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Astronomical Observatory: Cool Images

Astr384 Photography Projects, Spring 2004

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Virgo Galaxy Cluster, Andrew Jordan

Virgo Galaxy Cluster

There are at least fourteen galaxies in the field, although some of them are fairly faint. The two bright ellipticals at center right are M86 and M84. In the top left comer is NGC 4473; below and to its right are NGC 4458 and 4461. Almost straight below 4461 is a faint smudge marking IC 3393. Above M86 is NGC 4402, which is an edge-on spiral with a pronounced dust lane. Below and between M86 and M84 is NGC 4387. Below and to its right is another fainter galaxy IC 3303. Straight below NGC 4387 is another edge-on spiral with a good dust lane: NGC 4388. To its left is NGC 4413, which has NGC 4425 to its upper left.

These form only a small part of the entire Virgo Cluster, which has around 3000galaxies, according to Burnham's Celestial Handbook. The cluster is about 42 million lightyears from our own galaxy and is composed mostly of spirals and some ellipticals(and a few irregulars "thrown in" for good measure). Half of the galaxies in the field are spiral or lenticular, the other half are elliptical.

This picture is a wonderful case in point for the nonuniform distribution of galaxies in the universe. Since all of these galaxies are part of the Virgo Cluster, further research could find how they affect each other gravitationally. Of course, this would not be complete, since there are thousands more galaxies in the cluster that would also have an effect. M86 has an x-ray gas tail (, and further studies could show what powers it—perhaps a supermassive back hole as in nearby (and more famous) M87.

The Processing

I had sixteen fields of three images each to map some of the Virgo Cluster. The images had 3x3 binning so that the final product would not be an enormous file. First I subtracted the biases and darks from all the images. No flats were taken in 3x3 binning, so I had to create my own. I took a median of all of the images and had MaxIm normalize the median, leaving only the diffraction rings caused by dust particles and other imperfections. I divided all the images by the final dark.

Next I had to combine each set of three images into one. To do this, I took the average of the three images. Some images, however, had cosmic rays in them; I had to take the median of their sets. When I finished this, I had sixteen images that were better than any of the original ones.

Before I could start mosaicing, I had to make the background field on all the images equal. I found the average background count for each image and then scaled all of them up (using Pixel Math) to the brightest image so that I would not lose much information. Now I had sixteen images with the same background level, but not necessarily the same star brightness. For each image, I found a star on the border and compared its pixel count to the same star in the neighboring field. If there were a big difference in the two counts, I scaled the fainter image up, while subtracting a constant to keep the background the same. I only had to do this three times, since usually the counts were very close.

I was then finally able to make the mosaic. This consisted mainly of aligning the images and blending them.

After completing the mosaic, I used a gamma function of 0.5, with the minimum value at 0 and the maximum value at 60000 to display the fainter parts of the images—small galaxies and outer edges of brighter ones—and to get rid of the residual background introduced by the mosaicing. Thus I had my final product.





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