QUANTIFYING OBSERVATIONAL PROJECTION EFFECTS USING MOLECULAR CLOUD SIMULATIONS

Abstract

The physical properties of molecular clouds are often measured using\nspectral-line observations, which provide the only probes of the clouds'\nvelocity structure. It is hard, though, to assess whether and to what extent\nintensity features in position-position-velocity (PPV) space correspond to\n"real" density structures in position-position-position (PPP) space. In this\npaper, we create synthetic molecular cloud spectral-line maps of simulated\nmolecular clouds, and present a new technique for measuring the reality of\nindividual PPV structures. Our procedure projects density structures identified\nin PPP space into corresponding intensity structures in PPV space and then\nmeasures the geometric overlap of the projected structures with structures\nidentified from the synthetic observation. The fractional overlap between a PPP\nand PPV structure quantifies how well the synthetic observation recovers\ninformation about the 3D structure. Applying this machinery to a set of\nsynthetic observations of CO isotopes, we measure how well spectral-line\nmeasurements recover mass, size, velocity dispersion, and virial parameter for\na simulated star-forming region. By disabling various steps of our analysis, we\ninvestigate how much opacity, chemistry, and gravity affect measurements of\nphysical properties extracted from PPV cubes. For the simulations used here,\nour results suggest that superposition induces a ~40% uncertainty in masses,\nsizes, and velocity dispersions derived from 13CO. The virial parameter is most\naffected by superposition, such that estimates of the virial parameter derived\nfrom PPV and PPP information typically disagree by a factor of ~2. This\nuncertainty makes it particularly difficult to judge whether gravitational or\nkinetic energy dominate a given region, since the majority of virial parameter\nmeasurements fall within a factor of 2 of the equipartition level alpha ~ 2.\n