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Messier 102 (NGC 5866)
Zachary Bruce




Messier 102 (or M102) is classified as a SO-3, or lenticular, galaxy in the Draco constellation. This type of galaxy is thought to be a remnant of a spiral galaxy whose star formation has slowed down or stopped completely. A lenticular galaxy has both a bulge at the center and a disc around it. In the image above, one can clearly see a disc of dust going through the middle and a prominent bulge in the center. There is some controversy over its discovery.  M102 was most likely discovered by Pierre Méchain in March 1781.  Pierre’s report caused Charles Messier to include it as number 102, but without giving it a position or verification.  However, about two years after the observation, Méchain retracted his discovery, stating that he made an error, and that he had measured M101 and thought it was a different object.  There are some problems with this retraction though; M101 and M102 are notably different. Today it is accepted by most that he had observed NGC 5866.

The distance to M102 is approximately 45 million light years, or 13.79 Megaparsecs, away. We can use this to find the physical size, which includes the major axis (long direction) and the minor axis (narrow direction), of the object. One can calculate this by first finding the coordinates of the end points for each axis, then use the Pythagorean Theorem to find the distance between the two. The major axis was 131.8 pixels and the minor axis was 47.71 pixels. Next, we need the size in radians. To do this, we have to multiply the pixel length by the number of arcseconds per pixel, then convert to radians. Since the image was taken with a binning of 2x2, there are 1.18 arcseconds per pixel. The major axis ended up being 7.541x10^-4 radians, and the minor axis was 2.729x10^-4. Finally, we must use the equation where l is the physical size, is the angular size in radians, and d is the distance. In the end, the major axis was calculated to be 3.394x10^4 light years, and the major axis was 1.228x10^4 light years.

Image Interpretation:

As you can see from the image above, the color of the galaxy is slightly yellow. This suggests that the stars in the galaxy are low mass, and have not yet reached the point in their lifespan where they become red giants. Because this is a lenticular galaxy, most of the star formation has ceased. However, there is some evidence of massive stellar formation, because the dust in the disk is warm. There must be some massive stars in order to heat the dust. The disk also shows evidence of warm ionized gas, which is another indicator of stellar formation. There is also strong evidence of rich cool gas in the galactic disk as well. Even though this gas is slowly being consumed by the stellar formation, it can be replenished by the mass loss of evolved stars. In summary, this dust lane or disk is made of dust and both warm and cool ionized gas.


Dust Lane Opacity:

So how much light does this dust lane block? To determine this, one must measure the intensity of the light while moving from the top of the galaxy to the bottom (along the minor axis). As expected, the intensity is lower in the middle due to the dust lane blocking the emitted light (see graph A). To find the light blocked by the dust, we can find the lowest point of the dip and read off the intensity. Then, if we can ignore the points of the dip, we can fit a curve to the points left over and find what the intensity is without any dust (Graph B). After doing this, one can find optical depth of the dust lane by using the equation , where I is the observed light intensity including effects of the dust, is the intensity if there was no dust lane, and τ is the optical depth. The optical depth is a value that shows how much extinction of light the dust is causing, meaning the higher the value of τ, the more light being absorbed. was calculated to be 4243, and I was 2172, both in units of counts. After solving for τ, the optical depth ended up being .4009 for the green (V) filter. The final calculation obtainable from this is column density, or the density of the dust lane. To find this, the formula is used, where is the cross-section (assumed to be 10^-9 cm^2), n is the number density, and l is the column length. To find the column density, it is necessary to solve for nl. By doing this, the column density was calculated to be 4.009x10^8 per cm^2.

Graph A: This is a graph of the light intensity as you move along the minor axis. Notice the dip in the center and that I is equal to the lowest point on that dip.


Graph B: This graph estimates what the light intensity of the galaxy would be along the minor axis if the dust lane was not present. The line fit of the points was Gaussian, and here is the peak of the curve (highest y value).


Data Reduction:

There were many steps when creating the final color image you see above. First, each filter (red, green, blue, and clear) had to be calibrated, taking out the dark, bias, and flat effects from each filter. After this, every image from each individual filter had to be combined together. By doing this, flaws from just one image are able to be taken out, and produce a higher quality color picture. Now, a single picture from each filter was obtained, each one of them being calibrated and combined. Now, we must combine these four images into one color image. To make a high quality picture, it is necessary to balance each of the filters correctly. For this image each filter had the same time of exposure. The ratio of each filter should be around 1.0 for Red, 1.5 for Green, and 5.4 to Blue. However, these ratios are not exact. One must adjust each filter until the image is right. It is not necessary to balance the clear filter because its only function is to add more detail to the picture. The final balance for the filters can be seen in the table below. Next, the color saturation needed to be adjusted. This was very important because without doing this, it galaxy was too bright and you were not able to see the dust lane going through the center.

Table for filter balance:

Intput Output Red Green Blue
Red .5    
Green   .9  
Blue     4.32



Right Ascension (J2000) 15:06:30
Declination (J2000) +55.46
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter

B, V, R, C (300s x 5)

Image dimension 595x439 pixels;12.97x9.60 arcminutes
Date/time observed March 22, 3:00-4:30 UT



The Messier Catalog-Seds. <>

 Hartmut Frommert. The Messier Catalog-Sed. <>

Jiang-Tao L, , Q. Daniel Wang, Zhiyuan L, Yang Chen. "DYNAMIC S0 GALAXIES: A CASE STUDY OF NGC 5866". <>



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