One of the surprises that the Hubble Space Telescope has given us over
its decades of observation is that galaxies have not always
simply existed as independent entities,
one from another, forming separately and remaining separate, that in fact
galaxies typically merge with other galaxies during cosmic history.
Now, this is a little counterintuitive because we know the universe is expanding,
so we would expect that galaxies
are moving apart from each other, which they generally are.
But what we know is that when galaxies are close enough
to each other, their mutual gravitational pull can dominate
and draw these galaxies together as one. We call that a merger.
And Hubble revealed for the first time in such detail
many of these cases where galaxies, particularly in the distant universe,
were in the process of merging with a neighbor galaxy.
These dramatic mergers are not unusual.
In fact, we believe now, thanks to Hubble and subsequent observations
that all galaxies typically go through at least one merger
in the process to becoming what they are today.
So here's some
examples of what Hubble has seen in terms of galaxies
in the process of merging or in the aftermath of merging.
So here you can see some examples of distorted shapes of galaxies.
You can see here and here none of these look like your typical
beautiful spiral or even spherical or elliptical shape.
They all have some irregularities to them, which are telltale signs
that they are either currently merging with another galaxy or are right
in the aftermath of merging with another galaxy.
One of those telltale signs are long tails.
So in some of these you see these interesting long
tail structures coming out from the galaxy itself.
That's often an aftereffect or an impact of galaxies gravitationally
interacting with each other and creating these long tail structures.
Another telltale sign is that you can sometimes see two nuclei
visible in the cores of some of these structures.
That means that there’s still two bright cores of the originally separate
galaxies that haven't quite completed merging together.
An irregular structure, as all of these have, is always a sign that something
has disrupted the original more symmetric structure of these galaxies.
And it's typically the interaction with a neighboring galaxy during
the process of merging,
Mergers can take millions of years.
So these are not quick processes and in fact,
sometimes they are very interesting little dances where the galaxies
can come close to each other and then begin to rotate around one
another before they eventually merge, or they can actually pass through one
another and then be pulled back again and again before they eventually merge.
So this can take a long time.
So we now know that
most galaxies have gone through mergers
and some galaxies are still on the path toward a future merger.
We know that our own Milky Way has a whole little swarm of dwarf galaxies
orbiting around that will eventually be swallowed into the Milky Way.
But we have a more traditional merger in line with the Andromeda
Galaxy, our nearest neighbor, a big spiral galaxy.
It's on a head on collision course with the Milky Way.
Hubble has ascertained
that this is going to happen probably in a few billion years.
So we don't need to think about it too soon.
But it is interesting that our universe is not finished.
We're still in the dynamics of forming and growing galaxies, even today.