This Picture of the Week features two interacting galaxies that are so intertwined, they have a collective name — Arp 91. This delicate galactic dance is taking place over 100 million light-years from Earth, and was captured by the NASA/ESA Hubble Space Telescope. The two galaxies comprising Arp 91 do have their own names: the lower galaxy, which in this image looks like a bright spot, is known as NGC 5953; and the ovoid galaxy to the upper right is NGC 5954. In reality, both of these galaxies are spiral galaxies, but their shapes appear very different because they are orientated differently with respect to Earth.

Arp 91 provides a particularly vivid example of galactic interaction. NGC 5954 is clearly being tugged towards NGC 5953 — it looks like it is extending one spiral arm downwards. It is the immense gravitational attraction of the two galaxies that is causing them to interact. Such gravitational interactions between galaxies are common, and are an important part of galactic evolution. Most astronomers nowadays believe that collisions between spiral galaxies lead to the formation of another type of galaxy, known as elliptical galaxies. These immensely energetic and massive collisions, however, happen on timescales that dwarf a human lifetime — they take place over hundreds of millions of years. So we should not expect Arp 91 to look any different over the course of our lifetimes!

Coordinates
Position (RA): 	15 34 33.16
Position (Dec): 	15° 11' 46.26"
Field of view: 	2.55 x 2.85 arcminutes
Orientation:    	North is 82.0° right of vertical

Colours & filters Band	Wavelength	Telescope
Optical G	477 nm	SDSS
Optical G	474 nm	Víctor M. Blanco 4-meter Telescope DECam
Optical R	644 nm	Víctor M. Blanco 4-meter Telescope DECam
Optical R	623 nm	SDSS
Optical I	785 nm	Víctor M. Blanco 4-meter Telescope DECam
Optical I	762 nm	SDSS
Optical V	606 nm	Hubble Space Telescope ACS