1. Interacting Galaxies
By: Emily Ducharme

2. Types of Interactions Galaxies can interact in several different ways:
Interactions between galaxies which don’t results in a merge
Close encounters (“Galaxy Harassment”)
Collisions
“Galactic Cannibalism”
Major mergers
Galaxy interactions of all types occur within galaxy clusters.
Looking back, interactions between galaxies were much more likely in the past (galaxies were much closer together).
Takes a very long time for any interaction to fully take place.
Penn State, 2011

3. Galaxy Interactions
The term “galaxy interactions” doesn’t always indicate a merger:
Interacting Galaxies: Galaxies can pass by each other with such a velocity so they don’t get caught up in each others gravity. This often has an effect on the structure of galaxies.
Bursts of star formation
Tidal “tail” formation
Stars and gas stripped from the galaxy
Swapping of contents (gas and stars)
Also called “Galaxy Harassment”.
In high population galaxy clusters, this is very common and can result in some very small galaxies with populations of stars unattached to any galaxy.
Most common affect of the interaction of galaxies: the rapidly varying gravitational forces compress the interstellar gas within a galaxy, resulting in bursts of star formation. This occurs all across the galaxy! These galaxies are called starburst galaxies.
Penn State, 2011

4. Galaxy Interactions
Colliding Galaxies: In some cases, collisions between galaxies occur without a merger as the result. Here, galaxies simply pass right through other galaxies and come out the other side.
A galaxies structure is affected by this occurrence.
In terms of the Cartwheel Galaxy, a smaller galaxy passed right through the centre of the galaxy, resulting in massive changes to its large-scale structure.
Density wave ‘rippling’ outward from the area of impact, creating young stars as it moves along.
Chaisson, McMillan, 2011
NASA, 2006

5. “Galactic Cannibalism”
“Galactic Cannibalism” is a term penned to describe the merging of galaxies in which one galaxy is much smaller than the other.
Very common within galaxy clusters.
It’s very likely that a large galaxy has ‘eaten’ several smaller galaxies.
Common to find large super-massive galaxies at the centre of a galaxy cluster
When this occurs there is very little change to the larger galaxy, whereas the smaller galaxy spirals inwards and is destroyed near the centre of its giant companion.
A small galaxy can spend a lot of time on the outside of the larger galaxy before it succumbs to the gravitational force and spirals inwards.
The smaller galaxy gets caught up in the halo of the larger galaxy. Its these interacting dark matter halos extending farther out from the galaxy than the visible matter which allows galaxies to get caught up with each other so easily.
University of Maryland, 2009
Chaisson, McMillan, 2011

6. The Mice Galaxy Image Courtesy of:
NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI),
G. Hartig (STScI), the ACS Science Team, and ESA

7. R. Kennicutt, 2004

8. Major Mergers
Major mergers occur between galaxies of comparable mass and a small relative velocity. These are the most violent of the mergers, resulting in the most extreme structural change.
When two large galaxies collide, they will not have enough velocity to get away from each other due to their large mutual gravitational force. They will fall into each other and merge into one large galaxy.
When this happens, there are several changes to the structure:
Burst of star formation (starburst galaxy)
Loss of large-scale structure (spiral -> elliptical)
Doubling of volume of stars
Long tidal ‘tails’ formed
Don’t occur in the centre of clusters, the galaxies will be moving too fast relative to each other. Tends to occur in looser groups.
Penn State, 2011

9. The Antennae Galaxy NASA, 2007
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration, 2006

10. Evidence of Major Mergers
A spectrum indicative of young stars (A type)
Tidal tails
Indicates that the presence of two galaxies (if the galaxy is also isolated it removes the possibility that they may have been formed by surrounding galaxies)
Galaxy isolation
Opposite tail motions
Single nucleus and body, single light distribution
These correspond to a complete merger
Surviving motions systems of gas (the nucleus moves at one velocity, the outer gas in another, completely different motion)
Merger occurred recently
Shortage of gas (speaks to a structure change to an elliptical galaxy)
Schweizer, 1982

11. Atoms for Peace Galaxy
ESO, 2010

12. The Milky Way-M31 Merger
In 4 billion years, our galaxy will undergo a major merger with the M31 galaxy (otherwise known as Andromeda).
The Andromeda galaxy appears to be falling into our galaxy.
Measurements have actually proven that this will occur.
The entire merger process will take 2 billion years.
Our solar system will be tossed to another part of the galaxy, but otherwise unchanged.
Van der Marel, et al, 2012

13. References
Schweizer F Colliding and Merging Galaxies. I. Evidence for the Recent Merging of Two Disk Galaxies in NGC The Astrophysical Journal. 252:
Van der Marel RP, Besla G, Cox TJ, Sohn ST, Anderson J The M31 Velocity Vector. III .Future Milky Way M31-M33 Orbital Evolution, Merging, and Fate of the Sun. The Astrophysical Journal. 753:9-30
2012. Hubble Shows Milky Way is Destined for Head-On Collision with Andromeda Galaxy. Astronomy. In Press.
Chaisson E, McMillan S (7th Ed.). Astronomy Today. San Francisco: Pearson Addison-Wesley. pp
Penn State Galaxy Interactions. Last Access: Oct. 16,

14. References
University of Maryland Introductory Astronomy: Galaxy Interactions.
Last Access: Oct. 16, 2012.