NASA’s Hubble Space Telescope has clicked yet another cosmic wonder — an interaction event of two galaxies that are so closely intertwined that they are collectively labeled as a single entity. Galactic interactions are not a rare phenomenon. In fact, a large number of galaxies show some form of interaction either with their satellites or on a much larger scale with other galaxies. The latter category can include events such as collisions that end up in a merger of galaxies or lead to bursts of star formation.
In scenarios of a collision and eventual merger, the larger galaxy might retain its shape after gobbling up the much smaller galaxy. The gravitational interaction between galaxies often leads to the formation of a larger irregular galaxy, but elliptical galaxies are also known to form as a result. Some of the best examples of galactic interactions are the Messler 81 group (includes the dominant M81 galaxy interacting with two smaller NGC 3034 and NGC 3077 galaxies) and the Cartwheel group which consists of four spiral galaxies in a beautiful ring outline.
The image captures by the Hubble Space Telescope shows the entity called Arp 91 which is located over 100 million light-years away from the Earth. Arp 91 consists of two spiral galaxies named NGC 5953 and NGC 5954. In the image above, NGC 5953 is the lower one with higher brightness and a more intact spiral shape. The oval-shaped galaxy at the top is NGC 5954. Interestingly, NASA notes that both the galaxies are spiral, but their shape appears oddly distorted due to their orientation in space with respect to Earth.
A quick glance at the image suggests that the oval-shaped NGC 5954 is extending one of its arms towards NGC 5954 due to the strong gravitational interaction between them. Scientists believe that galaxies form arms because of their rotation around a central axis and another factor called density waves. These arms happen to be an active site of star formation and usually appear brighter because of the stars inhabiting those regions. Interestingly, a recent study revealed that one of the Milky Way’s arms is broken.
As per a research paper titled ‘Photometric and Kinematic Traces of an Interaction’ that appeared in the Astronomical Society of the Pacific, the two galaxies in Arp 91 system are separated by a distance of 5.8 kiloparsecs and have prominent star-forming regions. But based on Infrared Astronomical Satellite (IRAS) data, NGC 5953 has a much higher rate of star formation per unit area compared to its interacting neighbor. Interestingly, NGC 5953 is said to fall under the Class II Seyfert galaxies, which are characterized by their bright core that is known as an active galactic nucleus (AGN) and happen to be one of the brightest sources of electromagnetic radiation in the cosmos. Interestingly, the Hubble telescope recently captured an image of one such Seyfert galaxy that looks like a giant cosmic eye with a bright center.