4C 18.68

Lore & Background

4C 18.68 was first discovered as a radio source in 1967. Its host galaxy is classified as elliptical due to its smooth appearance with no signs of spiral structure. Narrowband imaging reveals an extended appearance with a faint halo and a faint curved tidal tail originating from its southeast side. High-resolution imaging shows irregularities, including isotopes on the southeast end of its axis splitting into two branches rather than a smooth ellipse pattern, suggesting a tidal interaction with its close western companion. Hubble Space Telescope imaging also shows a bright emission region of doubly ionized oxygen lying away from its nucleus.

Reader's Guide

The significance of 4C 18.68 lies in its combination of extreme size and active galactic nucleus properties. As one of the largest and most massive galaxies known, it provides a laboratory for studying galaxy formation and evolution. Its radio source is relatively young, with an estimated age of 107 years, and exhibits a compact, curved structure embedded within a 60 kiloparsec radio halo. The radio halo shows fractional polarization, with the greatest polarization located in its outer edges. The curved radio jet, with a moderate velocity of 0.7c and an orientation at 20 degrees half-opening cone angle, is precessed, displaying ejections of material at relativistic speeds. The precession period of the jet is estimated to be 50,000 years. The galaxy's ongoing tidal interaction with its western companion and the irregular morphology of its southeast axis further highlight its dynamic state, making it a key object for studying the interplay between galaxy mergers, radio activity, and jet dynamics.

Did You Know?

• 4C 18.68 has a diameter of 257 kiloparsecs, making it one of the largest and most massive galaxies known.
• Its radio jet has a precession period estimated at 50,000 years.