Abstract
It is unable to obtain the disk thickness of a face-on spiral galaxy by measuring its surface brightness distribution. Here, in order to obtain the disk thickness of faced-on spiral galaxies, we determine the scale heights of their disks using an analytical method, based on the asymptotic solution of Poison's equation of gravitational potential in a 3-D galactic disk under the equiangular logarithmic spiral type of density perturbation. For measuring the key parameter, namely the innermost position of spiral arms (forbidden radius r0), and free from the effect of light contamination of galactic bulge, an improved image processing method is used in this study by subtracting a two-component (disk+bulge) brightness model from the observed galaxy image. On the basis of some fitted and measured fundamental parameters of spiral structures, we obtain the disk thickness and other parameters for the two face-on spiral galaxies: one normal spiral galaxy (S) and one spiral bar galaxy (SB), and their ratio parameters (the scale ratio of bulge and disk rb/rd, and the ratio of disk scale and thickness rd/H) are also derived. By using this improved method of two-component subtraction, it is easy to obtain the forbidden radius r0, which is smaller than that obtained by measuring directly the original galaxy image, thus the obtained effective thickness of galactic disks becomes thinner.
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