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Physics & Astronomy Department

Astr212 Project: Light profile of galaxy NGC 7331 (The Deer Lick Group)

NGC 7331 GalaxyContents: This image is the spiral galaxy NGC 7331 as viewed from our Celestron 16 telescope at Calvin College. This galaxy closely
resembles our own Milky Way galaxy and the Andromeda galaxy. NGC 7331 is approximately 46 million light years away with a radius of about 32,000 light years which makes it visible only with the aid of a telescope.

NGC 7331 was discovered in 1784 by William Herschel. NGC 7331 was later named "The Deer Lick Group" in honor of an incredible night for viewing at Deer Lick Gap, just off the Blue Ridge Parkway in the North Carolina mountains.

NGC 7331 is a spiral galaxy composed of a large number of stars which create the light that makes it visible to observers. The Deer Lick Group has been used for radio observations which searched for supernovae remnants. Through unsuccessful searches, nuclear sources were found which were emitted from the galaxy, pointing to the theory that a massive black hole (5*10^8 Mo) resides within NGC 7331.

The objective for our project was to capture a high quality grayscale image of the galaxy NGC 7331, and analyze the light data from the image. We also determined the approximate inclination angle of the galaxy to be 63 degrees, or almost edge on, which is consistent with the 60-70 degree range found by other sources. The inclination angle of a galaxy is the angle between its plane and the sky plane, so with an inclination angle is 0 degrees we would be viewing it face on, and with an angle of 90 degrees we would be viewing it edge on. Dark spots
in the galaxy also show where the highest concentrations of dust are because the dust blocks the light.

NGC 7331 Galaxy

Processing: This image was created by Andrew Vanden Heuvel with the help of Derek Wright and Nathaniel Witte. It was created using 20 fifteen second exposures that were taken on November 15, 2002. All the images were taken with a clear filter and dark and flat field corrections were applied. We used CCDSoft to optimize our final image, and DS9 to gather light profile data. We analyzed the light profile data and created a light curve in Microsoft Excel. This project is of interest to us because we can use our light curve data to find the mass distribution of the galaxy's light-emitting matter. The equation that best fits the log of our light curve data is y = -0.2205x + 2.5396. From this equation we calculated a scale length function of e^(-r / -.2205) which gives NGC 7331 a scale length of 4.535 kpc. The scale length is important because it is the distance over which light levels decrease by a factor of 1/e. The brightness decreases slower in galaxies with larger scale lengths. (Elmegreen p.93)

Orientation and Scale: In the image North is up and East is toward the left. The image is 9.42 by 6.22 arc minutes. The galaxy is located at 22h37m0.60s, +34d25^Ò00^Ô (Epoch 2000), units are the celestial coordinates.

Additional resources:
http://www.seds.org/messier/xtra/ngc/n7331.html
http://www.clifty.com/scott/DSO/7331.html

Team: Andrew Vanden Heuvel, Derek Wright, Nathaniel Witte