The optical morphologies of galaxies in the IllustrisTNG simulation: a comparison to Pan-STARRS observations (1809.08239v2)

Abstract: We have generated synthetic images of $\sim$27,000 galaxies from the IllustrisTNG and the original Illustris hydrodynamic cosmological simulations, designed to match Pan-STARRS observations of $\log_{10}(M_{\ast}/{\rm M}{\odot}) \approx 9.8$-$11.3$ galaxies at $z \approx 0.05$. Most of our synthetic images were created with the SKIRT radiative transfer code, including the effects of dust attenuation and scattering, and performing the radiative transfer directly on the Voronoi mesh used by the simulations themselves. We have analysed both our synthetic and real Pan-STARRS images with the newly developed $\tt{statmorph}$ code, which calculates non-parametric morphological diagnostics -- including the Gini-$M{20}$ and concentration-asymmetry-smoothness (CAS) statistics -- and performs two-dimensional S\'ersic fits. Overall, we find that the optical morphologies of IllustrisTNG galaxies are in good agreement with observations, and represent a substantial improvement compared to the original Illustris simulation. In particular, the locus of the Gini-$M_{20}$ diagram is consistent with that inferred from observations, while the median trends with stellar mass of all the morphological, size and shape parameters considered in this work lie within the $\sim$1$\sigma$ scatter of the observational trends. However, the IllustrisTNG model has some difficulty with more stringent tests, such as producing a strong morphology-colour relation. This results in a somewhat higher fraction of red discs and blue spheroids compared to observations. Similarly, the morphology-size relation is problematic: while observations show that discs tend to be larger than spheroids at a fixed stellar mass, such a trend is not present in IllustrisTNG.

Overview of "The optical morphologies of galaxies in the IllustrisTNG simulation: \ a comparison to Pan-STARRS observations"

This paper presents a comprehensive paper comparing the optical morphologies of galaxies simulated in the IllustrisTNG hydrodynamic cosmological simulation with those observed in the Pan-STARRS survey. Authored by Rodriguez-Gomez et al., the research employs synthetic imaging techniques and rigorous morphological analysis to assess the performance of the IllustrisTNG model in producing realistic galaxy morphologies when compared to observational data.

Simulation and Imaging Methodology

The work begins by generating synthetic images of approximately 27,000 galaxies from both the IllustrisTNG and the original Illustris simulations. These images aim to replicate Pan-STARRS observations of galaxies with stellar masses ranging from $\log_{10}(M_{\ast}/{\rm M}_{\odot}) \approx 9.8$ to $11.3$ at $z \approx 0.05$. The paper employs the skirt radiative transfer code to incorporate realistic photon scattering and attenuation due to dust, performed on a 3D Voronoi mesh that mimics the simulation’s computational structure. This approach ensures the synthetic images accurately reflect the complex interplay of light and dust within simulated galaxies.

The paper also introduces a secondary synthetic imaging pipeline based on the galaxev stellar population synthesis code for galaxies showing negligible dust content. This enables efficient and accurate modeling while optimizing computational resources.

Morphological Analysis and Findings

Using the newly developed statmorph code, the paper conducts an evaluation of both synthetic and observed galaxy morphologies through several diagnostic measures, including Gini--$M_{20}$ and concentration--asymmetry--smoothness (CAS) indices, as well as 2D Sérsic profile fitting. Statmorph provides an advanced framework for the analysis of morphological features in both synthetic and observational datasets.

The analysis reveals that IllustrisTNG shows substantial improvement over its predecessor, Illustris, with the Gini--$M_{20}$ locus and concentration index trends aligning closely with observed data (Figure 1). IllustrisTNG galaxies exhibit realistic representations of early-type, late-type, and potentially merging systems as per observed trends, indicative of the advantages provided by the reparametrized galactic wind and AGN feedback models in the simulation.

Contradictory Outcomes

Despite the successes, certain discrepancies are noted. IllustrisTNG struggles with the morphology--colour relation, resulting in an overproduction of red discs and blue spheroids, which deviates from observational data. Similarly, the morphology--size relation is problematic, as expected larger disc sizes are not evident.

Implications and Speculations

While the IllustrisTNG model's improvements are noteworthy, the challenges encountered could provide insights into the underlying galaxy formation processes. The paper suggests that the lack of accurate representation of merger-induced starbursts and AGN activity might require further refinements in resolution or model tuning. It proposes an exploration of mechanisms to enhance merger-driven quenching, potentially benefiting both starburst and AGN feedback models.

Conclusion and Future Directions

Rodriguez-Gomez et al.'s findings highlight the importance of utilizing synthetic observations to bridge theoretical simulations and empirical data. The detailed examination provides a robust platform for further developments in galaxy formation and evolution theory, particularly in light of upcoming surveys and advanced telescopes like JWST and LSST.

This work establishes a foothold for continuous refinement of large-scale simulations and emphasizes the necessity for collaborations between observational and theoretical astrophysics to decode the complexities of galaxy morphologies. As computational capacities grow, future iterations of such simulations promise more accurate modeling and deeper insights into the cosmic phenomena governing galaxy evolution.