This composite image of the Whirlpool Galaxy and it’s nearby companion galaxy overlays radio astronomy data from the Very Large Array with optical data. The image in white shows how the galaxies appear to optical telescopes: one giant spiral galaxy with a smaller one hanging off an arm. The VLA sees a much bigger picture. The blue overlay reveals the the cast-off gases that were once in the outer spiral arms of these galaxies which have been pulled apart as the smaller galaxy has moved passed the larger one.
This is an odd galaxy known as NGC 1487. It’s not a single galaxy but two or more galaxies in the act of merging. Each of the old galaxies has lost almost all traces of its original appearance as the stars and gas have been thrown about by gravitational interactions. Unless one of the merging galaxies is very much bigger than the other(s), galaxies are always disrupted by the violence of the merging process, so it’s essentially impossible to determine exactly what the original galaxies looked like or how many of them there were. In this case, it may be that this NGC 1487 is the merger of several dwarf galaxies that were previously part of a small group.
Although older yellow and red stars can be seen in the outer regions of the new galaxy, its general appearance is dominated by bright blue stars that probably formed in a burst of star formation triggered by the merger.
In this image, galaxy NGC 2799 appears to being pulled into the center of its neighbor NGC 2798. Interacting galaxies such as these may eventually merger or form a unique pairing. For now, stars from NGC 2799 seem to be falling into NGC 2798 almost like droplets of water.
Galactic mergers usually take place over time scales of several hundred million to a billion or more years. While one or both of the galaxies may cease to exist as an independent entity, the vast space between stars means that stellar collisions are unlikely, so the individual stars typically drift past each other. Our Milky Way is on track to merge with the Andromeda galaxy in four billion years or so.
This animation shows a #D rendering of a gas halo observed by ESO’s Very Large Telescope superimposed over an older image of a galaxy merger obtained with ESO’s Atacama Large Millimeter Array. The halo of hydrogen gas is shown in blue, and the ALMA data is shown in orange. The halo is bound to the galaxy, which contains a quasar at its center. The gas in the halo provides the perfect food source for the supermassive black hole at the centre of the quasar.
The redshift on these objects is 6.2, meaning we see them as they were 12.8 billion years ago.