NGC 680

Lore & Background

NGC 680 was discovered by William Herschel on September 15, 1784. It appears asymmetrical, likely due to interactions with other galaxies. Tidal forces have disturbed its shape, creating arcs, shells, and an asymmetric envelope, along with a patchy dusty feature. The galaxy's hydrogen gas forms two large diffuse stellar plumes, possibly tidal tails: one extending to the southeast and the other to the northwest, with projected lengths of 75 and 60 kiloparsecs, respectively. Despite these disturbances, NGC 680 does not have a kinematically decoupled nucleus.

The galaxy's characteristics suggest it was created by the merger of two galaxies, possibly gas-rich spirals, based on the amount of surrounding gas. The lack of detection of molecular gas indicates the merger took place 1–2 billion years ago. The merger remnant is classified as a 'fast rotator,' suggesting either that the galaxy retained the angular momentum of the original galaxy or that the two merged in a way that attained angular momentum. The mean age of the stellar population in the central 0.3 kiloparsecs is 7.8 billion years, while for the central 2.6 kiloparsecs it is 6.5 billion years. Ionised gas has been detected in the nucleus, probably created by gas emitted from stars at the end of their lives. An arc of bluer, younger stars has been detected east of the galaxy.

NGC 680 is a member of the NGC 691 group, which includes IC 163, NGC 678, NGC 691, NGC 694, IC 167, and NGC 697. It forms a pair with NGC 678, located 5.5 arcminutes to the northwest, and IC 1730 lies 3.7 arcminutes northwards. Hydrogen emission surrounds both NGC 680 and NGC 678, and an HI tail extends toward NGC 691. This gas was probably stripped by NGC 678 after a close encounter with NGC 680. However, deep photography has not detected a stellar bridge between NGC 680 and NGC 678, nor between NGC 680 and IC 1730.

Reader's Guide

NGC 680 serves as a key example of a merger remnant in a galaxy group, illustrating the processes that shape elliptical galaxies over cosmic timescales. Its asymmetric envelope, tidal plumes, and lack of molecular gas point to a merger that occurred 1–2 billion years ago, providing a timeline for the transformation of gas-rich spirals into an elliptical. The galaxy's classification as a 'fast rotator' offers insight into how angular momentum is preserved or acquired during mergers. The detection of ionised gas in its nucleus and a younger stellar arc to the east suggests ongoing or recent star formation activity, possibly triggered by the merger or interactions with nearby companions. As a member of the NGC 691 group, NGC 680's interactions with NGC 678 and other members—evidenced by shared hydrogen emission and an HI tail—highlight the dynamic environment of galaxy groups and the role of tidal stripping. The absence of a detected stellar bridge between NGC 680 and its neighbors underscores the complexity of such interactions, where gas may be exchanged without visible stellar connections. Overall, NGC 680 contributes to the understanding of galaxy evolution, merger dynamics, and the interplay between galaxies in a group setting.

Did You Know?

• The galaxy is about 100,000 light years across, similar in size to the Milky Way.
• It has two large diffuse stellar plumes extending 75 and 60 kiloparsecs, likely tidal tails.
• The merger that formed NGC 680 occurred 1–2 billion years ago, based on the lack of molecular gas.