Galaxy Pairs and Groups

Single, isolated galaxies are rather the exception, owing to the hierarchical structure of the density distribution in the early universe (after the Big Bang). Galaxies mostly come in pairs, groups or even large clusters. Due their motion in the common gravitational field they repeatedly come close to each other, which often leads to gravitational merging, i.e. the coalescence of galaxies.

M 81/82 (NGC 3031/34)
Maksutov-Newton, Canon 1100Da, 16.6 hours

In this example two galaxies, which in projection are about 10 million ly apart, orbit each other over a period of several billion years. The interaction has induced strong and prominent spiral arms in M 81 (bottom) as well as ejection of ionised gas in M 82 (top).

M 81 group of galaxies

Evostar 72, Canon 1100Da, 4.2 hours

M 81, M 82, NGC 2976 and NGC 3077 form a group of galaxies, together with a number of dwarf galaxies (outside of the field shown here). At the western edge of the image the galaxies NGC 2959 and NGC 2961 are visible, which are located further in the background.

NGC 4627, NGC 4631, NGC 4656
Maksutov-Newton, Canon 1100Da, 5.0 hours

This interesting group of galaxies consists of three galaxies, NGC 4631 (top middle), NGC 4627 (immediately above), and NGC 4656 (lower left). These are in gravitational interaction, thereby orbiting each other over a period of several billion years, with the influence of the interaction evident in the two large galaxies. NGC 4656 is strongly bent, NGC 4631 exhibits intense star formation, which is also responsible for its conspicuous blue colour. It shows a lot of dust along its plane, which forms pronounced vertical structures that emanate from the disc. This material (dust and gas) is mainly ejected by the pressure of supernova explosions within the disc. The small elliptical galaxy NGC 4627 appears to be structureless as it lacks recent star formation. Its interstellar gas has been almost completely converted into stars. The distance of this galaxy group is about 30 million ly.

Leo Triplet: M 65 (NGC 3623), M 66 (NGC 3627), NGC 3628
Maksutov-Newton, Canon 1100Da, 7.2 hours

The Leo Triplet, 30 million ly away, consists of the three galaxies NGC 3628 in the north-east, NGC 3627 (M 66) in the south-east, and NGC 3623 (M 65) in the south-west. They are gravitationally interacting, which can be recognized in the perturbations of their discs. In particular, NGC 3628, which is seen exactly edge-on, clearly shows such perturbations. Its disc is strongly warped and even has a bifurcation.

NGC 4633/34

Maksutov-Newton, Canon 1100Da, 11.1 hours
The galaxy pair NGC 4633/34 is located at the periphery of the Virgo Cluster of galaxies, at a distance of 62 million ly. NGC4634 is seen exactly edge-on, and NGC4633 is also strongly inclined w.r.t. the plane of the sky. Their gravitational interaction is not very strong, as there is no evidence for any strong deformation of their disks or any bridge of matter connecting them. Their centres are about 60000 ly apart in projection. They are most likely further apart in space.

NGC 7331 + Stephan's Quintet

Maksutov-Newton, Canon 1100Da, 19.8 hours

This image shows an interesting area in the sky, in which NGC 7331 (top left) is about 40 million ly away from us. There is a group of galaxies forming the so-called Stephan's Quintet. This galaxy group is about 300 million ly distant, and the group members are all gravitationally bound. The brightest galaxy in this group, NGC 7331 (the larger one with the bluish colour), is located much closer to us, namely 35 million ly, and is accidentally projected onto the compact galaxy group in the background. However, the group remains a quintet, because further to the east (left) one can see another galaxy, which physically belongs to the other four. One can clearly see tidal structures in the group caused by the gravitational interaction.

M 51 (NGC 5194/95)
Maksutov-Newton, Canon 1100Da, 9.8 hours

This galaxy, at a distance of 25 million ly, is seen in close gravitational interaction with a second, smaller galaxy. The two galaxies orbit each other over periods of a couple of billion years. In the course of this process the smaller partner, NGC 5195, obviously loses a large number of stars, as is evident in the stellar streams running away from it. The main galaxy, NGC 5194, is featured by prominent spiral structure. The spiral arms are also a consequence of the gravitational force imposed on it by the smaller partner. North of NGC 5194 kind of a wave pattern apears, with a plume extending towards the west. These are tidal streamers caused by the gravitational interaction, which has pulled billions of stars out of the satellite galaxy. The two galaxies will have merged in a few billion years.

NGC 4038/39
Maksutov-Newton, Canon 1100Da, 15.9 hours

Located at a distance of 65 million ly, the galaxy pair NGC 4038/39, also called the 'Antenna Galaxies', is in a state of gravitatonal merging, i.e. the two galaxies have moved towards each other and are now penetrating each other. They have produced long tidal tails in this process, which reach out 180000 ly (in projection!) into intergalactic space. In the central region, the star-formation rate is enormously high, owing to the permanent collision of gas clouds. Larger galaxies have run through this kind of process at least once in their liffetime.