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Astronomical Observatory: Cool Images

Astr212 Galaxy Projects, Spring 2005

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M100: A Grand Design Spiral Galaxy, Chris Beaumont


Galaxies are social objects. Even a brief survey the sky will reveal that they reside in groups, called galaxy clusters. The nearest such ensemble is the Virgo Cluster, of which M100 is a prominent member. Lying 58.7 million light years distant, M100 is 26 kly in diameter, about 80 percent the size of our own Milky Way Galaxy. A thoughtful study of both this image and others of M100 can reveal much about the nature of galaxies in general.

What this image has to say

 The most immediate and striking feature of this galaxy is its majestic spiral structure. Because of its easily identifiable arms, it is classified as a Grand-Design spiral galaxy. The display is especially inspiring because the galaxy is oriented in such a way that we see it nearly face on (By contrast, spiral galaxy M104 of the Virgo Cluster is seen nearly edge on, a dramatic difference).

The most dominant source of light in the galaxy is the core (called the galactic bulge). The region is incredibly bright and yellow. The light is produced by a strong concentration of globular clusters surrounding a central black hole. Extending out of the bulge are the dominant spiral arms. A careful look at the spiral arms reveals that they are composed of three layers. The innermost layer is marked by dark lanes of dust. This material (mostly Hydrogen gas) is being compressed as it enters into the spiral. As it passes through the arm, the hydrogen is compressed enough for new stars to form. The energy given off by these new stars ionizes the surrounding Hydrogen cloud, causing it to glow red. Beyond this red layer is the outer blue layer. By the time the newly formed stars leave the arms, the most massive and energetic members have run out of stellar fuel and have exploded. Without this energy source, the hydrogen gas cools, and we see the blue light given off by the remaining stars. There is also a knotty structure to the arms, indicating regions of concentrated star formation. The full picture reveals at least 5 other galaxies in the same field of view. Many of these are Dwarf Ellipticals or Lenticular Galaxies in the Virgo Cluster. This field is a microcosm for galactic distribution in general; spiral galaxies are much less common than smaller Elliptical and Lenticular galaxies.

To extract more about the specific dimensions and orientation of M100, we use a more quantitative analysis. First, each pixel in the image covers an angular distance of 1.18”. Furthermore, since the distance to the Virgo Cluster is known to be 18 Mpc, trigonometry reveals that each pixel corresponds to a linear distance of 102.8 pc (1Mpc = 1,000,000 pc = 3.26 million light years). This allows us to describe dimensions in terms of actual units of length.

Next, we turn to the shape of M100. Though M100 is likely circular in shape, it appears slightly oval shaped because of its slight inclination away from the plane of the sky. It is possible to discern how much the galaxy is tilted away from us by analyzing to what degree its shape deviates from a circle. To do this, we measure the natural log of the galaxy’s brightness as a function of distance from the galactic center along two lines, its major and minor axes. The result is shown below:


M100 Light profile


M100 Minor Axis


After an initial rapid decrease from the center, brightness drops exponentially for both axes. However, because the minor axis appears shortened, it falls off more rapidly. To quantify this, we first find an equation for the line that describes the diminishing brightness. We then define the scale length, or the distance from the center where the line has fallen to 1/e of its value at the center. Again, both scale lengths would be identical if the galaxy were not inclined at all. The difference in scale lengths allows a measurement of inclination to be made. Inclination is found using the equation

i=cos -1[b/a]

where a is the major axis scale length, and b is the minor axis scale length. Using this equation for M100, the inclination is measured to be 22 degrees.

The fact that the inclination of M100 is small is significant. Note that the measurement of scale length depended on knowledge of the distance to the Virgo Cluster. Though many calculations of objects in the Virgo Cluster rely on this distance, M100 is unique in that astronomers have relied on the galaxy to measure this distance directly by observing supernovae and Cepheid variables. M100 is well suited for this because, due to its small inclination, the location of events within the galaxy are unambiguous and easily discernable (as an analogy, it is easier to observe a chess game from directly above the board, as opposed to looking at it edge on).

Beyond Visible

To gain a deeper understanding for the dynamic nature of M100, it is fruitful to study other images in non-visible light. For example, the image below shows the central region of M100 in ultraviolet:

M100 UV

M100 in UV (false color)


M100, Optical Comparison (false color)

Most prominent in the image is the presence of UV radiation surrounding the center of the galaxy. Since UV light is more energetic than visible light, its presence points out areas of high activity in the galaxy. Most commonly, UV light is an indication of recent star formation (large stars emit a great deal of UV radiation, but do not live long past the stellar formation process). Thus, this central UV presence is called an area of circumnuclear star formation. Apparently, M100 is producing stars more actively than most galaxies. Astronomers who study such images of M100 in UV are gaining deeper insight into how star formation evolves over time.


In contrast to the UV image, the picture of M100 below was taken in the near-infrared part of the spectrum:

M100 near infrared

This image shows a spiral structure different to what we see in the optical; here, a central bar is visible. The light from this image is from older K and M type stars which emit most of their light in the infrared. These stars tend to follow the overall mass distribution in the galaxy. The reason that we don’t see the bar in the optical picture is because this region is optically dominated by the star formation discussed above. Since the infrared light from the K and M stars follow the overall mass distribution in M100, the barred spiral arm indicates a global density wave near the center of the galaxy. Again, this is of interest to astronomers who study the nature of star formation.


Elmegreen, D.M. 1998, Galaxies and Galactic Structure (New Jersey: Prentice Hall)

Elmegreen, D.M. 1981, A Near Infrared Atlas of Spiral Galaxies. Astrophysical Journal Supplement Series, 47, 229

Knapen, J.H. et al 1995, The striking near-infrared morphology of the inner region in M100, Astrophysical Journal Letters, 443, 73

Kutner, M. L. 2003, Astronomy: A Physical Perspective, 2nd ed. (Cambridge: Cambridge University Press)

Students for the Exploration and Development of Space, M100

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.


Observation Details
RA (J2000) 12h 22m 55.2s

Dec (J2000). 15° 49' 23"

This is in the constellation Virgo. North is up and East is to the left.
Scale The image is 16.4 by 12.5 arcminutes, which is 86.3 by 65.6 kpc at the distance of M100 (18 Mpc)
Exposure time per filter
12 x 300s
13 x 300s
7 x 300s
7 x 300s
Dates of observation
2005 Mar 1 2005 Mar 8

2005 Feb 8 2005 April 5

2005 Feb 8 2005 March 7 2005 April 5
2005 Feb 8 2005 March 7 2005 April 5
Processing details: All images were taken with the 16 inch telescope at the Calvin Rehoboth Robotic Observatory. Data in each of the red, blue, visual (green) and clear filters were calibrated using dark, bias, and flat fields to remove systematic artifacts from thermal noise, optical effects of the telescope, and digital artifacts. All the images from each filter were then aligned and combined with MaxIM’s Sigma Clip algorithm to improve the signal/noise ratio. An unsharp mask was applied to the clear image, which had the best S/N. Next, the combined image from each filter was combined with the others to produce a color picture. Color balance was selected so as to make a majority of background stars appear white. Next, a non-linear transform was applied to the image to enhance the fainter regions without overexposing the bulge. Finally, the color saturation was boosted to 120% and bad pixels were removed using MaxIM’s bad pixel processing tool.





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