Astr212 Galaxy Projects, Spring 2009
Colliding Galaxies (NGC 3395+NGC 3396),
The two galaxies are on a collision course. The galaxy on the left is NGC 3396 and the galaxy that is slightly lower on the right is called NGC 3395.
NGC 3395, a smaller galaxy 22.0 Mpc away with a linear size of 9.2 Kpc, is yet another galaxy in a huge universe full of galaxies. Each galaxy, however, is still unique in itself and interesting in its own way, kind of like people. Galaxies are each made up of billions of stars and are usually part of a family of galaxies called a cluster or, if the family is big, a super cluster. The shape and size of a galaxy is really determined by what is surrounding that galaxy and how much gravitational pull the surrounding matter can place on it. Galaxies consist of a core, usually a black hole, around which the rest of the matter in the galaxy orbits. When we talk about the rest of the matter, we are covering a lot of different things. A galaxy consists of large amounts of stars, planets, dust, gas, and lots of dark matter. Dark matter makes up most of the matter in the galaxy, around 70 percent, and is located primarily in what is called the halo of the galaxy, which is really just a big invisible sphere that marks edge of the galaxy matter. There are a few stars and nebulas in the halo but the majority of visible matter is located in galaxy plane, the big disk that one usually thinks of when talking about galaxies.
The most interesting thing about this galaxy is its collision course with its neighbor NGC 3396. As one can see in the picture they are very close to hitting each other and this creates some interesting effects in both galaxies. Because they are so close they have a strong gravitational pull on each other which enhances the amazing spiral arms that are so famous in other galaxies. There is already one arm forming on the bottom of the 3395 which makes this an interesting snapshot of how arms and tails develop. We can see the spiral arm of NGC 3395 because we see the galaxy more face on and so get a better view. NGC 3396 is at more of a side view, which is why it does not look as interesting as its counterpart since we are seeing the galactic plane, which we will see as more of a line. The picture above shows primarily a blue color inside the galaxy except for the bright core in the center. Blue light is on the shorter wavelength end of the visible spectrum which means a hotter source for the light. This points toward younger newly formed stars, they are hotter than older stars and tend to burn out quickly and so give off a blue light.
To develop this photo we took four preliminary photos in each filter to get an idea of how many images we needed to take in each filter and for how long. We ended up with 19 photos in the clear filter, 3 in the red filter, 4 in the green, and 7 in the blue. The background light was then removed with different biases, flats, and darks depending on what day the image was taken. A bias, flat, or dark are simply images of different levels of background light that can be used to remove specific background light from the images. Once all of the images were calibrated each of the filters were combined into a single image, giving us one crisp image in each filter. These images were then combined into the single image seen above.
What Other Scientists are Saying
Other scientists have also observed these two colliding galaxies. NASA has collected a lot of data on these galaxies.They classify NGC 3395 as an SAB (rs) cd pec galaxy, which is really a way of saying scientist cannot decide what to classify it. SAB means it is in-between a barred spiral galaxy and a normal spiral galaxy, an (rs) classification is the middle point between an (r) and an (s) classification, which are indications of whether or not there is a ring around the core of the galaxy. A cd classification is also an intermediate in-between c and d. A pec classification stands for peculiar which says it does not really fit with any of the standard classifications used right now. So NGC 3395 is really a little bit of everything.This is reasonable though since it is still in the beginning stages of a collision with another galaxy.
Brassington, Read, and Ponman (2005) write extensively about observations of the two galaxies in the X-ray spectrum. The key points of their paper are the locations of the point sources of emission, the amount and location of the emission, how many point sources there are and what this says about the galaxies. The main conclusion of the paper is there are 16 point sources inside and around the galaxy but the diffuse gas is responsible for only about 32 percent of the luminosity for the whole structure. This is probably because the gas has not had enough time to break through the galactic disk which is an indication of a relatively young collision in its early stages of evolution.
This image is from the Chandra telescope and is an optical image with X-ray contours laid on top of it. Looking at the contours you can get an idea of how spread out the gas is in each galaxy. In NGC3396 you can see that the x rays emissions are concentrated in the middle indicating some kind of structure there. NGC3395 has sort of the same thing but not to the extent that 3396 has. There are several point sources visible in this image particularly in 3395 and an X ray bridge is visible between the two galaxies which shows they have already begun to interact with each other. The point sources are probably remnants of old supernovae which heat up the gas enough to emit X rays.
Below is a picture of just NGC 3395 in the ultraviolet from Hancock Weistrop Eggers & Nelson (2003). It is rotated about 90 degrees with east pointing in an upwards direction and north pointing right, whereas in our optical picture north points upward and east points to the left. The knots are the denser darker parts of the picture and represent where emission in the UV is strongest. The main source of UV light is newly formed stars so these dark spots are a good indicator of star formation. The largest dark spot is not the center of the galaxy but is actually to the right and slightly below the center of 3395. This is because star formation is the primary emitter of UV so these spots indicate a star forming region, but not necessarily a brighter spot in optical which is why the core is not a part of this. In the color picture above, the dark spot would be to the right and up.
We used the GalCrash galaxy collision program to simulate the collision of these two galaxies to see what will happen. In our calculation one of the galaxies was1.2 times bigger than the other, and they were spinning in opposite directions. We had friction on, the green galaxy was oriented at an angle of 115 degrees relative to the red. The closest they came to each other was 10 kpc, which is pretty similar to the current distance between 3395 and 3396 which is about 12.5. The collision that took place in our simulation looked, at one stage, a lot like the actual collision is now so it should give us a good approximation of what will happen.
The next two pictures below are the light profile of NGC 3395. The x-axis is the radius or distance from the center and the y-axis is the brightness level at that point. The top graph is of the minor axis and the bottom is of the major axis. The minor axis is pretty un-eventful, looking at the graph one can see the brightness increase as the distance to the center decreases like it should since the core is the brightest part of the galaxy. The major axis is more interesting. Just like the minor axis the brightness increases the closer one gets the center, on one side however there is a jump in brightness that is not mimicked on the other side. This is the arm that is forming on the southern side of NGC 3395. There is no arm on the northern side, which in our color picture is toward the top, because NGC 3396 is interacting in a way that doesn't allow it.
Brassington, Read, Ponman. 2005 CHANDRA OBSERVATIONS OF THE INTERACTING GALAXIES NGC 3395/3396. Monthly Notices of the Royal Astronomical Society, Volume 360, Issue 3, pp. 801-815 <http://adsabs.harvard.edu/abs/2005MNRAS.360..801B>
Hancock, Weistrop, Eggers, & Nelson 2003. STAR-FORMING KNOTS IN THE UV-BRIGHT INTERACTING GALAXIES NGC 3395 AND NGC 3396.The Astronomical Journal, Volume 125, Issue 4, pp. 1696-1710 <http://adsabs.harvard.edu/abs/2003AJ....125.1696H>
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)
|| 300 seconds in CBVR 40 exposures
March 14, 2009(CBVR)