Filamentary Hierarchies and Superbubbles II: Impact of superbubbles and galactic dynamics on filament formation and fragmentation

Abstract

Large scale phenomena in spiral galaxies such as shear, supernovae, and magnetic fields all contribute to the formation and subsequent evolution of filamentary structure and star formation within them. In this paper, we analyze the properties and dynamics of filaments in a simulated Milky Way-like galaxy from Zhao et al. 2024. Using filament and superbubble structure analysis codes, we investigate the roles of galactic shear, supernovae and superbubbles, and magnetic fields on the stability and fragmentation of filaments. We find that local shear has little effect on filament stability and the largest structures at outer radii of the disk may be more likely to be dissipated by shear than supernovae. Filaments are largely parallel to the magnetic field, which plays a significant role in filament stability. By measuring the ratio of surface pressure on a filament to that on its central spine, $χ_f=P_{surf}/P_{central}$, we find that filaments with $χ_f \le 1$ are dominated by their own self gravity and have a strong tendency to be gravitationally supercritical, whereas those with $χ_f > 1$ are either transitory or in the act of being formed. Finally, we investigate the role of ISM pressure on filament dynamics and stability as a function of galactic radius, finding considerable changes in filament stability and the accompanying star formation rates in the inner versus outer regions of the disk.

Figures (16)

• Figure 1: Different temperature projection cuts of the gas in the galaxy from our pillbox projection. Top row:From left to right, we show cold gas ($<$200 K) with filaments overlaid and the warm gas between 200K and 10$^4$K with filaments overlaid. Bottom row: From left to right, we show all the gas hotter than $10^4$K, and the hot gas in the bubbles from perch.
• Figure 2: Length PDFs of the filaments identified from different preparations of our galaxy simulation data. The top left panel shows our original filament population from PillsworthRoscoe2025a, top center shows the filaments identified in a projection limited to distances of +/-427 pc of the midplane (pillbox projection) of the galaxy and the top right distribution shows the filaments identified in the pillbox projection with hot superbubbles masked out of the data. Bottom left and bottom right panels show the distributions for our column density cuts of $\sim10^{22}~\rm{cm}^{-2}$ (titled High) and $\sim7\times10^{21}~\rm{cm}^{-2}$ (titled Low). The power-law fits for each distribution are plotted in black.
• Figure 3: Mass PDFs of the filaments identified from different preparations of simulation data. Top left are the "original" filaments from PillsworthRoscoe2025b. The top middle panel is the distribution from the pillbox projection, while the top right shows the distribution of the pillbox projection with hot superbubbles masked out. Bottom left and bottom right panels show the mass distributions for our column density cuts of $\sim10^{22}~\rm{cm}^{-2}$ (titled High) and $\sim7\times10^{21}~\rm{cm}^{-2}$ (titled Low).
• Figure 4: Comparison of our original filament population's line masses PillsworthRoscoe2025a, PillsworthRoscoe2025b with the filaments from our pillbox projection. In the case of the original population, we show subcritical filaments as the grey line and supercritical as the black, dashed line. For the pillbox case, we show subcritical filaments in blue and supercritical filaments in green.
• Figure 5: Theoretical critical vs. measured line mass joint plots for the filaments in our population. Left: The magnetic critical line mass. Right: The shear critical line mass. The black line in each shows the critical ratio of 1, separating the plane into sub-critical and supercritical regions.