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Astr212 Galaxy Projects, Spring 2007

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Siamese Twins (NGC4567 and NGC4568)

David Hanson


The Siamese Twins or the galaxies known as NGC4567(top) and NGC4568(bottom) are two incredibly close galaxies in the Virgo Cluster. They reside roughly 16.5 Mpc(53 million light years) away. NGC4567 is has an inclination angle of roughly 25 degrees, so it appears to be almost face-on. NGC4568 has an inclination angle of roughly 70 degrees, so astronomers see a more side-on view then a face-on view; however they can still see many of its unique and beautiful details. NGC4567 and NGC4568 are called the Siamese Twins because they appear to be remarkably similar and only 11kpc apart; however on a closer view these galaxies show several differences. Both are of type SAbc which means they have clumpy and loosely defined arms, but NGC4568 has intense star formation in the reddish areas close to its bulge; NGC4567 also has most of its star formation near it's bulge, but quite a bit of star formation can be found in it's 2 deep blue arms. Blue arms indicate compressed dust clouds caused by the dense spiral arms. The bluish look is caused by the hotter OB stars that will probably burn out before they can separate themselves from the spiral. NGC4568 has two arms as well; however, they are less defined and are harder to see due to it's inclination angle. To get a better view of NGC4568's spiral arms, look at the near infrared picture below. Each galaxy has a remarkable different size; NGC4567 is roughly 4.2 kpc across, while NGC4568 is 7.4 kpc across. These appear to be tiny galaxies when compared our own galaxy the Milky Way which has a diameter of roughly 50 kpc, but on further review if we were able to look at them with a larger telescope we would find their actual size is probably larger.

Far Ultraviolet and Near Ultraviolet image:


The FUV (blue) and NUV(yellow) image above was taken by the high powered satellite telescope GALEX. From this image we are able to learn about star formation in the galaxy by looking at the blue clumps. The wavelength of the FUV spectrum is between 200nm-10nm and NUV spectrum is from 400nm-200nm, so only the youngest hottest stars can emit such radiation. Since young stars generally die out before they move much, we can infer that the blue are areas of intense star formation.

Near-Infrared image:

Siamese Near Infrared

The near Infrared image above was taken by the 2MASS telescope in H-band (1.65micron meter wavelength) of the infrared spectrum. The reason astronomers image in the infrared is to view objects without the domination of the youngest hottest stars. The near-infrared allows astronomers to view the cooler stars and red giants without interference of dust emission. The above infrared image shows a plethora of cool stars throughout each galaxy and accentuates the form of spiral arms to show the densest areas of cooler stars and dust. Notice the difference between this image and the FUV/NUV image. The FUV/NUV image was more clumpy due to the areas of intense start formation, this image is much smoother due to the viewing of cooler red main sequence stars which tend to live much longer thus are able to traverse the spirals of the galaxy. With this picture, the general shapeliness of the galaxy is easily seen due to the smooth view given by the near infrared.

Light Profiles:

A light profile is a measurement of brightness taken along the major and minor axis's of a star. By using pictures of the stars and measuring pixel brightness, astronomers are able to see the light curve of the galaxy along each of the axis's. Key things to look for include linear lines; they represent visible disk of the galaxy. A central cusp in the light profile show's the galaxy's bulge, so the larger the cusp proportional to the disk, the larger the bulge of a galaxy is compared to the rest of the visible galaxy. The scale lengths found for other spiral galaxies in Astr212 were between 0.6 and 1.8 kpc. NGC4567 and NGC4568 are both on the small side with the largest scale length being 1.09 for NGC4568's major axis and the smallest being the 0.39 scale length of NGC4568's minor axis. NGC4567's scale lengths are are very close to each other due to low inclination angle.


NGC4567 Light Profile MajorAxis

On the major axis a definitive cusp can be seen right along the left edge of the plot. This cusp signifies the bulge of the galaxy. The long straight fit line shows the disk of the galaxy. The scale length was roughly 0.61 kpc with about 3% uncertainty

NGC 4567 Light Profile MinorAxis

On the minor axis the cusp is much less definitive. It shows a smaller disk than previous, which is true because it is on the minor axis of a galaxy. The scale length is 0.563 kpc with about 2% uncertainty which shows that the inclination angle is small. The large hump in the middle of the graph is interference from NGC4568.


