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Astr212 Galaxy Projects, Spring 2009

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Barred Spiral Galaxy (NGC 2903), Josh Orndorff


NGC 2903 is a barred spiral Galaxy similar to the Milky Way (the galaxy in which we live). It is a little over 20 million lightyears away from the Earth, and 63 thousand lightyears across.

Color and Structure of the Image
Unlike everyday photography, astronomical photographs are made by taking pictures of an object in several different color filters, and then adding them together after all the desired colors are taken. this has the distinct benefits of showing each color individually, thereby highlighting cosmic phenomena that tend to emit light in specific colors, as well as allowing humans to see the galaxy at non visible wavelengths. The image above is a true-color image which means that all of the filters used were at visible wavelengths, and further that if something appears blue in the picture that it really is emitting blue light. This does not mean however that if you simply put your eye up to the eyepiece of a telescope that is pointed at NGC2903 that you would see a blue-purple object like above. Because I have images in each filter, I am able to emphasize whatever colors I want in order to point out important characteristics of the galaxy.

I've chosen to highlight the bluish purple color in this photograph because it highlights the kinds of stars that are present in this galaxy. These stars are likely young hot stars as the light that they are emitting is blue rather than red.

Multi-wavelength Analysis -- The X-Ray Halo
NGC2903 Halpha and X-ray

The image above is courtesy of D. Tsch¨oke, G. Hensler, and N. Junkes (2003). The picture itself is the H-alpha emission, and the overlaid contours show the full PSPC bandwidth. More detail can be seen in their paper: An X-ray halo in the “hot-spot” galaxy NGC 2903.

Light Profile

NGC2903 Light Profile

The diagram above shows the brightness at each point along the bar of the galaxy and extending out beyond the ends of the bar. The high central spike is the central bulge of the galaxy, but the most interesting features are the two lower spikes about one and a half kiloparsecs out from the center. These bright spots occur at each end of the bar and are quite notable and somewhat common. After the end of the bar, the light dies down slowly until it fades into sky's background. This illustrates the fact that galaxies don't have hard definite edges, but rather that material gradually become less dense.


Some Technical Detail about Data Reduction
As I mentioned previously, I started by taking several images in several color filters, red, blue, clear, and visible specifically. The next step was to subtract out noise and artifacts due to the physical conditions of the telescope when the images were taken including using dark, flat, and bias calibration. Next I combined all of the pictures in a given filter, and finally combined all the filters to make a color picture emphasizing the blue light.


Right Ascension (J2000) 09:32:10
Declination (J2000) 21:30:03
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 8X300 seconds in C
5x300 seconds in R
8x300 seconds in V
15x300 seconds in B
Dates observed

14 February, 2009
21 February, 2009
02 March, 2009