The influence of magnetic fields in Cloud-Cloud Collisions

TL;DR

This study investigates how magnetic fields influence star formation during head-on cloud–cloud collisions using isothermal SPH MHD simulations of two $500\,M_{\odot}$ clouds. By varying the initial field strength and collision velocity, it identifies two primary morphologies—Hub Filament and Spiders Web—and shows that magnetic fields delay fragmentation, broaden filaments, and shift outcomes toward centrally condensed clusters via competitive accretion. The findings quantify how magnetism governs filament widths, star-system formation timescales, and the stellar mass function, revealing a transition boundary that moves toward hub formation as fields strengthen. These results highlight magnetic fields as a key regulator of both the morphology and timing of star formation in GMC collisions, with implications for the prevalence of monolithic clusters and high-mass star formation in magnetized environments.

Abstract

Cloud-cloud collisions are expected to trigger star formation by compressing gas into dense, gravitationally unstable regions. However, the role of magnetic fields in this process is unclear. We use SPH to model head-on collisions between two uniform density clouds, each with mass $500 \,$M$_{\odot}$, initial radius 2 pc, and embedded in a uniform magnetic field parallel to the collision velocity. As in the nonmagnetic case, the resulting shock-compressed layer fragments into a network of filaments. If the collision is sufficiently slow, the filaments are dragged into radial orientations by non-homologous gravitational contraction, resulting in a $\textit{Hub Filament}$ morphology, which spawns a centrally concentrated monolithic cluster with a broad mass function shaped by competitive accretion and dynamical ejections. If the collision is faster, a $\textit{Spiders Web}$ of intersecting filaments forms, and star-systems condense out in small subclusters, often at the filament intersections; due to their smaller mass reservoirs, and the lower probability of dynamical ejection, the mass function of star-systems formed in these subclusters is narrower. Magnetic fields affect this dichotomy quantitatively by delaying collapse and fragmentation. As a result, the velocity threshold separating $\textit{Hub Filament}$ and $\textit{Spiders Web}$ morphologies is shifted upward in magnetised runs, thereby enlarging the parameter space in which $\textit{Hub Filament}$ morphologies form, and enhancing the likelihood of producing centrally concentrated clusters. Consequently, magnetic fields regulate both the morphology and timing of star formation in cloud-cloud collisions: they broaden filaments, delay the onset of star formation, and promote the formation of $\textit{Hub Filament}$ morphologies, monolithic clusters and high-mass star-systems.

Figures (10)

• Figure 1: False-colour surface-density maps looking along the $x$ axis and therefore face-on to the layer, showing the evolution of the filamentary network for collisions at $\Delta u_{\rm o}\!=\!3.6\,{\rm km\,s^{-1}}$. Left panels: no magnetic field, $B_{\rm o}\!=\!0$. Right Panels: strong magnetic field, $B_{\rm o}\!=\!5\, \upmu{\rm G}$. Each panel has dimensions $1{\rm pc}\,\times\,1{\rm pc}$. Times from the start of the collision are shown in the bottom right corner of each panel; each sequence spans $0.08\,{\rm Myr}$ and ends at $t_{10\%}$. The colour scale is logarithmic. For gas with solar composition, $\,1\,{\rm g\,cm^{-2}}$ corresponds to $\sim2.1\!\times\!10^{23}\,{\rm H_2\,cm^{-2}}$. Red dots mark the locations of star-systems. Maps have been generated using splashprice2007splash.
• Figure 2: False-colour surface-density maps at $t = t_{10\%}$, looking along the $x$ axis and therefore face-on to the layer. Left panels: no magnetic field, $B_{\rm o}\!=\!0$. Right Panels: strong magnetic field, $B_{\rm o}\!=\!5\, \upmu{\rm G}$. Top Panels: lower collision velocity $\Delta u_{\rm o}\!=\!2.4\,{\rm km\,s^{-1}}$. Bottom Panels: higher collision velocity $\Delta u_{\rm o}\!=\!3.6\,{\rm km\,s^{-1}}$. The initial magnetic field, $B_{\rm o}$, and the collision velocity, $\Delta u_{\rm o}$, are given in the top lefthand corner of each frame, and the value of $t_{10\%}$ in the bottom righthand corner. The colour scale is logarithmic. For gas with solar composition, $\,1\,{\rm g\,cm^{-2}}$ corresponds to $\sim2.1\!\times\!10^{23}\,{\rm H_2\,cm^{-2}}$. Red dots mark the locations of star-systems. Maps have been generated using splashprice2007splash.
• Figure 3: The normalised distribution of filament widths ( fwhm) for different magnetic field strengths: Left Panel, no field, $B_{\rm o}\!=\!0$; Middle Panel, intermediate field, $B_{\rm o}\!=\!3.3\,\upmu{\rm G}$; Right Panel, strong field $B_{\rm o}\!=\!5\,\upmu{\rm G}$. The mean, standard deviation and median of the fwhm, are shown in the top righthand corner of each panel. Each panel represents the results obtained from all the experiments obtained with that $B_{\rm o}$ value, i.e. three realisations for each value of $\Delta u_{\rm o}$.
• Figure 4: Zoom-in false-colour surface-density maps showing the filamentary structures and magnetic field lines in the vicinity of representative star-systems: Top Panel, a star-system formed at the intersection of three filaments; Bottom Panel, a star-system formed in the middle of a filament. The colour-scale representing surface-density is logarithmic, and the white lines indicate the direction of the lateral magnetic field, $B_{yz}$. Red dots mark sink locations.
• Figure 5: The normalised distribution of angles, $\theta_{\hbox{\tiny SB}}$, between the filament spines and the local magnetic field at $t_{10\%}$. This plot derives from all the experiments (three realisations for each combination of $B_{\rm o}$ and $\Delta u_{\rm o}$).