A new study has led to the classification of NGC 6872 as the largest known spiral galaxy. The galaxy measures some 522,000 light-years across and resides 212 million light-years from earth. The stellar system exists in close proximity to the small disk galaxy IC 4970, which is thought to have recently collided with the larger system.
The above photograph was created through a combination of visible light images from the European Southern Observatory's Very Large Telescope, infrared images taken by NASA's Spitzer Space Telescope, and far-ultraviolet data from NASA's Galaxy Evolution Explorer (GALEX). The image shows a (relatively) small disk galaxy above the NGC 6872, known as IC 4970, which is “interacting” with the larger, spiral galaxy, contributing to its unique size and appearance.
NGC 6872 was already ranked among the largest known stellar systems, but by studying ultraviolet data obtained through the GALEX mission, the team was able to confirm the existence of a large number of hot, young stars. Without this data, the team of astronomers from the US, Chile and Brazil would have been unable to detect the full scale of the galaxy, which is thought to be some five times the size of the Milky Way. The position of the younger stars on the outer reaches of the galaxy's spiral arms suggests that they were formed as a result of the system's collision with IC 4970 some 130 million years ago.
Many large galaxies, including our own, are thought to have formed through collisions similar to the one seen between NGC 6872 and IC 4970. What is interesting about the spiral galaxy (other than its sheer size) is that the interaction between the two galaxies may well be the causal factor in the creation of another stellar system. The system in question is at the far end of the north-eastern arm of NGC 6872, and is thought to be the creation of a tidal dwarf galaxy.
The tidal dwarf galaxy seen here is thought to have been created by the collision of NGC 6872 and IC 4970
Another example of the collision of stellar systems was discovered in July 2012 by the Hubble Space Telescope. The distant spiral galaxy BX 422 was found to be remarkably well-formed for its age, something that has been tentatively attributed to its close proximity to and past collision with a companion dwarf galaxy.
An analysis of Hubble measurements has also shown that the Milky Way is on a collision course with the Andromeda galaxy, our nearest neighbor. There's no need to worry just yet though, as the collision isn't expect to take place for another four billion years.