REAL-SPACE AND POSITION-VELOCITY REPRESENTATIONS

Figure 1 shows various aspects of the ISM simulations. Figure 1a shows the projection of the density field in z-direction. Figure 1b shows 16 cuts of the density field along the z-direction, with a separation of 7 pixels between cuts. Figure 1c shows cuts along the same fixed-z planes for the z-component of the velocity. Finally, figure 1d shows the velocity channels of the corresponding position-velocity (PV) data cube, with a resolution of 16 channels. The PV cube is obtained assuming an optically thin gas, the z-coordinate containing the density-weighted velocity histogram for each (x,y)-pixel. A number of points are worth noting:

• Localized structures in the PV maps do not necessarily correspond to localized structures in physical space, and viceversa. For example, the structure labeled A in the 11th velocity channel in fig. 1d is seen to result from the superposition of two density structures also labeled A in cuts 5 and 11 of the 3D density field (fig. 1b). Conversely, expanding shells (caused by the modeled star formation) are localized in physical space, but extended in PV space, such as the structures labeled B in the same figures.

• The structure seen in the velocity channels of fig. 1d is subjectively more similar to the velocity cuts shown in fig. 1c than to the density cuts shown in fig. 1b, or the projected density field of fig. 1a, indicating that the process of ``selecting'' the density by its velocity imprints features of the velocity distribution onto that of the density. Although further quantification of this effect is necessary, it can be understood as a possible consequence that the structure seen in the velocity channels actually originates from histograms of velocity, albeit weighted by the density.