1. Interactions Between Galaxies

2. Loners Interactions were once believed to be unimportant and rare.
Galaxies were thought to
be isolated loners, well
separated from one another
and scarcely interacting.

3. Au Contraire! There are several important things to note:
Galaxies are bigger relative to their separations than stars are (and they have dark matter halos as well!) so are more likely to ‘bump into’ one another, or at least come relatively close.
Galaxies are relatively “soft:” -- when one passes by another one, the tidal forces can distort them quite dramatically, pulling off tidal tails. Stars are ‘more concentrated’
Galaxies do not live in isolation: they are found in groups
and clusters, strongly interacting by mutual gravity.

4. Consider ‘Relative Sizes’
On average, the typical separation between stars in the solar neighbourhood is about 10 light years – roughly 100 trillion kilometers. That is about 100 million times the size of the Sun. M31, our nearest big neighbour galaxy, is 2 million light years away. That is only about 20 times the diameter of the Milky Way, even before we consider the dark matter halo!

5. Are These Three Loners, in Empty Space
Are These Three Loners, in Empty Space? To the eye, maybe… …but we don’t see everything!

6. The Same Three Galaxies at Radio Wavelengths This shows us where
the hydrogen gas is.
There are clear signs
of real interaction!
(The halos of dark
matter extend even
farther!)

7. Moreover, Galaxies are Sociable!
Our own Milky
Way is part of
the so-called
‘Local Group’

8. ... But There Are Also Huge Clusters of Galaxies
They contain quite literally thousands of galaxies!

9. Of Course, Galaxies Must Move
If you simply put a bunch of galaxies down in empty space, they would feel each other’s gravitational influence and move. We cannot see the associated changes in position – they take too long! But as before we can determine velocities (from the Doppler shift of the light of the galaxies) and work out the current states of motion. We can also make computer models of how they should interact.

10. Early Computer Models of Interactions
The Toomre brothers,
working in the 1970s.
Not well-received at the time!

11. …and again

12. Myriads of Real Examples Galaxies clearly do interact!

13. The Milky Way is a Cannibal
The Milky Way is a Cannibal! - it is swallowing up the Sagittarius dwarf galaxy

14. So Too is M31!

15. More Evidence?
We look at the most remote galaxies (those in the Hubble Ultra-Deep Field) to see if galaxies were different in the long-ago past.

16. The Conclusion
We do see important differences! In the remote past, there were more small galaxies and fewer large ones. There is also evidence for active interactions and collisions. It seems that over time, small galaxies merge to form larger ones, as our computer models imply.

17. The Modern Understanding
In general, big galaxies form gradually, from the mergers of considerably smaller building blocks in a hierarchical fashion.
There is now strong evidence (some of it from my own research) that the biggest elliptical galaxies may form from the dramatic merger of two or more big disk-like or spiral galaxies.

18. Case in Point: M87 a giant elliptical galaxy in the Virgo Cluster

19. Our Own Galaxy’s Fate
Our own Milky Way and the Andromeda galaxy will almost certainly merge in a billion years or so. (They are currently approaching each other at a speed of about 300 km/sec.)