On the Effects of Projection on Morphology

We study the effects of projection of three-dimensional (3D) data onto theplane of the sky by means of numerical simulations of turbulence in theinterstellar medium including the magnetic field, parameterized cooling anddiffuse and stellar heating, self-gravity and rotation. We compare thephysical-space density and velocity distributions with their representation inposition-position-velocity (PPV) space (``channel maps''), noting that thelatter can be interpreted in two ways: either as maps of the column density'sspatial distribution (at a given line-of-sight (LOS) velocity), or as maps ofthe spatial distribution of a given value of the LOS velocity (weighted bydensity). This ambivalence appears related to the fact that the spatial and PPVrepresentations of the data give significantly different views. First, themorphology in the channel maps more closely resembles that of the spatialdistribution of the LOS velocity component than that of the density field, asmeasured by pixel-to-pixel correlations between images. Second, the channelmaps contain more small-scale structure than 3D slices of the density andvelocity fields, a fact evident both in subjective appearance and in the powerspectra of the images. This effect may be due to a pseudo-random sampling(along the LOS) of the gas contributing to the structure in a channel map: thepositions sampled along the LOS (chosen by their LOS velocity) may varysignificantly from one position in the channel map to the next.Comment: 6 figures. To appear in the March 20th volume in Ap