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Interaction of Galaxies

Many galaxies are members of groups or clusters. Since groups and clusters contain so many galaxies relatively close together, they sometimes collide with each other. Although galaxy collisions are common, stars in each galaxy are so far apart that collisions between stars are very rare. Even if galaxies don't actually collide, though, they can still affect one another. When two galaxies pass close to one another, the force of gravity they exert on one another can cause both galaxies to bend out of shape. Both crashes and near misses between galaxies are referred to as "interactions."

Andromeda and Milky Way Interaction

Andromeda is the largest galaxy in the Local Group, which includes the Milky Way, the Traingulum Galaxy and a lot of other dwarf and irregular galaxies. It is the nearest major galaxy to our very own Milky Way galaxy. Andromeda is approximately 2.5 million light years away from Earth. Andromeda was first cataloged by Charles Messier and thus is also known as Messier 31 or M31. Andromeda Galaxy is about 220,000 light years in diameter which is almost twice the estimated diameter of Milky Way Galaxy.

Due to the expansion of the universe, galaxies are moving away from the Milky Way. However, Andromeda is moving towards us at the speed of 120 km/s. Andromeda and Milky Way Galaxy were formed very close to each other after the Big Bang. The expanding universe led these to galaxies to separate from each other but the gravitational attraction between the two still existed. Now, after billions of years, the two galaxies are falling back together and the gap between the two is closing at 500,000 km per hour.

There have been a lot of research and predictions carried out regarding the collisions with the help of computer simulations. But I would like to take a different route of ‘imaging’ of the Andromeda galaxy, studying them and forming my own predictions regarding the behavior of the two galaxies during the collisions using my knowledge of calibrating and radio imaging.


Collection of Data

I am going to use National Radio Astronomy Observatory (NRAO) Data Archive System to observe the data collected by the (Very Large Array) VLA in various bands and arrays. NRAO archive has a list of sources that has been observed at different times. I want to use the different data obtained by observing Andromeda at different times. Consideration of important factors like the time of observation and resolutions in order to select the best set of data is really important.

Calibration of Data

After downloading all the data sets, I am going to use Astronomical Image Processing Systems (AIPS). AIPS is a software package for calibration, data analysis, image display, plotting, etc on astronomical data. During this process, I will have to make sure that I chose the correct bands (L, C or X) and arrays. I am going to flag bad data and calibrate the rest of the data that I find suitable. However, I want to try and calibrate in all the bands and array if possible since I am trying to collect as much data as possible.

Imaging of Source

For this process, I am going to use Difmap. Difmap is a program developed for synthesis imaging of visibility data from interferometer arrays of radio telescopes world-wide. Its prime advantages over traditional packages are its emphasis on interactive processing, speed, and the use of Difference mapping techniques. After finishing my calibration in AIPS, I am going to load the file in Difmap and begin the process of data inspection and reduction and finally obtain the radio image of the source, in this case, Andromeda.

After calibration and imaging of all the data sets for Andromeda, I am going to spend time and observe all the images carefully and try and find a pattern or even a little difference in the images from the data collected by VLA at different times.


I am expecting to see some pattern or difference in the various radio images that I make so that they tell me more about how they are moving towards our Milky Way and what model are they forming while doing so. This is not going to be easy because we don’t have a lot of data sets. After the last class, I realized that data sets found in NRAO are mostly very recent (the latest would be around 50 years). The images I obtain are going to look very much the same and thus it is going to be very difficult to even find any pattern at all.

Therefore, I will eventually have to use computer simulations to generate and observe the interaction between the Milky Way and Andromeda.


NRAO Data Archive System,

Astronomical Image Processing System,


Cox, T.J., Loeb, Abraham, 2008, The Collision between the Milky Way and Andromeda, Monthly Notices of the Royal Astronomical Society, Volume 386, Issue 1, pages 461–474, May 2008,

My Questions

  • How far is the closest Active Galactic Nuclei (AGN) from Milky Way? Is the nearest AGN going to collide and merge with Milky Way and make it active? If yes, what will be the transition from a non-active to an active galaxy be like?
    • -Specifically: the Milky Way transition from AGN to not AGN; find which AGN is closest, is it even moving towards us, it's velocity, etc.; maybe even just looking into techniques to measure a galaxies velocity, direction etc; look into models of our eventual collision with Andromeda (would that collision happen before our potential collision with an AGN? what would that mean?-that is a huge question, you don't necessarily have to consider it); maybe even just narrow it down to the Milky Way's interaction with Andromeda --Rnakaba 20:06, 30 September 2011 (UTC) (plus Pratham and Anisha)
      • Really seems like two possible approaches here: simulation (which would be pretty demanding), OR looking for parallel/analogous situations out there in the cosmos. In the latter case, seems like the important focus would be on choosing the characteristics of situation whose influences you want to assess most pointedly...? Kwoods 03:47, 11 November 2011 (UTC)
    • -Look into the properties of active and non active galaxies separately, look more into different collisions and see how they were different from each other and what are the specific things that you can measure. How does an active galaxy become non-active?-- Jiaying, Kittie
  • If photon is the carrier of electromagnetic force and graviton of the gravitational force, how can we sense these forces if neither photons or gravitons can't escape black hole?
    • Hmmm... This question and your following question are interesting, because they are more theoretical physics questions than they are experimental. Almost all of our discussion in class has been about experimental questions. But a theoretical physics question needs to be approached differently - it is more like a math question. You start with a question which is a point of curiosity. But then you need to learn more to try to refine exactly what it is you are asking. The question serves as a guide to the theoretical physics you need to learn, which will probably end up modifying your question, which will lead you to learning other physics, and other questions... With this sort of research question, you can't really know where it is going to end up until you start. And you can't set out the hypothesis from the beginning like you would with an experiment, because learning enough to make a good hypothesis is really part of the research. So maybe the next step here is to identify what you would need to know in order to refine the question. -- Andrew
      • I think this is complicated by the fact that gravitons are (so far) purely hypothetical (if they do exist, they're supposed to be more absolutely massless than photons, I think?). But I'm also a bit unclear on what you're asking -- these forces are influenced by blackholes beyond their 'event horizon' even if no particles can escape from within that horizon. Do we 'sense' black holes directly? Perhaps one could say that we can't? Kwoods 03:47, 11 November 2011 (UTC)
  • I read somewhere that after the big bang, there occurred an expansion that traveled faster than the speed of light. I have always been skeptical about the statement. But today read an article today about how European Organisation for Nuclear Research (CERN) claim that they have recorded neutrinos that traveled faster than speed of light. So could the first statement about the 'speedy' expansion after the big bang be true? Are the captured neutrinos going to prove Einstein's theory of relativity wrong?
    • I'm no cosmologist, but is the expansion of universe itself something that can actually be compared to speed of light when light's travel can only be within the space-time continuum? The CERN thing is very interesting (and newsy): here's one comment on that issue from xkcd.Kwoods 01:11, 29 September 2011 (UTC)
      • A bit more research, and I realize that there are two issues here. The speed of expansion of the universe is not constrained by the speed of light. Special relativity just says velocity of objects WITHIN universe relative to other objects in universe can't exceed speed of light -- but the expansion of the universe itself -- 'metric expansion' -- is not so constrained by Einstein. So, I'm thinking the question about the CERN neutrinos is probably not going to bear on faster-than-light expansion of universe (or vice versa). Too bad. Kwoods 03:47, 11 November 2011 (UTC)