UGC 8058: The extraordinary galaxy Markarian 231 was discovered in 1969 as part of a survey searching for galaxies with strong ultraviolet radiation. It has long tidal tails and a disturbed shape. Results from the first spectrum showed clear signs of the presence of a powerful quasar in the centre that made Markarian 231 unique in the Markarian sample. Markarian 231 has maintained its reputation as an exceptional object since those early observations and continues to be a favourite target in all wavelength regimes. Its infrared luminosity is similar to that of quasars, making it one of the most luminous and powerful known ultraluminous infrared galaxies. Although the emission of many ultraluminous infrared galaxies appears to be dominated by energetic starbursts, Markarian 231 has been repeatedly identified as an exception and many pieces of evidence point toward an accreting black hole as the major power source behind the enormous infrared luminosity. Although the primary power source behind the incredible far-infrared luminosity of Markarian 231 is almost certainly an active nucleus, the galaxy is also undergoing an energetic starburst. Most dramatically a nuclear ring of active star formation with a rate estimated to be greater than 100 solar masses per year has been found in the centre. UGC 8058 is located about 600 million light-years away from Earth. This image is part of a large  collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.

Name: MRK 231, UGC 8058
Type: Local Universe : Galaxy : Type : Interacting
Category: AnniversaryGalaxies
Constellation: Ursa Major
Distance: 550 million light years
Release date: 24 April 2008, 15:00
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

IC 1623: IC 1623 is an interacting galaxy system that is very bright when observed in the infrared. One of the two galaxies, the infrared-bright, but optically obscured galaxy VV 114E, has a substantial amount of warm and dense gas. Warm and dense gas is also found in the overlap region connecting the two nuclei. Observations further support the notion that IC 1623 is approaching the final stage of its merger, when a violent central inflow of gas will trigger intense starburst activity that could boost the infrared luminosity above the ultraluminous threshold. The system will likely evolve into a compact starburst system similar to Arp 220. IC 1623 is located about 300 million light-years away from Earth. This image is part of a large  collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.

Name: IC 1623, VV 114
Type: Local Universe : Galaxy : Type : Interacting
Category: AnniversaryGalaxies
Constellation: Cetus
Distance: 250 million light years
Release date: 24 April 2008, 15:00
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

ESO 239-IG002: ESO239 is most likely the result of a cosmic collision or a lengthy merger process that will eventually result in an elliptical galaxy. The messy intermediate stage, captured here, is a galaxy with long, tangled tidal tails that envelope the galaxy's core. This image is part of a large  collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.

Name: ESO 239-2
Type: Local Universe : Galaxy : Type : Interacting
Category: AnniversaryGalaxies
Constellation: Grus
Distance: 550 million light years
Release date: 24 April 2008, 15:00
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

NGC 6621, NGC 6622: Arp 81 is a strongly interacting pair of galaxies, seen about 100 million years after their closest approach. It consists of NGC 6621 (to the right) and NGC 6622 (to the left). NGC 6621 is the larger of the two, and is a very disturbed spiral galaxy. The encounter has pulled a long tail out of NGC 6621 that has now wrapped behind its body. The collision has also triggered extensive star formation between the two galaxies. Scientists believe that Arp 81 has a richer collection of young massive star clusters than the notable Antennae galaxies (which are much closer than Arp 81). The pair is located in the constellation of Draco, approximately 300 million light-years away from Earth. Arp 81 is the 81st galaxy in Arp's Atlas of Peculiar Galaxies. This image is part of a large  collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.

Name: Arp 81, NGC 6621, NGC 6622
Type: Local Universe : Galaxy : Type : Interacting
Category: AnniversaryGalaxies
Constellation: Draco
Distance: 300 million light years
Release date: 24 April 2008, 15:00
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and W. Keel (University of Alabama, Tuscaloosa)

2MASXJ09133888-1019196: 2MASXJ09133888-1019196 comprises two interacting galaxies that are both disturbed by gravitational interaction. The wide separation of the pair -approximately 130,000 light-years - suggests that the galaxies are just beginning to merge. Together the two galaxies form an ultraluminous infrared system, which is unusual for the early stages of an interaction. One possible explanation is that the one or both of the components have already experienced a merger or interaction. Giant black holes lurk at the cores of both galaxies, which are found in the constellation of Hydra, the Sea Serpent, about 700 million light-years away from Earth. This image is part of a large  collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.

Name: 2MASX J09133888-1019196
Type: Local Universe : Galaxy : Type : Interacting
Category: AnniversaryGalaxies
Constellation: Hydra
Distance: 700 million light years
Release date: 24 April 2008, 15:00
Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)