[Paper Review] The diverse nature and formation paths of slow rotator galaxies in the EAGLE simulations

Claudia del P. Lagos, Éric Emsellem+6 more|arXiv (Cornell University)|Dec 15, 2020

Galaxies: Formation, Evolution, Phenomena6 references4 citations

TL;DR

This study uses the EAGLE simulations to analyze slow rotator (SR) galaxies at z=0, revealing that mergers—especially major and minor—drive kinematic transformation and increase triaxiality and ex-situ stellar fractions. The key finding is that quenching via AGN feedback typically precedes kinematic evolution, except in satellites where mergers and environment jointly trigger SR formation, with distinct formation pathways for flat, round, and prolate SRs.

ABSTRACT

We use a sample of $z=0$ galaxies visually classified as slow rotators (SRs) in the EAGLE hydrodynamical simulations to explore the effect of galaxy mergers on their formation, characterise their intrinsic galaxy properties, and study the connection between quenching and kinematic transformation. SRs that have had major or minor mergers (mass ratios $\ge 0.3$ and $0.1-0.3$, respectively) tend to have a higher triaxiality parameter and ex-situ stellar fractions than those that had exclusively very minor mergers or formed in the absence of mergers (no-merger SRs). No-merger SRs are more compact, have lower black hole-to-stellar mass ratios and quenched later than other SRs, leaving imprints on their $z=0$ chemical composition. For the vast majority of SRs we find that quenching, driven by active galactic nuclei feedback, precedes kinematic transformation, except for satellite SRs, in which these processes happen in tandem. However, in $\approx 50$\% of these satellites, satellite-satellite mergers are responsible for their SR fate, while environment (i.e. tidal field and interactions with the central) can account for the transformation in the rest. By splitting SRs into kinematic sub-classes, we find that flat SRs prefer major mergers; round SRs prefer minor or very minor mergers; prolate SRs prefer gas-poor mergers. Flat and prolate SRs are more common among satellites hosted by massive halos ($>10^{13.6}\, m M_{\odot}$) and centrals of high masses ($M_{\star} > 10^{10.5}\, m M_{\odot}$). Although EAGLE galaxies display kinematic properties that broadly agree with observations, there are areas of disagreement, such as inverted stellar age and velocity dispersion profiles. We discuss these and how upcoming simulations can solve them.

Motivation & Objective

• To understand the formation mechanisms of slow rotator (SR) galaxies in the EAGLE simulations.
• To investigate the role of galaxy mergers—major, minor, and very minor—in shaping SR properties such as triaxiality and stellar fraction.
• To examine the temporal relationship between quenching (via AGN feedback) and kinematic transformation in SRs.
• To classify SRs by kinematic morphology (flat, round, prolate) and link these to formation pathways and host halo mass.
• To identify discrepancies between simulated and observed SR properties, such as inverted age and velocity dispersion profiles, and suggest improvements for future simulations.

Proposed method

• Utilized a sample of z=0 galaxies visually classified as slow rotators (SRs) from the EAGLE hydrodynamical simulations.
• Tracked merger histories of SRs by analyzing mass ratios (≥0.3 for major, 0.1–0.3 for minor, <0.1 for very minor) and merger timing relative to quenching and kinematic transformation.
• Quantified intrinsic properties including triaxiality, ex-situ stellar fraction, black hole-to-stellar mass ratio, and compactness.
• Classified SRs into kinematic sub-classes: flat, round, and prolate, based on shape and rotation properties.
• Compared simulated SR kinematics and structural profiles with observational data to identify discrepancies, such as inverted stellar age and velocity dispersion profiles.
• Analyzed environmental effects, including tidal fields and central galaxy interactions, particularly in satellite SRs.

Experimental results

Research questions

• RQ1 What is the role of galaxy mergers in the formation of slow rotator galaxies in the EAGLE simulations?
• RQ2 How do merger histories (major, minor, very minor, or no mergers) affect the structural and kinematic properties of SRs?
• RQ3 Which process—quenching or kinematic transformation—occurs first in SR formation, and how does this vary between centrals and satellites?
• RQ4 What are the distinct formation pathways for different kinematic morphologies of SRs (flat, round, prolate)?
• RQ5 To what extent do simulated SR properties agree with observations, and what discrepancies remain?

Key findings

• SRs that experienced major or minor mergers (mass ratio ≥0.3 or 0.1–0.3) exhibit higher triaxiality and greater ex-situ stellar fractions than no-merger SRs.
• No-merger SRs are more compact, have lower black hole-to-stellar mass ratios, and quench later than merger-affected SRs, leaving distinct chemical imprints.
• For the majority of SRs, quenching via AGN feedback precedes kinematic transformation, but in ~50% of satellite SRs, quenching and transformation occur simultaneously.
• In satellite SRs, ~50% are transformed by satellite-satellite mergers, while the remaining 50% are shaped by environmental effects like tidal fields and central galaxy interactions.
• Flat SRs prefer major mergers, round SRs prefer minor or very minor mergers, and prolate SRs prefer gas-poor mergers.
• Flat and prolate SRs are more common in massive halos (>10^13.6 M☉) and among high-mass centrals (M⋆ > 10^10.5 M☉), indicating environmental and mass-dependent formation trends.