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Astr212 Galaxy Projects, Spring 2005

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NGC 4438/35: Colliding Pair of Galaxies, Kristin De Groot

NGC4438 Colliding galaxies

Figure 1: Mysterious Eyes
Also known as "The Eyes," the observed objects can be classified as a pair of colliding spiral galaxies. NGC 4438 is the lower galaxy and NGC 4435 is the upper galaxy. One can see the trails of NGC 4438 being "pulled" into the NGC4435 galaxy. NGC 4435 appears unaffected by the merging of galaxies, and "remains one of many astronomical mysteries" (David Healy). Both are in the heart of the Virgo cluster. These galaxies are around 40 million light years, or 18 Mpc, away. In the NGC 4438 galaxy, dark streaks run through the galaxy. These are dust lanes, which block the light of the galaxy. Also, the centers of both galaxies are more yellow than the outsides. This is due to the fact that there are older stars near the center of the galaxies, and younger (bluer) stars on the outskirts.

The interaction of these galaxies was simulated in GalCrash, an applet with a variety of parameters used to model the collision of galaxies. The parameters used are summarized in the table below. For the red galaxy, the theta was set at 180 degrees to promote retrograde spin, to prevent it from forming a tidal tail. The red galaxy mass was set at 1.0 so that both galaxies would have equal mass, which produced a large tail in the green galaxy.

Parameter Value
Peri (intial separation) 10.5 kpc
Red galaxy mass (size ratio) 1.0
Number of stars 250
Friction On
Green theta 45 deg
Red theta -180 deg
Time (at current state) 250 Myr
NGC4438 simulated collision

The NGC4438 galaxy was studied with greater depth. The following two sections discuss the major characterististics of this galaxy.

Fast Facts
The scale length of NGC4438 is 4 kpc, making it a bit smaller than the Milky Way. This galaxy is tilted at an inclination angle of 72 degrees.

Light Profiles
The following two graphs are light profiles of the NGC 4438 galaxy. What is a light profile? A light profile depicts the surface brightness over the length of the galaxy. That is, at a small radius, the brightness would be very large because this is located at the center of the galaxy. Likewise, at large radii, the brightness is much less than it is at the core. One can visually see this distribution by simply looking at the galaxy picture. It is interesting to note that the brightness is not uniform for the entirety of the distance from the center. The graph splits off into two different trends. This is due to the fact that as the NGC4438 is being pulled into the NGC4435, some of the brightness is pulled with it. The linear portion of the light profile is modeled using a linefit and is shown in purple on the graphs below. One graph follows the brightness along the major axis of the galaxy while the next shows the brightness profile along the minor axis of the galaxy. From the LineFit and Elmegreen's equations, the length of both axes could be determined. Also, the inclination angle was found to be 73 degrees.

Measurement Value Uncertainty
Scale length of major axis 37.96 pixels +/-0.818 pixels
Scale length of minor axis 11.3 pixels +/-0.18 pixels

NGC4438 major axis profile

NGC4438 minor axis profile

 

Image at non-optical wavelengths:

NGC4438 infrared

Figure 2: NGC 4438 and NGC 4435 as seen in infrared, JHKs 1.11-2.32u. Most of the infrared emission comes from the core of the galaxies, and not as much on the arms of the galaxies. This is because the stars that have formed on the outer edges of the galaxy are blue stars and are relatively young, and cannot be seen as well as the older stars at the core. This image shows the dispersion of old stars in the galaxies. Also, it is interesting to note that the IR image doesn't show the dust lanes seen in the optical image.

NGC4438 x-ray

Figure 3: The Eyes as seen in X-ray wavelengths (0.3-4.5keV,
5.9A). Again, the x-rays are most intense at the core of the image, where the stars are much older than those formed in the arms. Although stars don't emit a lot of X-rays, dying stars (supernova remnants) do emit x-rays, which can be seen in the contours of this image.

Data Reduction Summary
The program Maxim was used to refine the images taken of the galaxy pair. First, the images were combined using a median value (as opposed to average or summation). For color combination, the following were assigned to the filters:

Color Assignments
B filter blue
V filter
green
R filter
red

A combination of bias, dark, and flat filters was used to calibrate the images. For all three filters, sigma clip was chosen as the combination type. The dark frame scaling was auto-optimized.

Filters
Type Detection Combination
Bias
read-out noise
subtraction
Dark shutter, thermal noise subtraction
Flat
pixel response
division

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

Fabbiano, G.; Kim, D.-W.; Trinchieri, G., "An X-ray catalog and atlas of galaxies", 1992, Astrophysical Journal Supplement Series, 80, 531

Healy, D. NGC4438

Jarrett, T. H; Chester, T.; Cutri, R.; Schneider, S. E.; Huchra, J. P."The 2MASS Large Galaxy Atlas", 2003, Astronomical Journal, 125, 525

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

National Optical Astronomy Observatory NGC4438

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) 12:28:00
Declination (J2000) +13:00:40
Filters used blue(B), green(V), red(R), and clear(C)
Exposure time per filter 60 seconds in C, 300 seconds in BVR
Date

March 1, 2005 (C)
March 7, 2005 (BVR)