Beam bias: The influence of beamwidth on observation of H I 'clouds' and the significance of derived cloud densities

Details Beam bias: The influence of beamwidth on observation of H I 'clouds' and the significance of derived cloud densities Beam bias: The influence of beamwidth on observation of H I 'clouds' and the significance of derived cloud densities

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aj....106.2580v&link_type=abstract

The Astronomical Journal (ISSN 0004-6256), vol. 106, no. 6, p. 2580-2586

Astronomy and Astrophysics

Astronomy

9

Beams (Radiation), Bias, Density (Mass/Volume), Diameters, Molecular Clouds, Radio Astronomy, Resolution, Width, Histograms, Tables (Data), Very Long Base Interferometry

Scientific paper

Typical published values for H I 'cloud' angular diameters are found to be three times the beamwidth used to map the structure. Estimates of H I cloud densities therefore suffer from a bias introduced by resolution because H I volume densities are generally derived by invoking spherical symmetry for the clouds. Published volume density estimates are shown to be defined by nH = 380 B'theta -1 d-1100 NH20, where B'theta is the resolution in arcminutes, d the cloud distance in units of 100 pc, and NH2O the observed column density in units of 1020 sq cm. This resolution dependency of cloud density implies that the influence of instrumental parameters on the H I data is more profound than has heretofore been recognized. Whatever the cause for this 'beam bias' (e.g., fractal structure in the H I), it may be difficult to extract physically meaningful parameters from the data unless specific models are offered to account for the observations. It is concluded that the physical interpretation of H I cloud diameter and density data should be treated with extreme caution.

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