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Astronomical Observatory: Cool Images

Astr384 Photography Projects, Spring 2004

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M104 Sombrero Galaxy, Shannon Fogwell

M104 Sombrero Galaxy

M104, also known as the Sombrero Galaxy or NGC 4594, is a type Sa spiral galaxy, located in the constellation Virgo. We view it from 6° above the plane of the galaxy, almost completely edge on. It was first discovered by Pierre Méchain in 1781 and catalogued as the 104th object in Messier’s famous catalog. It has a right ascension of 12h 39m 59.3s and a declination of -11°37’22” (J2000). The galaxy has angular dimensions of 8.7 x 3.5 arcmin and it is at a distance of about 50 million light-years. At this distance, its angular diameter corresponds to a linear diameter of about 82,000 light-years. With a redshift of 0.003416, which means it is traveling away from us at a velocity of 1091 km/s, it was one of the first objects found to have such a high redshift, suggesting that it was not a part of the Milky Way, but some other galaxy. It is thought to contain a supermassive black hole at the center of the galaxy. At 8.6th magnitude, the Sombrero Galaxy can be easily seen in a moderate telescope.

In this image, we can see the bright core of the galaxy, the huge dust lanes, and even some hints of the faint spiral structure in the disk of the galaxy. The reddish tint of the core of the galaxy shows that it contains mostly older, redder stars. We can see the evidence for the tightly wound spiral arms which give the Sombrero its type Sa classification. The prominent dark lane across the center of the galaxy is the composed of dust that is obscuring light from behind it. The faint glow above and below the central plane of the galaxy is part of the halo of stars and star clusters that surround the galaxy.

Processing Technique

The first step in this project was to take a series of images of M104, the Sombrero Galaxy. I took sets of 300 second exposures with 1x1 binning, seven each in the red and visual filters, sixteen in the blue filter, and ten in the open filter. More images were taken with the blue filter than red and visual to make up for its lack of sensitivity. Once the images were taken, I started processing, using MaxIm DL 4.0, by subtracting the ghosts from all the images and then proceeding to calibration. I used the appropriate bias, dark and flat-field frames to calibrate all the images. The next step was to align all of the sets of images. Before combining the sets of images, I went through them to check for quality and decided to discard open images 1, 8, and 9, visual image 6 and red image 5 due to what looked like poor tracking. The blue images were all of equal quality so I kept all of them. I then combined each set of images using the sigma clip output option. Next I used the remove bloom function, manually boxing those stars that were overexposed and needed the blooming removed. The next step was to align each of the four combined images in preparation for color combining, using two-star align. Then, before color combining the images, I first applied MaxIm’s digital development filter on the open image, with the filter type set to FFT-low pass, FFT hardness set to custom with a cutoff of 2% and chose the background and midlevel values manually. Finally then, for the color combining, I used the four-color (LRGB) option, with the conversion color space set to RGB with background auto equalize enabled and selected the appropriate files. I set the values for r, g, and b to 1.7, 2.6, and 17 respectively, with the luminance weight set to 100%. The last step then was to apply a gamma function to the image to bring out both the bright core and the fainter structure in the outer parts of the galaxy. For this I chose a gamma value of 0.9, an input range of 750 to 10000, and output range set to 16-bit. This gave the best overall view of all parts of the galaxy.





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