Gas-Star Misaligned Galaxies: Dynamics & Evolution

• Gas-Star Misaligned Galaxies are systems where the stellar and gaseous rotational axes differ, indicating external accretion, mergers, or disturbance.
• Observations and simulations show misalignment fractions vary with morphology, with early-type galaxies exhibiting up to 45% misalignment due to bulge-dominated dynamics.
• These galaxies often feature central starbursts, angular momentum cancellation, and enhanced AGN fueling, providing key insights into galaxy evolution and morphological transformation.

Gas-star misaligned galaxies are systems in which the rotational axes (or angular momentum vectors) of the stellar and gaseous components are not aligned, leading to measurable differences in kinematic position angles between stars and gas. These misalignments can manifest over a range of angles, up to and including counter-rotation (180° misalignment), and are interpreted as evidence for recent or ongoing external accretion, dynamical disturbance, or secular evolution processes. Observational and theoretical efforts have characterized the incidence, structural properties, formation channels, and evolutionary implications of such systems across the galaxy population, especially using integral field spectroscopy (IFS) from large surveys and cosmological simulations.

1. Incidence and Morphological Dependence

The incidence of gas-star misalignment is strongly dependent on host galaxy morphology, star formation status, and gas content. Surveys such as the SAMI Galaxy Survey and MaNGA consistently find that approximately 11–12% of galaxies with measurable stellar and gas kinematics exhibit a misalignment angle |ΔPA| ≥ 30° between the kinematic major axes of stars and gas components (Bryant et al., 2018, Ristea et al., 2022, Zhou et al., 2022). However, this fraction is highly non-uniform across morphological types:

Galaxy Type	Misalignment Fraction
Early-type (ETG)	30%–45%
S0 (lenticular)	10%–15%
Late-type (LTG)	3%–5%

Misalignments are most prevalent in structurally concentrated, bulge-dominated systems with high Sersic index (n > 2–2.5) and low stellar spin parameter λ₍Re₎ (Ristea et al., 2022, Zhou et al., 2022). These trends persist regardless of total stellar mass but correlate with lower gas fractions and reduced specific star formation rates (sSFR), especially in quiescent systems. The global properties of misaligned galaxies, such as higher Sersic indices and lower λ₍Re₎, indicate dispersion-dominated dynamics and morphological transformation toward early-type characteristics.

2. Formation Mechanisms and Dynamical Pathways

The predominant physical mechanisms leading to gas-star misalignment are external gas accretion, galaxy mergers, and environment-driven gas disturbance, with minor contributions from secular processes and AGN-driven outflows.

• External Gas Accretion: The dominant channel, especially in isolated or group environments, involves the acquisition of gas from the cosmic web, filaments, or gas-rich satellites whose angular momentum is misaligned with that of the host stellar disk (Jin et al., 2016, Khim et al., 2020, Cao et al., 2022, Bao et al., 4 Mar 2025). The gas-gas collision and subsequent redistribution of angular momentum drive gas inflow and often centrally concentrated star formation in star-forming and green-valley misaligned galaxies.
• Mergers: Both major and minor mergers contribute, particularly in quiescent systems, by delivering gas with a different angular momentum orientation or by disturbing the dynamical axis through orbital angular momentum transfer (Li et al., 2019).
• Environment-driven Mechanisms: In clusters, ram-pressure stripping and interactions with the intracluster medium can induce misalignments, but these cases are relatively rare and are more often reflected in gas kinematic disturbances rather than lasting misaligned structures (Bryant et al., 2018, Khim et al., 2019).
• AGN-driven Outflows: Up to 8% of cases may result from outflows associated with Seyfert-like AGN, temporarily decoupling the gas kinematics from the underlying stellar rotation (Ristea et al., 2022).
• Secular Evolution: Smooth accretion and dynamical instabilities may create moderate misalignments, but these have shorter lifetimes due to efficient viscous drag and gravitational torques (Khim et al., 2020).

For S0 (lenticular) galaxies, misaligned gas acquisition is established as an additional bulge-growth and morphological transformation channel (Zhou et al., 2023). Central gas infall and star formation facilitate bulge build-up, while the depletion of outer cold gas and redistribution of angular momentum cause the fading of spiral arms.

3. Gas Dynamics, Star Formation, and Angular Momentum Exchange

The dynamical interaction between misaligned and pre-existing gas leads to efficient angular momentum cancellation and strongly affects both the gas distribution and star formation profiles:

• Gas Inflow and Central Starbursts: Angular momentum redistribution from gas-gas collisions triggers inflow, resulting in enhanced central SFR and younger stellar populations in the inner regions of star-forming and green-valley galaxies (Jin et al., 2016, Xu et al., 2022, Zhou et al., 2022).
• Morphological Changes: Associated with these processes are higher Sersic indices (n ~ 3–4 for misaligned SF and GV galaxies versus n ~ 2 for controls), lower λ₍Re₎ in all misaligned types, smaller effective radii, and lower HI/M(H₂) detection rates (indicating gas depletion or redistribution) (Zhou et al., 2022, Zhang et al., 15 Sep 2025).
• HI Profile Transformation: External gas accretion leads to a shift from double-horned to single-peaked HI profiles in centrally HI-deficient quiescent galaxies, reflecting central HI “filling-in” from inflow; smaller changes are seen in already central HI-enriched star–forming galaxies (Zhang et al., 15 Sep 2025).

