NGC 4618

Lore & Background

NGC 4618 is a distorted barred dwarf galaxy whose structure is classified as Sm, indicating a vague resemblance to spiral galaxies. It is often called a Magellanic spiral because of its similarity to the Magellanic Clouds. Unlike most spiral galaxies, NGC 4618 possesses only a single spiral arm, which gives it an asymmetric appearance. This peculiarity led to its inclusion in the Atlas of Peculiar Galaxies as one of three nearby examples of galaxies with single spiral arms, though many similar galaxies have since been identified.

The galaxy's asymmetric structure may be the result of a gravitational interaction with NGC 4625, as such asymmetry is common among interacting galaxies. However, observations of neutral hydrogen gas in NGC 4618 and NGC 4625 imply that only some of the gas outside the optical disks of NGC 4618 is affected by the interaction. This suggests that the one-arm shape may instead form as a result of processes intrinsic to the galaxy itself.

One supernova has been observed in NGC 4618: SN 1985F, a Type Ib supernova of magnitude 16, discovered by Alex Filippenko and Wallace L. W. Sargent on 28 February 1985.

Reader's Guide

NGC 4618 holds significance as a nearby example of a galaxy with a single spiral arm, a rare structure that challenges typical spiral galaxy morphology. Its inclusion in the Atlas of Peculiar Galaxies highlights its role in cataloging unusual galactic forms, and the subsequent identification of many similar galaxies suggests that such asymmetry may be more common than once thought. The galaxy's interaction with NGC 4625 provides a case study in how gravitational forces can shape galactic structure, yet the neutral hydrogen observations introduce ambiguity: the limited extent of the interaction's influence on the gas outside the optical disks implies that internal processes may play a larger role in creating the single-arm shape. This unresolved tension between external and internal mechanisms makes NGC 4618 a valuable object for understanding galaxy evolution and the origins of asymmetry. The detection of SN 1985F, a Type Ib supernova, adds to its legacy as a site of stellar death and contributes to the study of supernova types and their host galaxies.

Did You Know?

• NGC 4618 is sometimes called a Magellanic spiral because it resembles the Magellanic Clouds.
• It has only a single spiral arm, making it one of three nearby examples in the Atlas of Peculiar Galaxies.
• The galaxy is interacting with NGC 4625, but the one-arm shape may also arise from internal processes.
• The only supernova observed in NGC 4618 is SN 1985F, a Type Ib discovered in February 1985.