NGC 4568 Light Profile Major Axis

On the major axis this galaxy has a less pronounced bulge but a much larger disk. The scale length is 1.097 kpc with 2% uncertainty.

NGC 4568 Light Profile Minor Axis

The minor axis shows almost no bulge and a scale length of 0.39 kpc with 2% uncertainty. This means the galaxy appears elongated because it is tilted relative to our line of sight.

A Simulation of Galaxy Collision :

By using the galaxy collision simulator at http://burro.astr.cwru.edu/JavaLab/GalCrashWeb/backgrnd.html

To do this simulation yourself go to the above web site and click Applet.

Set the Following Properties:

  • Red Theta: 55.0
  • Red Phi: -80
  • Green Theta: 45.0
  • Green Phi : 55.0
  • Peri: 11kpc
  • Red Galaxy Mass: .6 (because NGC4567 is roughly .6 the size of NGC4568)
  • Number of Stars: 2000
  • Friction: on (checked)

I have been able to simulate the collision of NGC4567 and NGC4568.

Initial Picture:

Collision Simulation - now


To get this view, click and drag to rotate the camera so the two galaxies look like they would from the earth's perspective. Note both galaxies should rotate clockwise. Your screen should look similar to my screen shot above. This is how the galaxies appear now. By running the simulation forward, you can see how the galaxies will look in the future.

Collision Simulation 276.3Myr

After 274.6 Myr the Tidal effect of the red (NGC4567) has ejected a small tail on the green(NGC4568). Since the green galaxy is slightly larger than the red galaxy the red will have a larger tail; however, we cannot see it as well due to our angle of observation. Tidal forces are the pull of gravitational forces on the near and far sides of spherical objects. The near sides feel more of the pull causing a stream of material between them; the far sides of the objects feel less of a pull causing material float out in a tail.

Collision Simulations - 432.0Myr

After 432.0 Myr the galaxies start to come back together. They are close to merging. Notice the large tails at the ends caused by the tidal effects of that first pass between them. It is also possible to see the large amounts of material between them as they start their merge together.

Collision Simulation - 544.3Myr

After 544.8 Myr the galaxies have made their final passes across each other and are very close to merging. Each galaxy has lost much of it's structure. The tidal effects have certainly created the two sub tails that look like the rabbit's head, while the long tails remain.

Collision Simulation - 707.7Myr

Finally after roughly 707.5 Myr the galaxies have become completely indistinguishable. Several of the tails still remain; however with time, they too will disappear. These two spirals have taken on more of an elliptical shape and will continue to do so for millions of years until the galaxies appear to be one big elliptical galaxy.

Current Research on NGC4567 and NGC4568:

Siamese - Radio

From the research paper by Daisuke Iono on the molecular regions of merging galaxies, the centers of the two nuclei of the two merging galaxies are only 6.5 kpc apart. They are about to begin their first pass of a merger. Daisuke reasons that they must be beginning their first pass due to the similarity in radial velocity, the lack of present tidal effects and the relatively small streams of matter between them. These HI diagrams above were taking using the VLA telescope at 1.6 X 1.1 kpc resolution. VLA is a radio telescope laid out over roughly 13 mi. It consist of 16 25m telescopes laid out in a Y-shape. Using a highly sensitive telescope like the VLA, scientist were able to take the above pictures of HI regions in NGC4567 and NGC4568. The emission between them is quoted as "remarkably unexceptional", so its commonplace to have such emission between close large galaxies. By looking at the HI diagrams above, we can see the Doppler shift of the galaxies relative to us. On the right picture notice the top of the galaxy has the number 2350 while the bottom has 2150. This means that the top is moving away from us because of it's larger velocity(km/s), while the 2150 indicates that the bottom is moving towards us. From the above pictures we can determine that the NGC4568 and NGC4567 are rotating clockwise.


