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Contour Generation

The galaxy data set contains galaxy image-files in the FITS format. To be able to create contours around different intensity levels of a galaxy, one needs to extract the intensity information for each pixel for the image. FITSIO, a subroutine, is an interface for reading or writing data files in FITS format. This package was written to provide an interface with FITS files without having to deal directly with the complicated internal details of the FITS files. Using FITSIO the pixel values were extracted from the galaxy image files in array format.

The following method was used to extract contours around different intensity values. All the pixel values greater than the desired value of contour intensity were set to one, and the values less than the desired contour intensity value were set to zero. This divided the image into two groups, where one group contained the pixels having intensity higher than the desired contour intensity and the second group contained the pixels having intensity lower than the desired contour intensity. Now, all the pixels having the value of one that were surrounded by at least one pixel of value zero were retained and all the other pixels were discarded. The only pixels that remained were the pixels at the boundaries of the groups constructing contours around the desired intensity value. The pixel coordinates were stored in a file.

Figure 7 shows examples of contours generated using the method described above. Each example contains one of the 50 contours generated for different intensity values for an elliptical galaxy (NGC 4374) and a spiral galaxy (NGC 4303).


 
Figure 7: NGC 4374 and NGC 4303
\scalebox{0.35}{\includegraphics{NGC4374.ps}} \scalebox{0.35}{\includegraphics{NGC4303.ps}}


next up previous contents
Next: Calculation of Fractal Dimensions Up: FRACTAL DIMENSIONS OF GALAXIES Previous: FRACTAL DIMENSIONS OF GALAXIES
Sandip Thanki
1999-07-29