Chemical enrichment in cosmological, smoothed particle hydrodynamics simulations

Abstract: (Abridged) We present an implementation of stellar evolution and chemical feedback for smoothed particle hydrodynamics (SPH) simulations. We consider the timed release of individual elements by both massive (Type II supernovae and stellar winds) and intermediate mass stars (Type Ia supernovae and asymptotic giant branch stars). We illustrate the results of our method using a suite of cosmological simulations that include new prescriptions for radiative cooling, star formation, and galactic winds. Radiative cooling is implemented element-by-element, in the presence of an ionizing radiation background, and we track all 11 elements that contribute significantly to the radiative cooling. We contrast two reasonable definitions of the metallicity of a resolution element and find that while they agree for high metallicities, there are large differences at low metallicities. We argue the discrepancy is indicative of the lack of metal mixing caused by the fact that metals are stuck to particles. We argue that since this is a (numerical) sampling problem, solving it using a poorly constrained physical process such as diffusion could have undesired consequences. We demonstrate that the two metallicity definitions result in redshift z = 0 stellar masses that can differ by up to a factor of two, because of the sensitivity of the cooling rates to the elemental abundances. We find that by z = 0 most of the metals are locked up in stars. The gaseous metals are distributed over a very wide range of gas densities and temperatures. The shock-heated warm-hot intergalactic medium has a relatively high metallicity of ~ 10^-1 Z_sun that evolves only weakly and is therefore an important reservoir of metals.

• The paper introduces a new SPH framework that integrates element-specific radiative cooling and timed chemical feedback from various stellar sources.
• It employs high-resolution 512³ particle simulations to evaluate convergence and highlight discrepancies in nucleosynthetic yield predictions.
• The study finds that most metals are locked in stars while gaseous metals span a wide range of densities, underscoring the complexity of cosmic metal distribution.

Chemical Enrichment in Cosmological SPH Simulations

The paper by R. P. C. Wiersma et al. presents an implementation of stellar evolution and chemical feedback in smoothed particle hydrodynamics (SPH) simulations, focused on capturing the timed release of elements by various stellar sources. The study integrates the roles of massive stars, represented by Type II supernovae (SNe) and stellar winds, alongside intermediate mass stars, contributing through Type Ia SNe and asymptotic giant branch (AGB) stars, in cosmological simulations. The approach is demonstrated using a suite of simulations that encapsulate new models for radiative cooling, star formation, and the influence of galactic winds.

Significantly, the implementation tackles radiative cooling on an element-by-element basis, influenced by an ionizing radiation background, while tracking the contributions of 11 elements deemed crucial to radiative processes. Through a series of simulations conducted within uniformly defined physical parameter sets, the research rigorously assesses the robustness and degree of chemodynamical predictions' convergence related to the model's elements, methodologies, and numerical resolutions. Highlighting discrepancies in nucleosynthetic yields sourced from existing literature, it is noted that abundance ratios achieve reliability only to a factor of two, diverging most markedly when ratios involving iron, whose supernova rate remains uncertain, are considered.

Two distinct metallicity definitions are scrutinized. It is highlighted that such definitions concur at high metallicities but deviate significantly at lower levels, a difference attributed to insufficient metal mixing, a consequence of the metals' inseparability from particles. The authors recommend against mitigating this (numerical) sampling problem with poorly constrained physical solutions like diffusion, due to potential unintended outcomes.

Utilizing high-resolution simulations with $512^3$ particles, the evolution of heavy element distributions is probed. By redshift $z=0$, a notable result is that the bulk of metals is sequestered within stars, though gaseous metals distribute across a breadth of gas densities and temperatures. The warm-hot intergalactic medium retains a moderately high metallicity (~0.1 $Z_\odot$) with weak evolution, establishing itself as a critical metal reservoir. Any comprehensive metal mass assessment must account for a diverse array of cosmic structures.

Implications and Future Directions

This rigorous framework for SPH simulations significantly enhances our understanding of cosmic chemical enrichment over time. Capturing element-specific radiative cooling rates advances our ability to model the intricate dynamics of star formation, feedback processes, and their large-scale consequences accurately. Furthermore, the study underlines the importance of robust sub-grid models to realistically replicate hydrodynamic interactions, fundamental to the distribution and evolution of metals in the universe.

Upcoming research in AI might benefit from these insights by advancing models used in predictive astronomy, potentially utilizing deep learning methods to refine the interpretation and prediction of large-scale cosmological phenomena. Comparative studies utilizing different initial conditions and varying galactic environmental parameters could reveal further refinements in our understanding of cosmic evolution via chemodynamical modeling.