From the perspective of any random star in a galaxy, a head-on galactic collision isn't a particularly violent event. Stars are spaced so widely apart that they won't actually collide with each other.
Instead, a galaxy collision is an impressive creation event, that is, the creation of stars and, in this case, black holes.
The composite of Hubble (optical) and Chandra (X-ray) observations of the colliding galaxies that make up Arp 147 capture the event. The Arp catalog, compiled in 1966, consists of "peculiar galaxies" that were spotted by Halton C. Arp with the 200-inch Palomar telescope and its 48-inch companion. This list is still providing astronomers with the wonders of galaxies in many shapes of disarray.
The Hubble image to the right above sets the scene: An elliptical galaxy has recently plowed through a spiral galaxy, creating a circular wave, or ring, of star formation in the latter. You can tell that star formation is fairly recent by the fact that the stars in this ring are still very blue.
The biggest, hottest, bluest stars are the first to die off in a population that formed at one time, so their continued existence is an important clue.
Now, at right, you get another view with the X-ray light collected with the Chandra X-Ray Observatory. The ring is seen again with many discrete sources. In fact, they are so bright that they must be stellar-mass black holes.
These are the remnants of the very biggest stars that have died off already, leaving black holes behind after an energetic supernova. If such a star had a close-enough companion star, then the black hole left behind is pulling material off of that star onto itself, releasing massive amounts of energy in the form of X-rays from the disk surrounding the black hole.
The galaxy on the left is not unperturbed either. It has a bright X-ray source in the center, indicating that there is some activity going on there as well. In the most likely scenario, gas that has been stirred up by the collision has fallen towards a supermassive black hole that resides in the center of the galaxy. Material rushing around as it "swirls down the drain" will also give off X-rays here.
The activity is not obvious from the optical picture alone, so the addition of information from another part of the electromagnetic spectrum is crucial to seeing the whole picture. Though no stars are colliding in this scenario, gas does collide, creating stars and all the impressive processes that come with them, changing the evolution of these galaxies forever.
Image: Composite of Hubble and Chandra images of Arp 147, as well as each telescope's contribution shown individually. Credit: X-ray: NASA/CXC/MIT/S.Rappaport et al, Optical: NASA/STScI
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