Two Formation Channels of Quenched Lenticular Galaxies in Field Environments: Insights from the NewHorizon Simulation

Core Concepts

This study, using the NewHorizon cosmological simulation, reveals that lenticular galaxies (S0s) in field environments primarily form through two distinct channels: mergers and counter-rotating gas accretion, both leading to gas angular momentum cancellation and subsequent quenching of star formation.

Abstract

• Bibliographic Information: Han, S., Jang, J. K., Contini, E., Dubois, Y., Jeon, S., Kaviraj, S., Kimm, T., Kraljic, K., Oh, S., Peirani, S., Pichon, C., & Yi, S. K. (2024). Exploring lenticular galaxy formation in field environments using NewHorizon: evidence for counter-rotating gas accretion as a formation channel. arXiv preprint arXiv:2411.05910v1.
• Research Objective: This study investigates the formation mechanisms of lenticular galaxies (S0s) in low-density field environments, focusing on the role of mergers and counter-rotating gas accretion.
• Methodology: The authors utilize the NewHorizon cosmological hydrodynamic simulation to analyze the evolution of two massive, star-formation quenched S0s. They examine the galaxies' physical properties, morphology, gas content, and merger histories to identify the key factors influencing their formation.
• Key Findings: The study identifies two primary formation channels for S0s: (1) Mergers, particularly minor mergers, trigger central gas inflow due to gravitational and hydrodynamic torques, leading to rapid star formation and subsequent quenching. (2) Counter-rotating gas accretion, through hydrodynamic collisions with pre-existing gas, similarly causes gas angular momentum cancellation, central gas inflow, and ultimately, quenching.
• Main Conclusions: Both mergers and counter-rotating gas accretion effectively contribute to S0 formation in field environments. The cancellation of gas angular momentum is crucial in both scenarios, driving gas toward the galactic center and leading to rapid star formation and quenching on short timescales (< Gyr).
• Significance: This research provides valuable insights into the diverse formation pathways of S0 galaxies, highlighting the importance of both internal and external processes in shaping their evolution. It also emphasizes the role of gas dynamics and angular momentum in galaxy formation and quenching.
• Limitations and Future Research: The study focuses on two specific S0 galaxies from the NewHorizon simulation. Further research with larger samples and different simulations is needed to confirm the generality of these findings. Investigating the impact of varying accretion histories and environmental factors on S0 formation is also crucial for a comprehensive understanding.

Stats

Two massive star-formation quenched S0 galaxies are selected as the main sample from the NewHorizon simulation.
S0 19 experiences two minor mergers (mass ratios of approximately 1:5 and 1:14) and a mini merger (mass ratio less than 1:20).
S0 26 undergoes a major merger with a mass ratio of approximately 1:4.
The quenching timescale (τQ) for S0 19 is 270 Myr after the first two minor mergers and 15 Myr after the mini merger.
The quenching timescale (τQ) for S0 26 is 550 Myr before the major merger and 280 Myr after the major merger.

Quotes

"We demonstrate in this study that S0 formation can be driven by both mergers and counter-rotating gas accretion."
"In both scenarios, a decrease in gas angular momentum plays a key role in building S0 morphology by relocating exterior gas to the galactic center."

Key Insights Distilled From

Exploring lenticular galaxy formation in field environments using NewHorizon: evidence for counter-rotating gas accretion as a formation channel

by Seongbong Ha... at arxiv.org 11-12-2024

https://arxiv.org/pdf/2411.05910.pdf

Deeper Inquiries

How do the formation pathways and characteristics of S0 galaxies in field environments differ from those in denser environments like galaxy groups and clusters?

S0 galaxies in field environments, as explored in the provided research paper, showcase distinct formation pathways and characteristics compared to their counterparts in denser environments like galaxy groups and clusters. Here's a breakdown:
Field Environments:

Formation Pathways: The research highlights two primary formation channels in field environments:

Mergers: Both minor and mini mergers can induce gravitational torques and hydrodynamic interactions, leading to central gas inflow, enhanced star formation, and eventual quenching.
Counter-rotating Gas Accretion: Accretion of gas with misaligned angular momentum disrupts the existing gas disk, causing similar effects as mergers, ultimately leading to an S0 morphology.

Characteristics:

Gas-poor: Field S0s tend to be gas-poor due to the rapid consumption of gas during their formation.
Kinematically Diverse: They can exhibit a range of kinematic properties, from rotation-dominated disks to pressure-supported systems, depending on their specific formation history.
Denser Environments (Groups and Clusters):

Formation Pathways:

Ram Pressure Stripping: As galaxies traverse the hot intracluster medium (ICM), they experience ram pressure that can strip away their gas content, quenching star formation and potentially leading to an S0 morphology.
Strangulation/Starvation: The removal of halo gas due to interactions with the ICM can cut off the supply of fresh gas, gradually halting star formation and transforming a spiral galaxy into an S0.
Mergers: While mergers are less frequent in denser environments due to high galaxy velocities, they can still occur and contribute to S0 formation.

