Hydrodynamical Simulations of Turbulence in Galactic Centers
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Hydrodynamical Simulations of Turbulence in Galactic Centers

  • Author(s): Salas, Jesus Mario
  • Advisor(s): Morris, Mark R;
  • Naoz, Smadar
  • et al.
Abstract

Turbulence is a prevalent phenomenon in the interstellar medium, and in particular, in the environment at the centers of galaxies. For example, detailed observations of the Milky Way’s Central Molecular Zone (CMZ) have revealed that it has a complex and turbulent structure. It is suspected that turbulence is responsible for maintaining the relatively high temperatures in the CMZ, as well as for suppressing star formation. Therefore, turbulence is a key phenomenon for understanding the dynamics and energetics of this region.

In galactic-scale simulations, turbulence is often modeled by invoking star formation and supernovae feedback. However, these phenomena do not appear to be sufficient for explaining the high-velocity dispersion observed in the CMZ, indicating that additional gas-stirring processes are likely to be operating. In this thesis, I present a new numerical method to driveturbulence in galactic centers. Instead of relying on a particular physical mechanism, I have adopted a Fourier forcing module, which has the advantage of being independent of the actual sources of turbulence, but which is adjustable to the scales needed to match observations. I have applied this method using a Smoothed Particle Hydrodynamics (SPH) code. This turbulence injection method is capable of balancing the self-gravity of gas in different physical scenarios, which allows the simulations to be run for long timescales, thereby enabling my study of the effects of turbulence on the gas dynamics.

First, I introduce this new numerical tool and present simulations of a simplistic model of the CMZ to showcase its performance and numerical convergence. I find that turbulence induces a flocculent spiral pattern similar to that observed in the centers of other galaxies. Furthermore, the driving module induces a turbulent viscosity which enhances mass inflowto the center.

To further show the flexibility and applicability of this method, I present larger-scale simulations of galactic nuclear rings. Nuclear rings are a common feature of galaxies which form due to the effects of bar potentials, and the CMZ contains such a ring. I find that turbulence thickens nuclear rings and enhances mass accretion towards the center, both effects causedby turbulent viscosity. This mass inflow could explain the feeding of material from the CMZ to the inner few parsecs of the Galaxy.

As a final example of my method’s versatility, I performed simulations of the Circumnuclear Disk (CND) at the center of the Milky Way. The transient vs long-lived nature of the CND is widely debated in the literature. Some authors have argued that the observed stream morphology and non-axisymmetric density distribution implies that the CND is a transientstructure. However, the results of my simulations suggest that these arguments are not correct. I find that turbulent perturbation on sufficiently large scales, such as those that would be produced by supernova blast waves, creates a disk with a non-uniform density structure, and promotes the creation of streams that endure for longer than a few dynamical timescales.

In conclusion, the work I present in this thesis shows that my injection method is a flexible tool that can be applied to a variety of physical scenarios. Furthermore, this method can be adapted to different hydrodynamical codes based on alternative hydrodynamics techniques.

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