Irregular galaxy

Lore & Background

Collectively they are thought to make up about a quarter of all galaxies. A large chunk of the Local Group is made up of irregular galaxies, most being distant and classified as dwarf irregulars. Irregulars are broken down into two more subcategories along with dwarf. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force, or may be young galaxies between spiral and elliptical status.

Irregular galaxies may also be formed in galaxy collisions. Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy, though there are also unusual cases of large irregulars like UGC 6697. Due to their small sizes, they are prone to environmental effects like colliding with large galaxies and intergalactic clouds.

Morphology and the Limits of the Hubble Sequence

Unlike the orderly spirals and smooth ellipses that dominate the Hubble classification scheme, irregular galaxies resist neat categorization. They lack a central bulge, show no spiral arm traces, and possess no axis of symmetry in their blue-light distribution. Their appearance is often described as chaotic and sporadic.

Within this broad category, astronomers have carved out meaningful subtypes. An Irr-I galaxy retains some internal structure—enough to hint at organization but not enough to slot it cleanly into the Hubble sequence. Those with faint spiral hints are labeled Sm, while those without any spiral signature become Im.

The Irr-II class represents the most extreme case: galaxies showing essentially no recognizable structure, frequently shaped by violent collisions or mergers. Dwarf irregulars, or dIrr galaxies, are defined by a luminosity threshold below ten to the eighth times the Sun's output. All irregulars are grouped as late-type objects, standing alongside spirals in contrast to the early-type ellipticals.

Origins, Formation, and Environmental Vulnerability

Irregular galaxies are far from rare outliers; collectively they account for roughly one quarter of all known galaxies, and they constitute a substantial fraction of the Local Group, where most appear as distant dwarf irregulars. Their origins are varied and often violent. Some were once orderly spirals or ellipses that became distorted by an uneven external gravitational pull, while others may represent young systems still transitioning between spiral and elliptical configurations.

Galaxy collisions are another well-established pathway to irregularity, and the resulting shapes often carry the scars of such encounters. Because most irregulars are small—typically around one-tenth the mass of the Milky Way—they are especially vulnerable to environmental disturbances, including encounters with massive neighboring galaxies and sweeps through intergalactic gas clouds. Many harbor abundant reservoirs of gas and dust, though this is not a universal trait, particularly among the dwarf irregular subclass.

Star Formation Without Density Waves

Star formation in irregular galaxies is both frequent and diverse, with some systems producing new stars at rates rivaling those of mature spirals. The fundamental physics of star birth remains the same everywhere, yet the triggers differ. Because irregulars lack the density waves that help organize gas in spiral arms, their star-forming regions are shaped more by the random turbulent motion of interstellar gas, the feedback from massive young stars, and the unusually prevalent giant HII regions that punctuate the interstellar medium.

This very absence of density-wave structure makes irregulars ideal laboratories for studying star formation in a more natural state, free from the organizing influence of spiral dynamics. Dwarf irregulars, in particular, are prized by cosmologists: their low metallicity and high gas content make them resemble the earliest galaxies that seeded the Universe, and they may serve as nearby analogs of the faint blue galaxies detected in deep-field surveys. These small systems also carry surprisingly large fractions of dark matter.

The Magellanic Clouds and the Spectrum of Irregularity

The Magellanic Clouds offer a fascinating case study in how galaxy classification can shift with deeper observation. The Large Magellanic Cloud, long catalogued as an irregular, has since been reclassified as a barred Magellanic spiral (SBm), revealing an underlying ordered structure that was previously obscured.

Its smaller companion, the Small Magellanic Cloud, still carries the Im designation despite containing a bar structure, illustrating how even partial order can leave a galaxy in the irregular category. More broadly, the so-called Magellanic type encompasses small spiral galaxies whose shapes are being progressively distorted by the gravitational pull of a larger neighbor, blurring the line between spiral and irregular. These cases remind us that morphology is a snapshot of a galaxy's dynamic history rather than a fixed identity.

Reader's Guide

There are three major types of irregular galaxies: An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. Subtypes with some spiral structure are called Sm galaxies. Subtypes without spiral structure are called Im galaxies. An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence, and is often shaped via collision or merger.

A dI-galaxy (or dIrr) is a dwarf irregular galaxy. Irregular galaxies with luminosities below fall into the dwarf irregular galaxy category. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe.

They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys. These types of irregular galaxies also have high amounts of dark matter. Irregular galaxies are considered late-type along with spiral galaxies as opposed to early type elliptical galaxies. Some of the irregular galaxies, especially of the Magellanic type, are small spiral galaxies that are being distorted by the gravity of a larger neighbor.