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The origin of Non-thermal Radio Filaments observed in the Galactic Centre

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  • Figure 1: The slice of the density, pressure, and magnetic field distribution (from top to bottom) along the z-axis of the comet-like filament in the ISM (left), harp-like filament in the ISM (middle), and the harp-like filament in the CMZ (right). The distributions are shown in the stellar rest frame. The arrows in the magnetic field distributions indicate the direction of the magnetic field.
  • Figure 2: The time-integrated synchrotron intensity for the three configurations considered in this project, shown in the laboratory frame. The intensity is normalised by the maximum intensity in each distribution. The comet-like and harp-like filaments (Adapted from Heywood et al. 2022, ApJ *925* 165 and Thomas et al. 2020 ApJ *890* 2) are also overlayed on the left of the distribution.

Keito Watanabe

The origins of numerous filamentary structures observed in the Central Molecular Zone (< 300 pc of the Milky Way) through non-thermal, radio emission have not yet been known to this date. Various theories exist, but a particularly promising origin are through the synchrotron radiation emitted from cosmic rays (relativistic charged particles) that are accelerated by bow shocks generated from massive runaway stars.

As a first step to verify this possible origin, we focus on two particular morphologies of such filaments (Comet-like and Harp-like) and numerically simulate stellar bow shocks in environments similar to the interstellar medium (ISM) and the Central Molecular Zone (CMZ). We numerically solve the magnetohydrodynamic equations using FLASH, a hydrodynamical code used to simulate astrophysical scenarios. Figure 1 shows the density, pressure, and magnetic field distribution for the three proposed scenarios for a massive star of 30 M and velocity of v = 30 and 100 km/s within a uniform magnetic field of B = 10 μG. They show that the magnetic fields dominate the dynamics of the formation of the bow shock with ISM-like environments but is balanced between the gas density and the magnetic fields for CMZ-like environments.

To compare with the observed filaments, we construct synthetic emission maps by assuming that the bow shock accelerates cosmic rays that can emit synchrotron radiation. Figure 2 shows the time-integrated synchrotron emission map of our three proposed models, overlayed with the observed filaments from the MeerKAT telescope. We show that the simulations qualitatively match the observed filaments but still show deviations due to the lack of cosmic rays within our simulations.

Our next steps are to include cosmic rays within our simulations by numerically modelling the acceleration of cosmic rays from bow shocks and dynamically coupling the transport equation of cosmic rays with the magnetohydrodynamic equations. In this way, we can more accurately construct synthetic emission maps that will allow us to better verify whether stellar bow shocks can be an origin of these filaments.