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Molecular clouds: spheroids, filaments or sheets?

Figure 1: 3D rendering of an example molecular cloud obtained from the SILCC-Zoom simulations of realistic molecular clouds. The blue surface outlines the entire sheet-like cloud at a number density of nH ~ 100 cm -3, while the red structure denotes a central dense filament at ~1000 cm -3.

Shashwata Ganguly

An important clue to the puzzle of how we obtain stars from molecular clouds is the geometry of the said clouds. Historically, the geometry of molecular clouds have often been assumed to be spherical for simplicity. While astronomers have always known that this is incorrect, it was considered a “good enough” assumption.

With high resolution maps of different molecular cloud complexes, and especially with the space observatory “Herschel”, it became increasingly clear that the interstellar medium (ISM) is not “blobby”, but rather more filamentary in nature (see Andre et al. (2014), Hacar et al. (2022) for general overviews of the filamentary ISM).

Even more recently, GAIA observations, that allow us to access both 3-D position and velocity information, seem to point to the fact that even that is not the end of the story. Using these proper motion information, several works have concluded that many famous filamentary molecular clouds are actually perhaps sheet-like in nature, with our previous understanding limited due to projection effects. Rezai Kh. et al. (2022) have concluded this for the California molecular cloud. Tritsis et al. (2022) reach similar results regarding the Musca molecular cloud.

The morphology of molecular clouds contains important information relating to how the clouds themselves form. Inutsuka et al. (2015) propose a theory where the molecular clouds form at the shells and intersections of expanding supernova bubbles. This picture can naturally lead to sheet-like molecular clouds tracing the shells of such bubbles.

A morphological analysis of forming molecular clouds in the SILCC-Zoom simulations seem to point to the same. The SILCC-Zoom clouds are simulations of realistic molecular clouds in an embedded galactic region. Example 3D rendering of a cloud morphology can be seen in Fig. 1. The overall cloud structure (shown in blue) is thin sheet-like, while the denser embedded structures are often more filamentary. The red structure in Fig. 1 shows one such dense filament. The structures were obtained using a dendrogram analysis, which here computes hierarchically nested 3D density contours.


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