"Near, mid, far infrared " Cool Cosmos . <http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irregions.html>.

"Ultraviolet".Wikipedia.org. <http://en.wikipedia.org/wiki/Ultraviolet>

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.

"The GALEX Ultraviolet Atlas of Nearby Galaxies". Gil de Paz A et. all . ArXiv Astrophysics e-prints. June 2006. <http://adsabs.harvard.edu/cgi-bin/bib_query?2006astro.ph..6440G>

"The 2MASS Large Galaxy Atlas" .Jarret, T.H.; Chester, T.; Cutri, R.; Schneider, S.E.; Huchra, J.P. , The Astronomical Journal: Volume 125 Issue 2 <http://adsabs.harvard.edu/cgi-bin/bib_query?2003AJ....125..525J>

Iono, Daisuke, Min S. Yun, and Paul T. Ho. "Atomic and Molecular Gas in Colliding Galaxy Systems.I. The Data." The Astrophysical Jornal Supplement Series 158 (2005): 1-38. NED. 1 May 2007 <http://www.journals.uchicago.edu/ApJ/journal/issues/ApJS/v158n1/61777/61777.web.pdf>.


Right Ascension (J2000) 12:36:32.8
Declination (J2000) 11:15:10
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter

9x300 seconds in C

6X300 seconds in B

3X300 seconds in V

2X300 seconds in R

Date observed

March 13, 2007

How To Make an Image:

(Taken from Astronomy @ Calvin College 212 Labs 2&7)


  1. To calibrate in Maxim, you first tell it the type and disk location of all the calibration files.   To do this, select the Process menu, then Set calibration.  This brings up the calibration window.
  2. If there are any groups in the upper part of this window, get rid of them by selecting them and clicking Remove group.
  3. At the top of the calibration window, check Calibrate Bias, Calibrate Dark, and Calibrate Flat, and make sure the other items are unchecked.  
  4. In the drop down menu on the left (just above “Group Properties”), select Dark.  Then click Add Group.  In the lower half of the window, click Add and navigate to your directory and select the appropriate dark file.    Under Dark Frame Scaling select Auto-Optimize.  Under Combine type, select Sigma Clip. 
  5. In the drop down menu on the left, select Flat, then click Add Group.  In the lower half of the window, click Add and navigate to your directory and select the appropriate flat file. Under Combine type, select Sigma Clip. 
  6. In the drop down menu on the left, select Bias, then click Add Group.  In the lower half of the window, click Add and navigate to your directory and select the appropriate bias file.
  7. Click Calibrate.
  8. Do this for each appropriate image file in each filter.
  9. This prepares the images so they have relatively similar pixel values. You don't want the images to look overexposed or have too much background fuzz.

Aligning the Images:

  1. Once finished calibrating, Select Process then Align. A Select Images window will appear; click add All then Ok
  2. The Align Images window will appear. For Align Mode select Manual 2 stars and pick the correct stars.
  3. Click OK
  4. Check by selecting View then Animate. If the image doesn't jiggle, you did it correctly.
  5. This process correctly aligns all the filters, so you are able to bring out the colors from each filter in the correct place.

Adjusting the Color:

  1. After you have completed the alignment step, Select Color from the menu, then Combine Color. Select the LRGB button and be sure to use the clear filter in the Luminance slot.
  2. For your RGB values insert 0.8, 1, 9, respectively.
  3. Select Color then Adjust Saturation
  4. Try to make the image represent it's original color. If the color of the background stars is not close to white, you should lower your color saturation.
  5. Once the saturation looks correct, Save it as a new file.

Brightness Balance:

  1. Go to File then Save As
  2. Under the file format select JPEG and under compression quality select 100%
  3. Click the Stretch button. In the box that appears select Gamma and Screen Stretch. In the preview window, scroll to the region of interest in you image and adjust the Gamma value until the image shows the bright structures and the faint structures
  4. The purpose of the previous step is to tone down the bulge in comparison to the rest of the image.