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Come Together

If you’re at all a fan of astronomy, you’ve probably marveled over the many beautiful photos of spiral galaxies that are out there. Like this one of NGC 3344 captured by the Hubble Space Telescope.

Galaxy in a spin
Credit: ESA/Hubble & NASA
But you may not know that galaxies have not always looked this way. The grand spirals we are so familiar with were formed over the course of billions of years by the collisions of smaller galaxies. Though, when spirals collide with a similar-sized galaxy, the disruption can cause them to merge and evolve into a giant elliptical galaxy. According to an article in Nature News, based on a survey of galaxy shape and structure (current to 2009), it is thought that “nearly all massive galaxies have undergone at least one major merger since the Universe was 6 billion years old.”

Giant elliptical galaxy, M87, located in the Virgo cluster, Credit: ESA/Hubble & NASA

When we look at very distant galaxies, we see a completely different picture. Older galaxies tend to be small and clumpy, often with a lot of star formation occurring in the massive clumps. The question of how these clumpy galaxies evolve and develop structure over time is a big open question in astronomy, and we hope that the powerful up-and-coming James Webb Space Telescope will help astronomers to learn more.

Clumpy Galaxy
Clumpy galaxy spied by the Hubble

Clumpy Galaxies
Clumpy galaxies spied by the Hubble

Other unanswered questions about galaxies include the following. How did the first galaxies form? How did we end up with the large variety of galaxies we see today? (We see not only organized and structured spiral galaxies in the modern universe, we also see giant ellipticals, and galaxies in a wide variety of shapes and sizes.) We now know that extremely large black holes live at the centers of most galaxies – but what is the nature of the relationship between the black holes and the galaxy that hosts them? There is also more to understand about the mechanisms that cause star formation whether it happens internal to a galaxy or because of a merger.

One thing we do know is that galaxies are still forming and assembling today. There are many, many examples of galaxies colliding and merging to form new galaxies. And in our own local neighborhood of space, the Andromeda galaxy is headed toward the Milky Way for a possible future collision – many billions of years from now!

Here’s an example of a galaxy (named IRAS 23436+5257) thought to be, because of its twisted, worm-like structure, the result of a collision and subsequent merger of two galaxies.

Galactic glow worm
Credit: ESA/Hubble and NASA, Acknowledgement: Judy Schmidt
There are thousands of amazing images of galaxy mergers out there. Here is a sampling.

Visible light Hubble data combined with infrared data from Spitzer, to create this stunning image of M51, the Whirlpool galaxy. M51, as you can see, is actually two interacting galaxies (formally NGC 5194 and 5195). Phil Plait has a good write-up on this image.

Credit: vdHoeven/NASA/JPL-Caltech/R. Kennicutt (Univ. of Arizona)/DSS
Here is Hubble’s view of the merging galaxies known collectively as II Zw 096. This is multiwavelength, from the far-ultraviolet to the near-infrared.

Hubble View of Galaxy Merger II Zw 096
“The Mice,” a pair of colliding galaxies, were given this name because of the long tails of stars and gas trailing each galaxy. They will someday be a single giant galaxy.

Hubble's newest camera takes a deep look at two merging galaxies
Hubble image, Credit: NASA, Holland Ford (JHU), the ACS Science Team and ESA
Interacting galaxies can create some unique and strange shapes, as seen in these Hubble images:

Interacting galaxies
Image: NASA/ESA/Hubble Heritage Team/STScI/AURA/A Evans/University of Virginia/NRAO/Stony Brook University/K Noll/J Westphal)
And if those weren’t enough for you, here are more! (Be sure to look at the larger image!)

Hubble - Interacting Galaxies
We’ll leave you with this video, done by Dr. Frank Summers, which shows a comparison of a simulation of a galaxy collision with five Hubble observations of galaxy collisions.



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