Saturday, April 10, 2021

Arp 78 galaxy

Arp 78 (also known as NGC 772 and PGC 7525) is a spiral galaxy, located at a distance of around 130 million light years in the constellation of Aries. It was discovered on the 29th of November 1785 by William Herschel.

The galaxy is around 200,000 light years across, thus it is two times larger than our own Galaxy. Furthermore, it is surrounded by several satellite galaxies. One of them is known as NGC 770, a dwarf elliptical galaxy. The distance between the two galaxies is around 100,000 light years, and the gravitational effects of NGC 770 are responsible for the disturbances that we observe in Arp 78. 

It is clear from optical images that the outer regions of Arp 78 feature an obviously disturbed structure. This has an one sided spiral arm-like morphology that appears to be due to tidal forces.

Observations in the strong ultraviolet have revealed emission that coincides with the elongated structure. Such a feature is due to massive stars that were formed in a starburst event. It is still not clear though if the age of the stellar population agrees with estimates of the age of a tidal driven starburst based on models (i.e, 2 billion years).

Finally, two more dwarf galaxies are nearby Arp 78, thus forming what is known as the NGC 772 group. The first is known as PGC 7826, a low surface bright spiral galaxy, located at a distance of 110 million years (from Earth). The second is known as PGC 7750, a dwarf irregular galaxy, located at a distance of 109 million light years.

Image: Composite optical image of Arp 78 taken with the Canadian French Hawaiian Telescope at Mauna Kea.

Image Credit: Image by Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum), "Canada-France-Hawaii Telescope / Coelum”

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