10.1.2 Fitting Overlapping Galaxies

By overlapping galaxies, we mean two (or more) galaxies, which have a small angular separation, so that it is not possible just flag the other galaxy. Therefore, the galaxies have to be fitted iteratively. GALPHOT does not support this per se, so one has to do it in a manual fashion. In the following, image is the original image, gal1 is the brightest of the two overlapping galaxies, and gal2 is the faintest.

1.

(a)

Using image, gal1 is fitted with gal2 flagged with a large radius. A model image based on the fit is subtracted from image to get image_mod. The background level in image is added to image_mod, so that the two has the same background levels, keeping photon statistics unaltered.

(b)

Using image_mod, gal2 is fitted with gal1 flagged with a large radius. A model image based on the fit is subtracted from image to get image_gal1 - the notation is, that gal1 is the galaxy left in the image image_gal1. The background level in image is added to image_gal1.

2.

(a)

Using image_gal1, gal1 is fitted with gal2 flagged with a small radius. A model image based on the fit is subtracted from image to get image_gal2. The background level in image is added to image_gal2.

(b)

Using image_gal2, gal2 is fitted with gal1 flagged with a small radius.

We found that the above 2 steps were enough. Otherwise the procedure could be continued further.

The iterative procedure were also used on galaxies with bright stars very close to the center. This gave a much better fit than if we had just flagged the star. This kind of star subtraction was done for 2 of the 34 galaxies fitted by me. Note the notation: we have 64 galaxies and 227 galaxy observations.