Formation and Incidence of Shell Galaxies in the Illustris Simulation (1706.06102v2)

Abstract: Shells are low surface brightness tidal debris that appear as interleaved caustics with large opening angles, often situated on both sides of the galaxy center. In this paper, we study the incidence and formation processes of shell galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We identify shells at redshift z=0 using stellar surface density maps, and we use stellar history catalogs to trace the birth, trajectory and progenitors of each individual star particle contributing to the tidal feature. Out of a sample of the 220 most massive galaxies in Illustris ($\mathrm{M}{\mathrm{200crit}}>6\times10^{12}\,\mathrm{M}{\odot}$), $18\%\pm3\%$ of the galaxies exhibit shells. This fraction increases with increasing mass cut: higher mass galaxies are more likely to have stellar shells. Furthermore, the fraction of massive galaxies that exhibit shells decreases with increasing redshift. We find that shell galaxies observed at redshift $z=0$ form preferentially through relatively major mergers ($\gtrsim$1:10 in stellar mass ratio). Progenitors are accreted on low angular momentum orbits, in a preferred time-window between $\sim$4 and 8 Gyrs ago. Our study indicates that, due to dynamical friction, more massive satellites are allowed to probe a wider range of impact parameters at accretion time, while small companions need almost purely radial infall trajectories in order to produce shells. We also find a number of special cases, as a consequence of the additional complexity introduced by the cosmological setting. These include galaxies with multiple shell-forming progenitors, satellite-of-satellites also forming shells, or satellites that fail to produce shells due to multiple major mergers happening in quick succession.

• The paper leverages the Illustris Simulation and a robust identification method to find that approximately 18% of massive galaxies at $z=0$ exhibit shell structures, with incidence increasing with mass.
• Formation of these shell galaxies in massive hosts is largely driven by relatively major mergers (ratio > 1:10) of progenitors accreted on low angular momentum orbits 4-8 Gyrs ago.
• Shells can survive for 2-4 Gyrs post-merger, making them valuable tracers of past galactic accretion history for observational astronomy.

Formation and Incidence of Shell Galaxies in the Illustris Simulation

The paper “Formation and Incidence of Shell Galaxies in the Illustris Simulation” by A. R. Pop et al. offers a comprehensive analysis of the origin and prevalence of shell galaxies within the context of a cosmological hydrodynamical simulation. Utilizing the data from the Illustris Simulation, the paper examines the factors contributing to shell galaxy formation and their frequency across different mass and redshift ranges.

Key Findings

1. Simulation Methodology: The Illustris Simulation is leveraged to model galaxy dynamics and assembly histories, giving rise to a diverse array of morphological structures that include stellar shells. Shell galaxies are identified via a robust two-step approach using visual analysis of stellar density maps and stellar history catalogs.
2. Incidence of Shell Galaxies: From a sample of 220 massive galaxies, defined as $\mathrm{M}_{\mathrm{200crit} > 6 \times 10^{12} \, \mathrm{M}_{\odot}$, approximately $18\% \pm 3\%$ exhibit shell structures at redshift $z=0$. Notably, the incidence increases with higher galactic masses, and shows a mild decrease at higher redshifts, likely due to shorter shell lifetimes in those conditions.
3. Merger Events as a Catalyst: A primary assertion of the paper is that shell formation in massive galaxies is largely driven by relatively major mergers, i.e., those with mass ratios greater than 1:10. These progenitors are typically accreted on low angular momentum orbits within a specific time window (4-8 Gyrs ago), suggesting that shells are remnants of hierarchical merging processes.
4. Phase Mixing Times: The paper assesses the phase mixing times of shells, suggesting that on average, shells can survive for 2-4 Gyrs post merger. However, mixing times are influenced by the mass of the host and the dynamical conditions, providing insight into the observable features of shell galaxies at different epochs.

Theoretical and Observational Implications

The findings of Pop et al. have significant implications for the theories surrounding galaxy evolution and dynamics. The preference for major mergers as a formation mechanism for shell galaxies suggests that intensive interactions are a gateway for inducing complex stellar morphology in massive ellipticals. Moreover, the impact of infall orbits and dynamical friction on shell formation aligns with previous theoretical models but also extends the understanding of orbital dynamics in real cosmological scenarios.

For observational astronomy, these insights necessitate careful consideration of redshift effects and environmental factors when cataloging shell galaxies. The paper highlights the importance of shell structures as tracers of galactic accretion history, which may unveil the nuanced merging pathways of host galaxies. Future surveys with heightened sensitivity to low surface brightness features are poised to refine measurements of shell incidence and further constrain the formation processes.

Speculations and Future Directions

While the paper articulates a compelling case for the role of major mergers in shell galaxy formation, future work may explore quantifying phase mixing times and the environmental dependency of shell visibility. The ongoing advances in cosmological simulations, coupled with observing platforms designed for low surface brightness studies, will enhance our ability to test these theoretical predictions across diverse galactic environments.

In conclusion, the paper by Pop et al. represents a significant stride in leveraging cosmological simulations to unravel the complexities of shell galaxy formation, providing pivotal insight into the hierarchical nature of galaxy assembly. As datasets grow and simulations become more refined, the understanding of these structures will undoubtedly deepen, offering a broader perspective on the iterative cosmic dance of galaxy evolution.