14 DAY TRIAL // The Hubble Space Telescope, currently operated by the NASA/ESA collaboration, celebrates 18 years since it was launched into space. In order to mark this moment, the Hubble collaboration is now launching the vastest amount of individual images ever released at once for public use - 59 new images - presenting dramatic galactic collisions, observed throughout the universe. Colliding galaxies often spell disaster for star systems, albeit they can also trigger new star formation processes and merges to create new galaxies.
Observations of the distant universe reveal that galactic collisions were much more frequent in the past that they are today. Galaxy merges in particular are a critical step in cosmic evolution, creating quasars, triggering new furious star formations and even stellar deaths. Even some of the most isolated galaxies bear the marks of their dynamic past and interactions with other galaxies.
The Milky Way, for example, is orbited by several hundred globular star clusters, each of them being possibly the remnant of a dwarf galaxy swallowed by our own. On the other hand, the Milky Way is currently on a collision course with its bigger sister, the Andromeda galaxy. Both are approaching each other at a speed of 500,000 kilometers per hour, meaning that they will be in direct contact in less than two billion years.
Gravitational interactions between galaxies during a collision usually complete a merging process in a few hundred million years. Although galactic collisions are relatively frequent in the visible universe, star collisions during these interactions are in fact extremely rare, mostly because the distances between star systems are extremely large. Powerful tidal effects are able to distort the old patterns inside the galaxies in order to create a new stable one inside the merger.
Because stars interact with each other only through gravitational forces, galaxies are able to alter the motion of stars within each other and to produce intricate patterns and varied effects while passing close to each other. The first sign of gravitational interaction is usually a bridge of matter approaching from both galaxies. As the collision is in course, long streams of gas and dust stretch out and wrap around both cores, often persisting for long periods of time after the merging has occurred.
After some time, the galactic cores start interacting, exchanging large masses of gas and dust in the process, thus triggering furious star formation, which further heats up the clouds of dust and gas to create some of the brightest objects in the universe in the infrared wavelengths of the electromagnetic spectrum.
The luminosity of such a galactic core could exceed that of the Sun several thousand billion times. Unlike the case of the Milky Way, such galaxies contain hot gas throughout the whole galactic disk, which also radiates bright light in the infrared spectrum. The energy emitted during the star formation process can reach several hundred solar masses per year, while our galaxy is able to generate only a few solar masses per year. The energy created by the accretion disks spinning around supermassive black holes in the galactic nuclei can reach up to a billion times the mass of the Sun.
During the gravitational interaction, intense star formation processes and bright emissions of infrared radiation are typical. Other signs may manifest themselves through galactic nuclei disruptions that can continue long after the interaction is over.
Some of the interacting galaxies presented in the images have been catalogued during the 1960s by Halton Arp, in an attempt to determine the shapes of the galaxies seen with the help of ground-based telescopes.
