4438/35: Colliding Pair of Galaxies,
Kristin De Groot
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.
galaxy mass (size ratio)
(at current state)
The NGC4438 galaxy
was studied with greater depth. The following two sections discuss the
major characterististics of this galaxy.
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.
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.
of major axis
of minor axis
Image at non-optical
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
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.
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
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.
Elmegreen, D.M. 1998, Galaxies and Galactic Structure (New Jersey: Prentice
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
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.
red(R), and clear(C)
||60 seconds in
C, 300 seconds in BVR
March 7, 2005 (BVR)