Project description

The THESAN-ZOOM project is a suite of zoom-in radiation hydrodynamic simulations that self-consistently model the formation of some of the first galaxies in the Universe across a broad mass range and follows them for 2 billion years (until redshift 3). Thanks to the high spatial resolution reached and the rich physics, the THESAN-ZOOM simulations capture the multi-phase inter-stellar medium (ISM) of galaxies, including the formation of cosmic dust and molecular hydrogen. In addition, they include a novel treatment of the radiation field sourced by distant galaxies, that for the first time is self-consistent inherited from the parent THESAN simulation box instead of relying on approximated treatments. The following sections include non-specialist and technical introductions to the simulation suite focusing on the unique aspects and central scientific themes of the THESAN-ZOOM project.

Numerical simulations have risen to be a third pillar of research in the last few decades, flanking theoretical and experimental (or observational) efforts. They are particularly relevant in astrophysics because nature prevents scientists from performing controlled experiments due to the large distances, physical sizes, long timescales, or extreme densities and energies involved. In fact, in cosmology the experimental setup cannot be altered as it coincides with part or all of the Universe, a disadvantage that is partially alleviated by the wealth of passive observational data collected by telescopes and detectors across the electromagnetic spectrum. Thus, numerical simulations provide synthetic and controllable versions of the Universe that effectively allow astrophysicists to better understand reality by transcending it.

The predictions derived from these virtual experiments however are limited by the size of our computers. Even the largest supercomputers in the world can not follow in all details the interconnected physical processes at play in the Universe, forcing a choice between simulating large volumes at low resolution or only few objects with high accuracy. THESAN-ZOOM does the latter using a technique called zoom-in. Like a camera, the simulation will focus most of its attention on a single galaxy or small group of galaxies.

In the aftermath of the Big Bang, the Universe was comprised of a hot dense plasma of matter and radiation. Cosmic expansion led to rapid gas cooling, with the cosmic microwave background (CMB) forever preserving the moments when electrons and protons recombined to form neutral hydrogen atoms. Throughout its infancy, the dim Universe then went through a process of structure formation induced by the growth of overdensities of dark matter, a still-unknown substance making up approximately 85% of the material in the Universe and only interacting through gravity. Galaxies started to form once rarefied gas could collapse into the central regions of dark matter halos. They then continued to grow and evolve according to the interplay between gravity, hydrodynamical forces, and feedback from stars and blackholes.

Eventually, within the first billion years, a complex landscape emerged in which various astrophysical phenomena govern the motion and properties of gas, connecting a hierarchy of large to small scale physics throughout the Universe. In particular, countless stars act as cosmic furnaces to not only shine as bright lights that illuminate their surroundings, but also inject large amounts of mass and energy into the interstellar medium (ISM) of galaxies through radiation pressure, stellar winds, and supernovae explosions. Furthermore, across their lifetimes they also fuse lighter elements into heavier ones, which can then form complex molecules like cosmic dust, and even planets. Some of these stars became black holes or other exotic objects at the end of their lives.

All of these processes are carefully included in the THESAN-ZOOM simulations. The gravitational and hydrodynamical forces are accurately calculated using the novel moving mesh approach with the AREPO code. The entire simulation volume is divided into individual volume elements, which roughly follow the motion of the gas. Most of these elements are inside or around the galaxies that are being zoomed on, while a small portion is placed outside representing all the out-of-focus regions of the simulated Universe. Thanks to the impressive resolution achieved, THESAN-ZOOM can follow all the relevant processes that not only form a galaxy, but also those that shape its resolved properties and its multi-phase ISM.

To make all of this possible, the simulations were run on one of the largest supercomputers in the world, the SuperMUC-NG machine at the LRZ (in Germany), employing more than 40 million hours of continuous calculations.

