What Happens When Two Galaxies Collide?

A galactic collision between NGC 2207 and IC 2163 (HST)

So what does happen? Will the stars crash into one another, sending out huge emissions of gamma radiation and gravitational waves? The effects of two galaxies meeting and colliding are actually a little more elegant than that – for starters, it’s most likely that none of the stars will meet due to the huge distances between star systems. Also, the merging of the systems will spark a huge campaign of star creation within the newly formed fertile gas clouds. So what will we see long after the galaxies have ripped each other apart? Simulations show huge arcs of tidally-formed dust and stars, looking strangely like the precursors to the galactic ghosts recently observed

On my daily trawl through the social book marking sites, I came across the NASA Astronomy Picture of the Day (April 20th) featuring the collision of two spiral galaxies NGC 2207 and IC 2163 (pictured above). It’s a superb image showing the ultimate collision in the cosmos. We can be pretty sure that only one galaxy will remain in a billion years after enduring the galactic blender has done its work (and my money is on the left one).

Simulated galactic collision (Summers, Mihos, Hernquist)

Well it makes a nice picture, but what is going on inside the galaxies? For a start, no stars were hurt in this cosmic collision. The space between systems is huge. If the Milky Way was to collide with Andromeda tomorrow, I doubt we on Earth will notice many short-term changes (although the night sky might look a little livelier). These collisions last for millions of even billions of years, and most of the collision effect comes from large-scale gravitational (tidal) forces. I recently came across a nice bit of research by Frank Summers (Space Telescope Science Institute), Chris Mihos (Case Western Reserve University) and Lars Hernquist (Harvard University), where two modelled galaxies are sent tumbling toward each other, colliding, coalescing and scattering stars and dust. I think it provides a good visualization as to what we can expect of the galactic collision at the top of this post.

Galactic cores merged, huge arcs of dust and stars are sprayed into space (Summers, Mihos, Hernquist)

See the full simulation by Summers, Mihos, Hernquist (2002) – .avi file (14 MB)

Tidal forces will cause the spinning galaxies to rip apart (the smaller galaxy will come off worse), scattering dust (blue dots) and stars (yellow dots). In this simulation, the investigators produced merging galaxies, gradually settling into gravitational harmony, galactic cores merging.

I recently reported on “killer galaxies” and “galactic ghosts”, and as the names suggest, something had died. Perhaps not died, more eaten alive. The long-term after effects of a huge collision have been observed in the form of large halos hanging over the location, looking like ghosts of galaxies past. These simulations go to some way to explain how these halos were formed. Tidal tails of dust and stars sprayed into space, cooling and dying to form an eerie reminder about the collision that occurred all those hundreds of millions of years ago.

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