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Spiral Galaxy M99
Merideth Beukelman

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East M99 Spiral Galaxy West


About 50 million light-years away (14.4 Mpc to be exact) is a beautiful, face-on spiral galaxy that goes by the name Messier 99. The region of the galaxy shown is approximately 15 kpc in diameter (about half the size of the Milky Way Galaxy), and it is located in the Virgo Cluster of galaxies. M99 was discovered by Pierre Méchain, a French astronomer, in the year 1781. Something peculiar one may notice about M99 is its unusual asymmetric shape. What qualifies it as asymmetric is the large lower south arm that looks quite different than the small, upper northern and eastern arms. The large southern arm extends much farther out that the other two arms we see in M99. There are only speculations about the cause of this strange phenomena, but one hypothesis is that the asymmetry of this galaxy may be a result of fairly recent interactions with other nearby galaxies in the Virgo Cluster.

Colors and Structures:

When looking at a picture of M99, one may wonder, "what is actually giving off light in this galaxy, and what is producing the different colors?" In this galaxy, many stars are giving off light, and the stars are also responsible for a lot of the different colors. The bright, blue light seen in M99 comes from new stars being formed in the long spiral arms where there is a high amount of concentrated gas. New stars are very bright and very blue, especially when compared to older stars, which can also be found in this galaxy. There is a very bright, yellow core at the center of this galaxy which is appropriately named the bulge. That bulge is made up of billions of low mass, low temperature, and low luminosity stars. They are very hard to spot individually in a picture like this, but when you put billions of them together, you get a magnificently bright bulge. You may also notice a few red clumps found within M99. Those are likely to be H II regions where the hydrogen gas has been partially ionized by very hot stars, and it is the partially ionized gas that emits the red color.

M99 has been classified by NED (NASA/IPAC Extragalactic Database) as a SA(s)c galaxy. The SA means that it is a spiral galaxy, the (s) means that it has no ring, and the c means that it has a small bulge, loose arms, and large clumps within the arms. All this is visible in the above picture of M99.

Multi-wavelength Comparison:

M99 Clear and HalphaOn left: Optical image from Calvin-Rehoboth Observatory. On right: red image (Halpha 6563 A spectral line) from KPNO 2.1m telescope (via NED on M99). Images on same angular scale.



This picture shows areas of where H II regions are located. It is zoomed in on the central area of M99. As mentioned earlier, H II regions are where the hydrogen gas has been partially ionized by hot, newly formed stars. Because the hydrogen has has been partially ionized, it gives off a wavelength that can be detected, and then a picture such as this can be produced. The very bright areas in this picture show where the H II regions are.

M99 Clear and radio (CO)On left: Inner part of galaxy optical image from Calvin-Rehoboth Observatory. On right: Radio image (CO 2.6mm spectral line) from NMA telescope (via NED on M99). Images on same angular scale.



This picture shows where carbon monoxide is located. This pictures is also zoomed in at the center of M99 and the dark areas in this picture show where the carbon monoxide is. The reason we care to know where carbon monoxide is located is because the location of carbon monoxide is strongly correlated with giant molecular clouds. Giant molecular clouds are areas where gas is very condensed. Many new stars form from giant molecular clouds, and once those new stars are formed, they produce H II regions like we saw in the previous picture.

Calculation of Pitch Angle:

I chose to examine the pitch angles at three different distances away from the galactic center for three different arms of M99. The pitch angle of an arm of a galaxy can be found by drawing circles different distances away from the center of the galaxy, and measuring the angle that is formed from that point on the circle and the angle of the arm at the intersecting point. What is expected to be found is that all the pitch angles, from the center to the outer parts of the arms, are all the same, or at least very close. What I found was interesting and unexpected. I thought it would be interesting to measure the pitch angle for M99 because of its irregular shape because the lower, southern arm looks quite different from the other two, and I thought that would be the arm that would deviate from the pitch angle. I found that it was instead the upper, northern arm seemed to be abnormal. The middle arm that extends out to the east had pitch angles of 24, 31, and 33 degrees at different distances from the core. The southern, large arm had pitch angles of 30, 35, and 30. The highest northern arm had pitch angles of 30, 30, and 45 degrees. I had an uncertainly of + or - 5 degrees. From these calculations it seems that the northern arm of M99 is the irregular arm in terms of pitch angle, and not the lower, southern arm like many would expect.


Elmegreen, Debra. Galaxies and Galactic Structure. Practice Hall, 1998. 15-20, 101. Print.

Frommert, Hartmut, and Christine Kronberg. "Messier Object 99." Students for the Exploration and Development of Space, 2 Sept. 2007. Web. Accessed 2 May 2011. <>.

Leach, Sid. "M99 Galaxy." 1997. Web. Accessed 2 May 2011. <>.

Méndez, Javier. "ING Picture Gallery: Galaxies". Isaac Newton Group of Telescopes, 13 Dec. 2010. Web. Accessed 2 May 2011. <>.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Right Ascension (J2000) 12:18:48
Declination (J2000) +14:25:00
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter

300 seconds in BVR and Clear. Number of exposures: B-8, V-3, R-4, Clear-11.

Date observed

February 9, 2011 (CBVR)
February 28, 2011 (CBV)
March 2, March 3, 2011 (R)
March 4, 2011 (C)

Data Reduction:

To get the images of my galaxy, I first did a test run of pictures to see if I needed longer exposure times for some filters, and if I could afford to have shorted exposure times for others. I adjusted from that data accordingly, and once I got my next set of pictures I began my work on combining them to one picture using Maxim DL software. I then used dark, flat, and biased photos to remove any artifacts or noise that is caused by the temperature, and other physical conditions of the telescope. Then I combined all the good images I had of my galaxy for each filter, and then I took those combined photos from each filter and combined them into the final image you see at the top of the page.



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