The radial gradient of Dₙ4000 in young misaligned S0s is positive (younger centers), in contrast to negative gradients in aligned and merger remnant S0s (Zhou et al., 2023). These observational diagnostics support scenarios of bulge growth through accretion-triggered central starbursts.

4. Environmental and Cosmological Context

Large-scale structure and environment play significant roles in both the incidence and evolutionary timescales of misalignments:

• Cosmic Web Connection: Gas-star misaligned galaxies are preferentially located in regions dominated by filaments and clusters but are statistically less dense and further from filament spines compared to aligned analogs (Bao et al., 4 Mar 2025). The gas spin vectors in misaligned galaxies tend to be perpendicular to the minor axis of the large-scale structure ($\vec{e}_3$), consistent with recent direct cold gas accretion from the cosmic web.
• Cluster versus Field: Misalignment fractions are similar in field and cluster environments for the overall galaxy population, but the physical drivers differ: accretion-driven in field/isolated galaxies, predominantly disturbance-driven in clusters (Bryant et al., 2018, Khim et al., 2019). Denser environments generally lengthen the dynamical settling time, prolonging the observable misalignment phase (Khim et al., 2020).
• Isolated Environments: Both ETG/Quiescent and S0 misaligned galaxies are often found in more isolated settings, which favors efficient external gas accretion and suppresses environmental quenching by ram pressure or strong tidal forces (Zhou et al., 2023, Jin et al., 2016).

5. Theoretical Interpretation and Simulations

Cosmological hydrodynamical simulations (e.g., Illustris, EAGLE, Horizon-AGN, FIREbox) provide robust frameworks for interpreting the formation and persistence of misaligned galaxies:

• Angular Momentum Dynamics: The misalignment angle $\theta$ between gas and stars is generally defined by $$\cos\theta = \hat{\mathbf{J}}_\star \cdot \hat{\mathbf{J}}_{\rm gas}$$, with significant deviations from unity indicating misalignment or counterrotation. The persistence of misalignment is controlled by a competition between gravitational torques (which realign the gas) and continuous external accretion (which can sustain misalignment) (Voort et al., 2015, Taylor et al., 2018, Casanueva et al., 2021).
• Settling Timescale: The analytic expectation for settling is $$t_s \propto R/V_{rot}(2\epsilon - \epsilon^2)\cos\phi$$, where $\epsilon$ is the ellipticity and $\phi$ is the initial spin misalignment angle (Ristea et al., 2022, Bryant et al., 2018).
• Morphology Dependence: Disky (rotationally supported) galaxies can realign infalling gas rapidly via strong gravitational torques and viscous drag, whereas triaxial or bulge-dominated systems tend to retain misalignment for Gyrs (Khim et al., 2020, Casanueva et al., 2021).
• Multiple Accretion Events: Rare systems with two misaligned gas disks have been observed, confirming that galaxies may undergo multiple discrete misaligned accretion episodes—these coexist only briefly due to efficient collisional dissipation (Cao et al., 2022).

6. Connection to Black Hole Accretion and Feedback

Kinematic misalignment between stars and gas has been directly linked to enhanced nuclear black hole activity:

• AGN Fueling: Misaligned or counter-rotating gas is funneled efficiently to galactic centers due to angular momentum loss through shocks and gravitational torques, leading to increased accretion rates onto supermassive black holes (SMBHs) (Raimundo et al., 2023).
• Observational Evidence: The fraction of galaxies hosting AGN or LINER activity is significantly higher in systems with large gas-star misalignments (ΔPA ≥ 45°) compared to aligned galaxies, at high statistical confidence.

7. Evolutionary and Cosmological Implications

The existence and characteristics of gas-star misaligned galaxies provide critical insights into galaxy assembly and transformation pathways:

• Bulge Growth and Morphological Transformation: Accretion-driven misalignment stimulates bulge growth and suppresses spiral arm formation/fading by removing cold gas from the outskirts, reshaping disk galaxies into S0s or more spheroidal types (Zhou et al., 2023).
• Timescales and Evolution: The lifetime of kinematic misalignment depends on internal structure (short for disks, long for spheroids) and environment (longer in groups/clusters due to repeated perturbations), typically ranging from ~0.75 Gyr (LTGs) to >2 Gyr (ETGs) (Khim et al., 2020, Casanueva et al., 2021).
• Feedback Effects: Misalignment episodes can regulate star formation via both AGN- and starburst-driven gas removal, contributing to rapid quenching and migration from the blue cloud to the red sequence in the color–magnitude diagram (Wong et al., 2015, Cenci et al., 2023).
• Large-Scale Structure Connection: The coupling between cosmic web orientation, gas spin, and misalignment underscores the importance of the cosmic environment in angular momentum acquisition and the non-linear evolution of galaxy spin vectors (Bao et al., 4 Mar 2025).

In sum, gas-star misaligned galaxies are paradigmatic of the complex interplay between external accretion, internal dynamics, star formation, black hole growth, and large-scale structure. Their properties—and the physical mechanisms governing their formation and persistence—serve as crucial diagnostics of the baryon cycle and morphological evolution in galaxies across environments and cosmic time.