Properties of galaxies reproduced by a hydrodynamic simulation (1405.1418v1)

Abstract: Previous simulations of the growth of cosmic structures have broadly reproduced the 'cosmic web' of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies due to numerical inaccuracies and incomplete physical models. Moreover, because of computational constraints, they were unable to track the small scale evolution of gas and stars to the present epoch within a representative portion of the Universe. Here we report a simulation that starts 12 million years after the Big Bang, and traces 13 billion years of cosmic evolution with 12 billion resolution elements in a volume of $(106.5\,{\rm Mpc})^3$. It yields a reasonable population of ellipticals and spirals, reproduces the distribution of galaxies in clusters and statistics of hydrogen on large scales, and at the same time the metal and hydrogen content of galaxies on small scales.

• The paper utilizes the Illustris hydrodynamic simulation to successfully reproduce the cosmic web structure and diverse properties of galaxies across various scales, resolving past discrepancies with observations.
• The simulation accurately matches observational data on galaxy morphologies, radial profiles of satellite galaxies in clusters (SDSS), and neutral hydrogen and metal content distributions (ALFALFA, DLA).
• A key finding is that baryonic processes, particularly AGN feedback, significantly influence the dark matter distribution, highlighting the importance of including these effects in future cosmological probes.

Insights on Hydrodynamic Simulations of Galaxy Formation

The paper, "Properties of galaxies reproduced by a hydrodynamic simulation," presents significant advancements in the simulation of the cosmic web and galaxy formation, resolving challenges that have previously hindered accurate modeling of galaxy morphologies and distributions. Utilizing the Illustris simulation framework, it demonstrates improved fidelity in reproducing the universe's structural and compositional features across varying scales.

The haLLMark of this paper is the hydrodynamic simulation initiated 12 million years post-Big Bang, tracing the cosmic evolution for 13 billion years across a volumetric space with a side length of approximately 106.5 Mpc. With over 12 billion resolution elements, the simulation discerns both dark matter and baryonic components, yielding results that resonate with observational data. This luminary achievement counters issues faced by earlier models which struggled with the diversity in galaxy morphologies and the accurate depiction of small-scale structures.

Key Contributions

1. Galaxy Morphologies: The simulation produces a realistic mix of galaxy types, including ellipticals and spirals, demonstrating an evolved capability to replicate a diverse galaxy population. The Illustris model accurately simulates the mix of star-forming and quiescent galaxies. It shows that past limitations were not due to failures of the ΛCDM paradigm but were primarily methodological.
2. Galactic Structures and Observational Alignment: The paper illustrates robust numerical alignment with structural and compositional aspects of galaxies within clusters. The radial profiles of satellite galaxies in clusters agree with observational data from SDSS, resolving prior discrepancies noted in semi-analytic models.
3. Hydrogen and Metal Content: The simulation accurately predicts the neutral hydrogen (HI) content and metal distributions across galaxies, aligning well with observational data from surveys such as ALFALFA and DLA studies. Previous simulations overestimated metallicity or presented a broad distribution inconsistent with observations, which Illustris addresses proficiently.
4. Baryonic Effects on Dark Matter: The paper finds that baryonic processes, particularly AGN-driven feedback, significantly affect the dark matter distribution even beyond the smallest scales. This has implications for cosmological probes relying on the power spectrum, pointing to a need for incorporating baryonic physics in future precision measurements.

Implications and Speculations for Future Developments

The results fortify the efficacy of hydrodynamic simulations in detailing galaxy formation and evolution, providing a benchmark for future work. Illustris's success foretells the evolution of increasingly comprehensive models, potentially resolving outstanding issues like the evolution of low-mass galaxies. However, challenges remain, particularly in ensuring fidelity in the timeline of stellar population development in lower-mass galaxies. Future simulations will need to integrate enhanced physical modeling of star formation and feedback processes, considering radiation pressure and stellar radiation fields.

This paper also underscores the importance of high-resolution, large-volume hydrodynamic simulations in providing insights into cosmic structure formation. As computational capabilities progress, this will inevitably lead to even more sophisticated models capable of addressing unresolved tensions and refining our understanding of cosmological phenomena.

In conclusion, the paper serves as both a landmark in computational astrophysics and a precursor to the future horizon of galaxy formation simulations, offering a detailed synthesis of cosmological and astrophysical processes on a grand scale.