium H, He chemistry as well as the equilibrium abundance of C I, C II, O I, O II, and CO in the irradiated ISM to capture the thermodynamics of all ISM phases, from cold molecular gas to hot ionized gas. Results.We evaluated the performance of the RIGEL model using 1 M_⊙resolution simulations of isolated dwarf galaxies. We found that the star formation rate (SFR) and interstellar radiation field (ISRF) show strong positive correlations with the metallicity of the galaxy. Photoionization and photoheating can reduce the SFR by an order of magnitude by removing the available cold, dense gas fuel for star formation. The presence of ISRF also significantly changes the thermal structure of the ISM. Radiative feedback occurs immediately after the birth of massive stars and rapidly disperses the molecular clouds within 1 Myr. As a consequence, radiative feedback reduces the age spread of star clusters to less than 2 Myr, prohibits the formation of massive star clusters, and shapes the cluster initial mass function to a steep power-law form with a slope of ∼ − 2. The mass-loading factor (measured atz = 1 kpc) of the fiducial galaxy has a median ofη_M ∼ 50, while turning off radiative feedback reduces this factor by an order of magnitude. Conclusions.We demonstrate that RIGEL effectively captures the nonlinear coupling of early radiative feedback and supernova feedback in the multiphase ISM of dwarf galaxies. This novel framework enables the utilization of a comprehensive stellar feedback and ISM model in cosmological simulations of dwarf galaxies and various galactic environments spanning a wide dynamic range in both space and time. 

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3. AMBER: A Semi-numerical Abundance Matching Box for the Epoch of Reionization

   https://doi.org/10.3847/1538-4357/ac5116  

   Trac, Hy; Chen, Nianyi; Holst, Ian; Alvarez, Marcelo A.; Cen, Renyue (March 2022, The Astrophysical Journal)

   Abstract The Abundance Matching Box for the Epoch of Reionization (AMBER) is a semi-numerical code for modeling the cosmic dawn. The new algorithm is not based on the excursion set formalism for reionization, but takes the novel approach of calculating the reionization-redshift field $z_{\mathrm{re}}\left(\mathit{x}\right)$assuming that hydrogen gas encountering higher radiation intensity are photoionized earlier. Redshift values are assigned while matching the abundance of ionized mass according to a given mass-weighted ionization fraction ${\overline{x}}_{\mathrm{i}}\left(z\right)$. The code has the unique advantage of allowing users to directly specify the reionization history through the redshift midpointz_mid, duration Δ_z, and asymmetryA_zinput parameters. The reionization process is further controlled through the minimum halo massM_minfor galaxy formation and the radiation mean free pathl_mfpfor radiative transfer. We implement improved methods for constructing density, velocity, halo, and radiation fields, which are essential components for modeling reionization observables. We compare AMBER with two other semi-numerical methods and find that our code more accurately reproduces the results from radiation-hydrodynamic simulations. The parallelized code is over four orders of magnitude faster than radiative transfer simulations and will efficiently enable large-volume models, full-sky mock observations, and parameter-space studies. AMBER will be made publicly available to facilitate and transform studies of the Epoch of Reionization. 

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4. Predicting sub-millimetre flux densities from global galaxy properties

   https://doi.org/10.1093/mnras/stac3451  

   Cochrane, R. K.; Hayward, C. C.; Anglés-Alcázar, D.; Somerville, R. S. (November 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Recent years have seen growing interest in post-processing cosmological simulations with radiative transfer codes to predict observable fluxes for simulated galaxies. However, this can be slow, and requires a number of assumptions in cases where simulations do not resolve the interstellar medium (ISM). Zoom-in simulations better resolve the detailed structure of the ISM and the geometry of stars and gas; however, statistics are limited due to the computational cost of simulating even a single halo. In this paper, we make use of a set of high-resolution, cosmological zoom-in simulations of massive ($$M_{\star }\gtrsim 10^{10.5}\, \rm {M_{\odot }}$$ at z = 2), star-forming galaxies from the FIRE suite. We run the skirt radiative transfer code on hundreds of snapshots in the redshift range 1.5 < z < 5 and calibrate a power-law scaling relation between dust mass, star formation rate, and $$870\, \mu \rm {m}$$ flux density. The derived scaling relation shows encouraging consistency with observational results from the sub-millimetre-selected AS2UDS sample. We extend this to other wavelengths, deriving scaling relations between dust mass, stellar mass, star formation rate, and redshift and sub-millimetre flux density at observed-frame wavelengths between $$\sim \! 340$$ and $$\sim \! 870\, \mu \rm {m}$$. We then apply the scaling relations to galaxies drawn from EAGLE, a large box cosmological simulation. We show that the scaling relations predict EAGLE sub-millimetre number counts that agree well with previous results that were derived using far more computationally expensive radiative transfer techniques. Our scaling relations can be applied to other simulations and semi-analytical or semi-empirical models to generate robust and fast predictions for sub-millimetre number counts. 

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5. Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass?

   https://doi.org/10.1051/0004-6361/202142323  

   Blondin, S.; Bravo, E.; Timmes, F. X.; Dessart, L.; Hillier, D. J. (April 2022, Astronomy & Astrophysics)

   Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit ( M Ch ≈ 1.4 M ⊙ ) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between M Ch and sub- M Ch events. In particular, it is thought that the higher-density ejecta of M Ch WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni  II ] lines in late-time spectra (≳150 d past explosion). Aims. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from M Ch and sub- M Ch progenitors. We explore the potential for lines of [Ni  II ] in the optical an near-infrared (at 7378 Å and 1.94 μm) in late-time spectra to serve as a diagnostic of the exploding WD mass. Methods. We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D M Ch delayed-detonation and sub- M Ch detonation models, we studied the synthesis of stable Ni isotopes (in particular, 58 Ni) and investigated the formation of [Ni  II ] lines using non-local thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code. Results. We confirm that stable Ni production is generally more efficient in M Ch explosions at solar metallicity (typically 0.02–0.08 M ⊙ for the 58 Ni isotope), but we note that the 58 Ni yield in sub- M Ch events systematically exceeds 0.01 M ⊙ for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction 57 Co( p ,  γ ) 58 Ni is the dominant production mode for 58 Ni in both M Ch and sub- M Ch models, while the α -capture reaction on 54 Fe has a negligible impact on the final 58 Ni yield. More importantly, we demonstrate that the lack of [Ni  II ] lines in late-time spectra of sub- M Ch events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong [Ni  II ] lines predicted in our 1D M Ch models are completely suppressed when 56 Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. Conclusions. [Ni  II ] lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among M Ch and sub- M Ch progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni  II ] lines would most likely result from a Chandrasekhar-mass progenitor. 

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