Simulating Galaxy Formation with the IllustrisTNG Model (1703.02970v2)

Abstract: We introduce an updated physical model to simulate the formation and evolution of galaxies in cosmological, large-scale gravity+magnetohydrodynamical simulations with the moving mesh code AREPO. The overall framework builds upon the successes of the Illustris galaxy formation model, and includes prescriptions for star formation, stellar evolution, chemical enrichment, primordial and metal-line cooling of the gas, stellar feedback with galactic outflows, and black hole formation, growth and multi-mode feedback. In this paper we give a comprehensive description of the physical and numerical advances which form the core of the IllustrisTNG (The Next Generation) framework. We focus on the revised implementation of the galactic winds, of which we modify the directionality, velocity, thermal content, and energy scalings, and explore its effects on the galaxy population. As described in earlier works, the model also includes a new black hole driven kinetic feedback at low accretion rates, magnetohydrodynamics, and improvements to the numerical scheme. Using a suite of (25 Mpc $h^{-1}$)$^3$ cosmological boxes we assess the outcome of the new model at our fiducial resolution. The presence of a self-consistently amplified magnetic field is shown to have an important impact on the stellar content of $10^{12} M_{\rm sun}$ haloes and above. Finally, we demonstrate that the new galactic winds promise to solve key problems identified in Illustris in matching observational constraints and affecting the stellar content and sizes of the low mass end of the galaxy population.

• The paper introduces the IllustrisTNG model that integrates magnetohydrodynamics to capture magnetic effects previously omitted in galaxy formation simulations.
• The model refines stellar feedback with isotropic galactic winds, modulated by dark matter velocity dispersion and local metallicity, to better match observed stellar masses.
• Calibrated against cosmological simulations, the revised framework achieves improved stellar-to-halo mass relations and galaxy size predictions aligned with astronomical observations.

Analysis of the IllustrisTNG Model for Simulating Galaxy Formation

The paper by Pillepich et al. introduces an updated model for simulating galaxy formation, known as the IllustrisTNG (The Next Generation) model. This model incorporates enhancements to both the physical processes considered and the numerical techniques employed within the framework of cosmological, large-scale simulations. The developments build upon the previous Illustris project, offering significant advances aimed at resolving its shortcomings and expanding the scope of simulated astrophysical phenomena.

Numerical and Physical Framework

The IllustrisTNG model operates using the {\sc Arepo} moving mesh code, benefiting from improvements in hydrodynamical schemes, including gradient estimation via the least-squares fit method and a second-order Runge-Kutta integration scheme. Moreover, the model now integrates magnetohydrodynamics (MHD) to capture the effects of magnetic fields on galaxy formation and evolution, which are resolved with the inclusion of a seed field at high redshift.

Key Components and Enhancements

The TNG model introduces substantial updates through three primary avenues:

1. Magnetic Fields (MHD): The incorporation of MHD in cosmological simulations is a novel feature in the TNG model. Previous simulations largely ignored magnetic fields, but this inclusion reflects their observed importance in governing gas dynamics and star formation processes within galaxies. Magnetic fields significantly influence the growth of stellar content through magnetic pressure and alteration of gas accretion predictions, revealing effects especially in larger haloes where the field impacts star formation suppression and gas dynamics.
2. Galactic Winds: A comprehensive refinement of the stellar feedback processes is achieved by altering the directionality, velocity, and energy scaling of galactic winds. In contrast to the bipolar approach in Illustris, TNG adopts isotropic winds with velocity modulated by local dark matter velocity dispersion and redshift scaling. These changes aim to better match observed galaxy stellar mass functions, particularly improving the model's performance at low masses by recalibrating wind parameters and introducing a minimum wind speed floor (350 km/s). The modulation of wind energy by the metallicity of star-forming gas cells is another sophisticated alteration promoting adaptive feedback response based on local environmental conditions.
3. Stellar and Black Hole Feedback: The TNG model maintains the stochastic nature of stellar formation and evolution but modifies the parameters affecting supernova feedback. Core collapse supernova and asymptotic giant branch (AGB) star contributions are recalibrated with updated yield tables to more accurately reflect mass and energy distribution during stellar evolutionary phases. The model also revises black hole (BH) dynamics, particularly addressing feedback modes to better mimic observed feedback effects at various accretion rates.

Results and Model Calibration

The authors calibrated the new model using a set of cosmological simulations intended to evaluate and constrain the outcome of the revised parameters. In comparing TNG to its predecessor, the model demonstrates resolved discrepancies such as those related to the stellar mass of galaxies, the cosmic star formation rate density, and the gas content of halos. The improvements result in a better alignment with observational data, particularly for the stellar-to-halo mass relations and galaxy sizes, both critical aspects where Illustris showed notable biases.

Implications and Future Prospects

The introduction of the TNG model provides a versatile tool for exploring pivotal aspects of galaxy evolution grounded in sophisticated simulations capable of capturing intricate physical phenomena. The adoption of MHD and revisions to feedback mechanisms enable enhanced predictive power, particularly for massive halos and low mass galaxy populations, extending the model’s applicability across cosmic epochs.

Despite these advances, the authors acknowledge certain limitations in numerical convergence and the sensitivity of model predictions to resolution, advocating for higher resolution simulations to further reduce these dependencies. Additionally, they highlight the open-ended potential for further enriching cosmic magnetic field studies and detailing the interplay with BH feedback.

The IllustrisTNG model thus represents a comprehensive enhancement that bridges several gaps present in cosmological simulations, offering a firm footing for future investigations that aim to reconcile theory with the expanding wealth of astronomical data. It sets the stage for the upcoming IllustrisTNG project, which promises to broaden the exploration of cosmic structure formation processes at unprecedented scales and fidelities.