Characteristics:

Red and Dead: S0s in dense environments are often characterized as "red and dead," indicating old stellar populations and little to no ongoing star formation.
Kinematically Distinct: They tend to have more pressure-supported kinematics, potentially due to the disruption of ordered rotation by interactions within the cluster environment.
Key Differences:

Environmental Influence: The dominant formation mechanisms in dense environments (ram pressure stripping, strangulation) are directly related to the presence of the hot ICM and frequent galaxy interactions, which are absent in field environments.
Gas Content: Field S0s might retain some gas, especially if formed through counter-rotating gas accretion, while those in denser environments are typically gas-deficient due to stripping mechanisms.
Star Formation History: Field S0s can experience rapid quenching episodes tied to mergers or gas accretion events, while those in clusters often undergo a more gradual decline in star formation.

Could other mechanisms, such as feedback from active galactic nuclei (AGN) or morphological quenching, play a significant role in the formation of S0 galaxies, even in the absence of mergers or counter-rotating gas accretion?

Yes, even without mergers or counter-rotating gas accretion, mechanisms like AGN feedback and morphological quenching could contribute significantly to S0 galaxy formation:

AGN Feedback:

Heating and Outflows: Powerful jets and winds driven by AGN can heat the interstellar medium, preventing gas from cooling and collapsing to form stars. This can effectively quench star formation, even in the presence of a gas reservoir.
Gas Removal: AGN outflows can also expel gas from the galaxy entirely, further reducing the fuel for star formation and potentially contributing to an S0 morphology.

Morphological Quenching:

Stabilized Disks: As galaxies evolve, they can develop more stable disk structures, such as bars or thick disks, that suppress large-scale star formation. These structures can inhibit gas inflow and reduce the efficiency of star formation in the disk.
Halo Gas Accretion: Changes in the morphology of the galaxy or its surrounding halo can influence the accretion of gas from the cosmic web. If the halo becomes less efficient at funneling gas onto the disk, star formation can gradually decline, leading to an S0-like state.
Importance in Absence of Mergers/Counter-Rotation:
In the absence of the more dramatic events of mergers or counter-rotating gas accretion, these mechanisms can provide alternative pathways for S0 formation:

Slow and Steady Quenching: AGN feedback and morphological quenching often operate on longer timescales compared to mergers, potentially leading to a more gradual transformation from a star-forming spiral to a passive S0.
Internal Evolution: These mechanisms highlight the importance of internal processes within galaxies in shaping their evolution. Even without external triggers, galaxies can undergo significant changes in their star formation activity and morphology.
Synergistic Effects:
It's important to note that these mechanisms are not mutually exclusive and can act in concert with mergers or counter-rotating gas accretion. For example, a minor merger could trigger AGN activity, which then further suppresses star formation and contributes to the final S0 morphology.

If gas angular momentum plays such a crucial role in galaxy formation, what are the implications for the formation and evolution of other galaxy types, such as spiral and elliptical galaxies?

Gas angular momentum is indeed a fundamental property influencing the formation and evolution of all galaxy types, including spirals and ellipticals:
Spiral Galaxies:

Disk Formation: High gas angular momentum is essential for the formation and maintenance of spiral disks. As gas cools and collapses, conservation of angular momentum leads to the formation of a rotating disk structure.
Spiral Arms: The interplay between gas dynamics, self-gravity, and differential rotation within the disk can lead to the formation of spiral arms, which are sites of active star formation.
Secular Evolution: Gas accretion with aligned angular momentum can sustain star formation in spiral galaxies over extended periods, allowing them to maintain their spiral structure.
Elliptical Galaxies:

Merger-Driven Formation: Major mergers, which disrupt gas angular momentum, are thought to be a primary formation channel for elliptical galaxies. The violent relaxation process during mergers randomizes stellar orbits, leading to a pressure-supported spheroidal morphology.
Early Gas Loss: Ellipticals are generally gas-poor, suggesting that they might have lost their gas content early in their history, potentially due to powerful AGN feedback or starburst-driven winds triggered by mergers.
Lack of Cold Gas Accretion: The hot halos of massive ellipticals can prevent the efficient accretion of cold gas, further limiting their ability to form new stars and maintain a disk.
Implications of Angular Momentum:

Diversity of Galaxy Morphologies: The varying amounts of gas angular momentum acquired during galaxy formation and evolution contribute to the diversity of galaxy morphologies observed in the Universe.
Star Formation Regulation: Gas angular momentum influences the rate and distribution of star formation within galaxies. High angular momentum promotes disk formation and sustained star formation, while low angular momentum can lead to centrally concentrated starbursts or quenching.
Galaxy Evolution Pathways: Understanding how gas angular momentum is gained, lost, and redistributed is crucial for deciphering the evolutionary pathways of different galaxy types and their connection to the broader cosmic web.