Hubble Space Telescope captures peculiar pair of interacting galaxies

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New Delhi: The Hubble Space Telescope has captured a pair of interacting galaxies that together bear the designation of Arp 72, at a distance of 160 million lightyears from the Earth in the direction of the constellation of Serpens Caput. Arp is a rather selective group of galaxies that has only two members, a large spiral galaxy designated as NGC 5996 and a smaller companion designated as NGC 5994.

The cores of the two galaxies, that are eventually expected to merge into one, are separated by a distance of 67 thousand lightyears. At the closest points, the galaxies are at a distance of only 40 thousand lightyears. For comparison, the distance between the Milky Way and its closest neighbour, Andromeda is 2.5 million lightyears. The distance between the Milky Way and the largest and brightest of its many satellite galaxies, the Large Magellanic Cloud is 162 thousand lightyears.

NGC 5996 is roughly the size of the Milky Way, and the interaction with its galactic neighbour may be the reason that the spiral shape of the galaxy has been distorted, because of the gravitational influence of NGC 5994. The interaction has also led to the formation of a very long and very faint ‘tidal tail’, that can be observed towards the top right of the image. This kind of tidal tail is common among interacting galaxies.

The diffraction spikes are a signature of Hubble

There are a few foreground stars that bear cross-shaped diffraction spikes. These spikes appear only over extremely concentrated and intense sources of light, and are formed by the light interacting with the internal support structure of the telescope. These spikes allow images captured by Hubble to be recognised at a glance. The target galaxies are sitting in a field of even more distant galaxies of all shapes and sizes, some of which are interacting.

The Altas of Peculiar Galaxies

Arp 72 is one of the space oddities catalogued by the American astronomer Halton Arp in his Atlas of Peculiar Galaxies. The atlas contains unusual interactions between galaxies and documents their individual peculiarities. Arp was motivated to create the Atlas to make it easier for astronomers to examine the evolution of galaxies, recognising limitations in the understanding of how galaxies changed over time.

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