Once the first stars — and subsequently galaxies — formed in the infant Universe, they began to pour large amounts of high-energy photons into the surrounding intergalactic medium. This gradually transformed the ambient neutral hydrogen gas into a hot highly-ionized plasma, a process known as cosmic reionization. Reionization represents the last phase transformation that the Universe went through and forever changed the stage upon which galaxies formed and evolved. Unfortunately, it is extremely challenging to gather data on this epoch as the large amount of time between us and when this transformation took place translates to truly astronomical distances. However, a detailed chronology and characterization of cosmic reionization will continue to emerge in the coming years.

The THESAN-ZOOM simulations will help us advance our knowledge regarding one fundamental unknown of cosmic reionization, namely the fraction of ionizing photons that manage to escape from a galaxy into their environment. This fundamental yet elusive parameter is key to understand when and which galaxies have powered the transformation of the hydrogen in the Universe into a hot plasma. The high spatial resolution of THESAN-ZOOM enables these simulation to capture in great detail the processing shaping the ISM of galaxies, including the low-density channels through which radiation is expected to escape, while the novel radiation-injection scheme ensures that photon escaping from the galaxies will encounter the correct physical condition even at the periphery or outside the galaxies.

The launch of the James Webb Space Telescope (JWST) has transformed our understanding of the first galaxies. Thanks to its high sensitivity and infrared capabilities, JWST is allowing us to carry out studies of galaxies deep into the epoch of reionization, when the Universe is only a few hundred million years old. Through exquisite imaging and spectroscopy, JWST allows for detailed characterisation of the properties of these galaxies though their rest-UV and optical emission, providing measurements such as their stellar masses, sizes and dust content. JWST is also obtaining new constraints on the timing of reionization, through measuring the imprint of neutral gas in the spectra of these distant galaxies. Finally, JWST has unveiled a number of surprising properties of galaxies in the first few billion years of the Universe, including a suprisingly large number of bright objects, the presence of massive red galaxies at very early epochs, a population of so-called Little Red Dots of unknown nature and large ionizing photon production efficiencies. These JWST observations are complemented by ground-based facilities like the Atacama Large Millimeter/submillimeter Array (ALMA) and the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT). In the future, the Extremely Large Telescope (ELT) will further enhance our understanding of the high-redshift Universe. The outstanding quality of these existing and upcoming observational datasets motivates a novel theoretical approach to simulating realistic counterparts of the first galaxies, with a view to understanding the physics responsible for forming the first galaxies. To achieve this goal, the THESAN-ZOOM simulations have been carefully designed to model the important processes of galaxy formation over a wide range of spatial scales, from large-scale fluctuations in the intergalactic ionizing background due to inhomogeneous reionization, to the physics of the small-scale, multi-phase interstellar medium.

We outline below a few of the key science areas that motivated the THESAN-ZOOM simulations:

• High redshift galaxy formation and JWST observations: JWST has transformed our ability to observe the formation and evolution of the first galaxies. However, gaining true understanding from such observations requires realistic models able to capture all major physical processes at play with sufficient resolution and that can, therefore, be faithfully compared with the observational data. The THESAN-ZOOM simulation suite provides such tool, incorporating radiation from both local and distant sources, models for molecular chemistry and cosmic dust an a multi-phase ISM. These are of paramount importance to bridge theory and observations, and ultimately to understand of how the first galaxies in the Universe emerged.
• Metal enrichment of the CGM: the production and transport of the first metals in the CGM of galaxies is an outstanding open question. THESAN-ZOOM is poised to provide a unique view of such process. The accurate galaxy-formation physics coupled with the self-consistent radiation field allows for accurate studies of when and how metals left their brithplace within galaxies, as well as observational evidence for such process.
• Ionizing radiation escape: By measuring the escape fraction of ionizing radiation for galaxies over a wide mass range and as a function of redshift, THESAN-ZOOM can predict what galaxy populations primarily drive cosmic reionization and assess how these predictions depend on simulation resolution and the details of the galaxy formation model. In addition, it will allow us to study the role of photoevaporation of absorbers of ionizing radiation and how satellite galaxies are affected by photoevaporation of gas.
• Lyα emission and transmission: The Lyα line of atomic hydrogen is among the strongest lines emitted by high-redshift galaxies, which makes it an excellent target for spectroscopic redshift determinations. JWST has observed such emission line to redshifts where, it was thought, it should not be visible. In addition, the properties of this line are being used to map out the size of ionised bubbles around the brightest sources. However, detailed modeling is needed to extract physical information from the observed emission. The THESAN-ZOOM project allows us to self-consistently connect the intrinsic Lyα emission from recombinations and collisional excitations, resonant scattering within the ISM/CGM, and frequency-dependent transmission through the IGM. Accurate Monte Carlo radiative transfer calculations for ionizing, Lyα, Hα, continuum, and other forms of radiation reveal the physical mechanisms at play in high-z LAEs and test if their properties are similar to those identified at lower redshifts, e.g. in terms of dust content and star formation. Combining the ability of THESAN-ZOOM to resolve the sub-structure of the ISM with the accurate large-scale reionization topology coming from the original THESAN creates a unique tool to study the properties of Lyα emitters, and to break model degeneracies and disentangle Lyα spectral features imprinted at every stage from emission to observation.

