What happens when two massive spiral galaxies -- each with a hundred billion stars -- slam into each other head-on at hundreds of kilometers per second?
[Click to embiggen; or go here to get the massive 15Mb TIF image.]
This is the unusual galaxy NGC 2623, seen in this newly-released and breathtaking Hubble Space Telescope image taken in 2007. We're seeing this vast smash-up caught in the act, a galactic collision already in progress. It appears frozen in time, but that is an illusion of distance: at a distance of a staggering 250 million light years the tremendous velocities of the collision are reduced to a motionless tableau on the human timescale.
But we see a large number of galactic collisions when we catalog the sky, and together with our knowledge of math and physics we have a good understanding of how these encounters play out.
When two massive galaxies approach each other, the gravity of each starts to affect the other. Call them Galaxy A and B. The side of Galaxy B closer to Galaxy A feels more gravity from it, so stars and gas are drawn toward it more strongly than the stars and gas on the far side of Galaxy B. The same is true in the other galaxy. As they get closer, this force strengthens, teasing out long ribbons of material -- called tidal tails -- that stretch in the direction of the other galaxy.
If the encounter is off-center, then the tails get curved when the galaxies pass, arcing either gently or severely depending on the speed, encounter distance, and mass of each participant. The Hubble image clearly shows the arcing tails from each galaxy in NGC 2623.
Incredibly, even though hundreds of billions of stars are involved, each individual star is far too small to suffer a physical collision. But gas and dust clouds are much bigger than stars (they can be hundreds of trillions of kilometers across, as opposed to stars which are a trifling million or so kilometers in diameter), so collisions between them are common. When clouds collide they collapse and undergo violent bouts of star formation. This too is clear in the image: the blue clumps in the tidal tails are vast regions of clusters of stars being born; over 100 such clusters have been identified in this image in the tail on the right alone.
Collisions like this blast out energy, not just in visible light, but at other wavelengths as well. In infrared alone, NGC 2623 radiates with the power of 400 billion times the Sun's energy. This makes NGC 2623 a ULIRG: an ultraluminous infrared galaxy. Although relatively rare locally, they are so common at great distance (and therefore earlier on in the age of the Universe) that they comprise as much as half of all the infrared background glow we see in the Universe. The huge amount of infrared comes from the collision itself; star formation produces prodigious amounts of dust which absorb ultraviolet light from newly-born stars and re-radiate it in the infrared. The collision also dumps gas and dust into the central supermassive black holes in the cores of the two colliding galaxies, which piles up in a flat disk outside the black hole, heats up hugely, and again glows brightly.
Astronomers are making a comprehensive study of such ULIRGs using a fleet of telescopes including Hubble, Spitzer, Chandra, GALEX, 2MASS, VLA, and even the venerable IRAS satellite which surveyed the sky in infrared in the 1980s, and in fact first discovered the ULIRGs.
Why study them? Because galaxies as large as our Milky Way almost certainly started off small and grew to their present size by colliding and merging with other galaxies. Studying ULIRGs is a way of examining how our galaxy came to be... and it's a glimpse of our future as well. In a billion years or more, we will suffer a massive collision with the Andromeda Galaxy. Our own clouds of gas and dust may smash into those in Andromeda, creating huge waves of star formation and blasting out light at all wavelengths. What will our fate be then? The Earth may survive -- the Sun will still be around for this event -- and the gravitational repercussions may toss us out of the new galaxy, or drop us down to the core.
It may seem academic, but astronomers thirst for understanding of these events. We want to know how we came to be, and where we are headed. That knowledge may have little or no practical use for our own survival (or at least not for a few million millennia), but for now, for today, we learn more about galaxies in general, more about the physics of cosmic collisions, and more about the interaction of gas and dust on a truly mind-numbing scale.
And of course, we get to gaze on lovely images, illusions of placidity and gentleness to be sure, but lovely nonetheless.
Image credit: NASA, ESA and A. Evans (Stony Brook University, New York & National Radio Astronomy Observatory, Charlottesville, USA)