4. Residual Images

To examine the inner structure of the central host galaxy, we create a residual image. The residual image is produced by taking the ratio of the original reduced image with a smoothed image. This has the effect of removing the large-scale structure in the image and emphasizing the small-scale structure.

4.1 FMEDIAN

We use the IRAF task FMEDIAN to smooth the original reduced image. FMEDIAN slides a two-dimensional window throughout the image. The dimensions of the window is specified by the user. For each pixel FMEDIAN centers the window on the pixel, computes the median of all pixels within the window, and replaces the center pixel with the median value. The window size is chosen to best emphasize the inner structure of the central host. For UGC 7576 and NGC 2685 we use a 10 by 10 pixel window.

4.2 IMDIVIDE

The IRAF task IMDIVIDE is used to divide the original reduced image by the FMEIDAN-smoothed image. IMARITH could also have been used but IMDIVIDE also has option to rescale output image. We do not rescale the output image in this case.

4.3 Inner Structure of PRGs

4.3.1 UGC 7576

The residual image in both B and R show a luminous warped disk-like strucuture in the central host of UGC 7576. Both images are shown below in Figure 7. The disk is evidence that the central host is an S0 type galaxy.

B Residual	R Residual
(a)	(b)
Figure 7. This figure shows the residual images for UGC 7576 in the B band (a) and R band (b)

4.3.2 NGC 2685

A luminous disk-like structure is also present in the U band residual image of NGC 2685. It is likely that the central host of this polar ring galaxy is also S0 type. The residual image is shown below in Figure 8. The color scale of the image has been inverted.

U band Residual
Figure 8. This figure shows the U band residual image of NGC 2685.

4.3.3 NGC 3718