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What dominates the energy budget of molecular cloud sub-structures?

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  • Figure 1: The hierarchical scales in the SILCC deep-zoom simulations. The larger box shows the cloud MC1-MHD, with two dynamically different regions shown as separate panes. The density slice, with the planar velocity field shown using arrows, suggests that MC1-MHD-Ra is more quiescent, gravity dominated; while MC1-MHD-Rc is being compressed by larger scale flows. The black contours in each case show one larger-scale dendrogram structure.
  • Figure 2: The virial balance of different dendrogram sub-structures, with the structure of the figure adapted from Dib et al., 2007. The x-axis represents the virial ratio, while the y-axis represents the sum of the gravitational (W) and the kinetic, thermal, and magnetic terms (ΘVT). The different quadrants represent conditions of being bound/unbound for the forming structures. Each point, color coded by their magnitude-wise largest energy term, represents a single dendrogram sub-structure. Almost all bound sub-structures are dominated by the interplay of ram pressure (light blue) and gravity (red).

Shashwata Ganguly
 

The interstellar medium (ISM) spans many orders of magnitudes in scales and is highly turbulent in nature. Molecular clouds, spanning few tens to hundreds of parsecs, represent the densest and coldest part of the hierarchical scale of the ISM. Yet, the process of how certain parts of this dense (~100 particles/cc), cold (~10 K) molecular gas ends up as stars (~1023particles/cc, ~5000 K) by climbing almost twenty one orders of magnitude in density is not well understood.

A crucial key to this puzzle is understanding the energy budget of the ISM gas at different scales. Stars are gravitationally bound. Yet the ISM on the largest scales is not. A key way to understand this transition from unbound to bound is by considering the energetic balance of self-consistently formed sub-structures inside molecular clouds at various scales. By investigating the scales at which gravity begins to dominate, we can understand the pathways of structure-, and eventual star formation inside parent clouds.

Figure 1 shows the hierarchy of different sub-structures obtained in the SILCC deep-zoom simulations, where we analyze structures formed with a 0.0078 parsec (~1600 astronomical units) resolution, starting from a large stratified galactic region of size ~500 parsecs.

We identify forming structures using a dendrogram algorithm – and consider the effect of various energy terms on their virial balance: gravity W (self-gravity EPE and tidal energy EPEext), and ΘVT, combining the effects of kinetic (volume: EKE, surface:EKEsurface), thermal (volume: ETE, surface: ETEsurface), and magnetic energy (volume: EB, surface: EBsurface) terms. From the relative contribution of the various terms, we can define a virial ratio α = WVT.

Using α and the sum of W and ΘVT, we can separate the different structures by their boundness. The dendrogram sub-structures, color coded by their magnitude-wise largest energy term, can be seen in Figure 2. The different quadrants describe different conditions – bottom left: bound by gravity; bottom right: bound by ram pressure, temperature gradient, or curvature in the magnetic field; top left: dispersed by tidal forces top right: dispersed by magnetic, thermal or turbulent pressure. 

We find that almost all the bound structures are bound by either self-gravity (red) or ram pressure (light blue), suggesting that the interplay between turbulence and gravity dominates the energy budget across different regions.