The wild dances of the interacting galaxies
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: April 24, 2008

From dramatic collisions that trigger frenetic bursts of star formation, switch on quasars or cause explosive stellar deaths, to stealthy mergers that result in completely new galaxies, the fifty nine spectacular new images of merging galaxies released today to mark the 18th anniversary of the Hubble Space Telescope shows that no two interactions are alike.

All 59 images can be downloaded from the Hubble website. Image: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).

The Hubble photo album shows snapshots of different instants in the hundreds of millions years it can take for the interaction process to complete. The interactions usually follow the same initial formula, driven by the tidal pull of gravity between two nearby galaxies, but the final product can vary significantly. Actual collisions between stars are rare because so much of a galaxy is empty space, but as the gravitational mesh linking the stars in each galaxy begins to interlock, strong tidal effects disrupt and distort the old patterns to produce seriously intricate effects.

Six snap shots of galaxies at different stages of merging. See text for explanations. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)

The image above shows different galaxies all at various stages of merging. The first tentative sign of an interaction is a bridge of matter as the gentle tugs of gravity tease out dust and gas from an approaching galaxy (frame 1). As the outer reaches of the galaxy impinges upon another, long streamers of gas and dust stretch out and sweep around the galaxy cores (frame 2). These long, ‘tidal tails’ can persist long after the main action is over (frame 3). Once the galaxy cores begin to interact, their gas and dust clouds are buffeted and pulled in all directions, resulting in shock waves that rip through the interstellar clouds (frame 4). Furious star burst formation is forced as gas and dust is siphoned into the active central regions (frame 5). Some of the galaxies show highly distorted features, with dust lanes crossing between the galaxies and long filaments of stars extending far beyond the central regions (frame 6). Even apparently isolated galaxies show tell-tale signs in their internal structure of previous merger events.

Just one of the 59 images released today. This image shows a highly symmetric spiral galaxy seen nearly face-on and partially backlit by a background galaxy. The foreground spiral galaxy has a number of dust lanes between its arms. Image: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama, Tuscaloosa).

Galaxy mergers, which are thought to be more common in the early Universe than they are today, are one of the main driving forces of cosmic evolution. Even our own Milky Way galaxy has been caught up in the seductive dance of the galaxies, containing the debris of many smaller galaxies it has encountered and devoured in the past, and currently absorbing the Sagittarius dwarf elliptical galaxy. The Milky Way will ultimately find its fate in the arms of the Andromeda galaxy; they are rushing towards each other at around 500,000 kilometres per hour and are set to collide in approximately two billion years time.