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The Role of Inner Halo Angular Momentum (Spin) in Shaping Dark Matter Bars in Milky Way Analogs

Shouvik Ghosh, Sandeep Kumar Kataria

TL;DR

The paper investigates how the inner-halo spin parameter $\lambda$ shapes the formation and evolution of dark matter bars in Milky Way–analog discs. Using N-body simulations with $\lambda$ in the range $0 \le \lambda \le 0.1$ (prograde) and $\lambda=0.1$ (retrograde), the authors quantify DM-bar triggering times, strengths $A_2/A_0$, pattern speeds $\Omega_p$, and morphological evolution over $\sim9.78$ Gyr. They find that higher prograde spin yields earlier DM-bar formation and stronger inner DM bars, with buckling events inducing transient strength drops in both stellar and dark components, followed by re-saturation and convergence of bar properties across models. The results emphasize halo spin as a key driver of disc–halo secular evolution and suggest observational imprints in inner-halo structure via lensing, consistent with cosmological simulations such as TNG50.

Abstract

Studies of galactic bars have primarily focused on stellar bars, since they can be directly observed through ultraviolet to infrared wavebands. Cosmological as well as idealised simulations reveal that the dark matter (DM) haloes interact with baryonic matter, primarily the stellar bars, dynamically by means of the exchange of angular momentum. In these simulations, the spherical DM halo dynamically responds to interaction with the stellar bar by reshaping its orbital structure in the proximity of the stellar bar, forming a bar-like configuration, called as the Dark Matter (DM) bar. Using N-body simulations of Milky Way analogs we discuss the role of inner halo angular momentum, measured as halo spin parameter λ of the dark matter halo, on formation and evolutionary characteristics of the DM bars. Our systematic study involves haloes with initial spin configurations ranging from λ = 0 to 0.1. The result conveys that DM bar formation and its characteristics are extensively dependent on the initial spin parameter λ of the DM halo. We demonstrate that the strength of the dark matter bar gradually increases with an increase in halo spin in long-term evolution, with a significant impact of stellar bar buckling on dark matter bar strength. The evolutionary characteristics of the DM bar are strongly influenced by the initial spin of the host halo.

The Role of Inner Halo Angular Momentum (Spin) in Shaping Dark Matter Bars in Milky Way Analogs

TL;DR

The paper investigates how the inner-halo spin parameter shapes the formation and evolution of dark matter bars in Milky Way–analog discs. Using N-body simulations with in the range (prograde) and (retrograde), the authors quantify DM-bar triggering times, strengths , pattern speeds , and morphological evolution over Gyr. They find that higher prograde spin yields earlier DM-bar formation and stronger inner DM bars, with buckling events inducing transient strength drops in both stellar and dark components, followed by re-saturation and convergence of bar properties across models. The results emphasize halo spin as a key driver of disc–halo secular evolution and suggest observational imprints in inner-halo structure via lensing, consistent with cosmological simulations such as TNG50.

Abstract

Studies of galactic bars have primarily focused on stellar bars, since they can be directly observed through ultraviolet to infrared wavebands. Cosmological as well as idealised simulations reveal that the dark matter (DM) haloes interact with baryonic matter, primarily the stellar bars, dynamically by means of the exchange of angular momentum. In these simulations, the spherical DM halo dynamically responds to interaction with the stellar bar by reshaping its orbital structure in the proximity of the stellar bar, forming a bar-like configuration, called as the Dark Matter (DM) bar. Using N-body simulations of Milky Way analogs we discuss the role of inner halo angular momentum, measured as halo spin parameter λ of the dark matter halo, on formation and evolutionary characteristics of the DM bars. Our systematic study involves haloes with initial spin configurations ranging from λ = 0 to 0.1. The result conveys that DM bar formation and its characteristics are extensively dependent on the initial spin parameter λ of the DM halo. We demonstrate that the strength of the dark matter bar gradually increases with an increase in halo spin in long-term evolution, with a significant impact of stellar bar buckling on dark matter bar strength. The evolutionary characteristics of the DM bar are strongly influenced by the initial spin of the host halo.
Paper Structure (15 sections, 6 equations, 10 figures)

This paper contains 15 sections, 6 equations, 10 figures.

Table of Contents

  1. Introduction
  2. Model Setup
  3. Results
  4. Stellar Buckling Strength
  5. Evolution of Bar Strength
  6. Stellar Bar Strength
  7. Dark Matter Bar Strength
  8. Pattern Speed
  9. Co-evolution of Bar Strength and Pattern Speed Over Time
  10. Stellar and Dark Matter Bar Angle
  11. Angular Momentum Over Time
  12. Morphological Evolution of DM Bar Structure
  13. Discussion
  14. Conclusion
  15. Acknowledgement

Figures (10)

  • Figure 1: Stellar buckling strength over time for increasing halo spin parameter
  • Figure 2: Evolution of stellar bar strength $A_2/A_0$ over time for increasing halo spin parameter
  • Figure 3: Evolution of dark matter bar strength $A_2/A_0$ over time for increasing halo spin parameter
  • Figure 4: Evolution of dark matter bar pattern speed over time for varying halo spin parameter
  • Figure 5: Co-evolution of bar strength ($A_2/A_0$) and pattern speed ($\Omega_{\mathrm{bar}}$) over time for dark matter bars in halos with varying spin parameters.
  • ...and 5 more figures