• Radiation transport: The propagation of radiation is handled using a moment-based approach to solve the radiation transport equations. We solve the set of coupled hyperbolic conservation equations for photon number density and photon flux. This set of equations is closed using the M1 scheme. The Riemann problem is solved at each cell interface by computing the flux using Godunov’s approach. We achieve second order accuracy by replacing the piecewise constant (PC) approximation of Godunov’s scheme with a slope-limited piece-wise linear spatial extrapolation and a half timestep, first order time extrapolation, obtaining the primitive variables on both sides of the interface. This approach is fully local and does not scale with the number of sources in the simulation box, a feature particularly appealing given the large amount of stellar particles present in the simulations.
• Treatment of external radiation fields: The simulations employ a unique framework that incorporates the impact of patchy reionization by adopting the large-scale radiation field topology from the parent THESAN simulation box rather than assuming a spatially uniform UV background. The maps of the radiation field of the parent simulation that were saved with high cadence, are interpolated in space and time to set the radiation field outside of the high-resolution region at each timestep. Inflowing radiation is then propagated into the high-resolution region using AREPO-RT.
• SMUGGLE-RT galaxy formation model: The simulations employ a state-of-the-art galaxy formation model, that models the multi-phase nature of the interstellar medium (ISM). It includes (i) feedback from supernova explosions and stellar winds, in the form of kinetic and thermal energy; (ii) the production and evolution of nine elements (H, He, C, N, O, Ne, Mg, Si and Fe), as well as the tracking of the overall gas metallicity; (iii) a six species non-equlibrium chemical network that allows the gas to cool down to 10 K; (iv) a model for dust production and desctruction; and (v) radiative transfer that accounts for both local and external sources. All of these elements are key to realistically simulating the properties of high-redshift galaxies.
• Simulations: THESAN-ZOOM is a comprehensive suite of high-resolution zoom-in simulations of 14 high-redshift (𝑧 > 3) galaxies selected from the parent THESAN simulation volume. They cover a wide range of halo masses from M_halo = 10⁸ - 10¹³ M_☉ at z=3, and are simulated at three different resolution levels. In the highest-resolution runs, the DM and baryonic mass resolutions reach 762 M_☉ and 142 M_☉, respectively, corresponding to spatial resolutions of ~140 cpc for DM and stars, and down to ~17 cpc for gas. We simulate the most massive halos at the standard resolution level (4x), halos up to M_halo = 10¹¹ M_☉ at increased resolution (8x), and M_halo < 10¹⁰ M_☉ at the highest resolution (16x). The different reaolution levels of the THESAN-ZOOM simulation set and the associated parameters are cataloged in the following table:

The suite also features additional runs designed to investigate the effect of varying numerical parameters and physical models on galaxy properties. This gives a total of 60 simulations, which are used to study the formation and evolution of high redshift galaxies.