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Astr212 Project: Light profile of M31

M31 spiral galaxy

Contents: M31 is the well-known Andromeda Galaxy. It is a Sb galaxy, that is, a spiral galaxy. It is 2.9 million lightyears away from us and its diameter is over 200,000 lightyears. The light from the galaxy is solely the result of stars fusing hydrogen. The reason it appears hazy is that its distance makes it difficult to resolve individual stars without using very high magnification. It has two smaller satellite galaxies, M32 and M110, that are gravitationally bound in orbit around it. They are visible only in telescopes and even then appear only as small fuzzies.

Since it appears to the naked eye as a faint hazy glow, the first discovery of M31 was possibly about as early as 905 AD or earlier by a famous Persian astronomer named Abd-al-Rahman. Another Persian, Al-Suf, was the first one to depict M31 in his Book of Fixed Stars, though he did not claim the discovery of M31. In December of 1612, Simon Marius was the first one to find and observe M31 with a telescope. An Italian, Giovanni Gastista Hodierna, did not realize that the galaxy was reported earlier, so he “rediscovered” M31 around 1654.

During that time of some centuries, M31 was mistaken to be the Great Andromeda Nebula (not Galaxy) and was assumed to be one of the closest nebulae to us. This mistaken belief was overturned by William Huggins, a pioneer in spectroscopy. He discovered the difference between the spectral lines of nebula and galaxy, thus establishing that the Andromeda Galaxy is not a nebula.

Procedure: The goal of this project was to calculate the effective radius of M31. The area bound inside the effective radius contains half the luminosity from the galaxy. Astronomers use the effective radius to classify galaxies into useful categories. They can use the relative sizes of the effective radii in different wavelengths to understand what is going on in the galaxy and how it compares to other galaxies of the same type. The effective radius is also useful for finding the mass and mass distribution in a galaxy.

In order to calculate the effective radius, we had to first create contour plot of the southwest region of M31, along the semi-major axis of the galaxy. We measured the radii (in pixels) of each of the contour levels along the galaxy’s axis and then converted the pixels to arcseconds, and then to kiloparsecs (using the distance-angle formula). DS9, a data-analysis program, gave us the brightness of each of the contour levels. We subtracted the brightness of the background from the brightness at each contour line and took the log (base 10) of each value. We plotted the radius to the one-fourth power against the log of the brightness at each of the radii. The slope of the resulting line gave us the value of the effective radius in kiloparsecs, by using the formula
log S = -3.331 * [(r/re)^(1/4) – 1] + log Se
We found the effective radius, over which the light curve of M31 drops off sharply, to be 6.6 kpc.

M31 light profile

Processing: The images were taken by Andrew Jordan and Lee Miller on the nights of October 5, 2002 and November 2, 2002 between 10 P.M. and 12 A.M. EDT. All images were taking with the 16-inch Celestron telescope using clear filters. Each image had at exposure length of 20 seconds. Dark subtractions and flat field corrections were applied to each image taken. Each field was then averaged and the resulting images were arranged into a mosaic with Fireworks. The contour levels and plots were made using DS9, and GSView was used to convert the pictures to jpeg for the photomosaic.

Orientation and Scale: M31 is located at 00h 42m 54s and 41° 17’ 14” in the constellation Andromeda. In the image north is up and east is to the left. The angular dimensions of the image are about 19 arcminutes by 12 arcminutes, corresponding to about 5 kpc by 3 kpc in the galaxy itself.

Interesting Links:
For more information on the Andromeda galaxy, go to: http://www.seds.org/messier/m/m031.html
For a good picture showing the outer regions of the galaxy, while overexposing the central bulge (what we imaged), go to:

Observers: Karen Lie (class of 2004, mechanical engineering). Lee Miller (class of 2005, physics). Andrew P. Jordan (class of 2005, physics).