Galaxies come in a variety of intriguing and beautiful shapes that reveal important details about their formation and evolution. While spiral and elliptical galaxies are the most common shapes we see in the nearby universe, there are a number of more unusual and rare galaxy types that excite astronomers. When searching for the rarest of these cosmic creatures, a few unusual suspects come to mind.
What are the main galaxy types?
Before examining some of the rarest galaxy shapes, it’s helpful to review the most common galactic forms first:
- Spiral galaxies like our Milky Way are characterized by a flat, rotating disk with well-defined arms of stars, gas, and dust curving out from the center. They make up about 77% of galaxies in the observable universe.
- Elliptical galaxies have a smooth, nearly featureless oval or spherical shape. They contain older stars and very little gas and dust. Ellipticals account for approximately 15% of observed galaxies.
- Lenticular galaxies look like a cross between elliptical and spiral galaxies. They have a bulge with a disk but no visible arms. About 5% of galaxies are lenticulars.
- Irregular galaxies have no well-defined shape. Their random, uneven structure is the result of gravitational interactions with other galaxies. Irregulars constitute only about 3% of known galaxies.
Beyond these basic morphologies, astronomers have discovered a number of unusually shaped galaxies that provide clues about galactic evolution, dark matter, and more. Let’s look at some of the strangest and most intriguing of these rare galactic specimens.
Ring Galaxies
Instead of a circular central region surrounded by spiral arms, ring galaxies have the bulk of their stars, gas, and dust arranged in a ring around an empty or almost empty center. It is estimated that less than 0.1% of observed galaxies have a ring morphology. The most well-known example is the Cartwheel Galaxy, located 500 million light years away in the constellation Sculptor. Its distinctive wheel-like shape is the result of a smaller galaxy crashing through the center around 300 million years ago.
Other ring galaxies, like Hoag’s Object in Serpens, likely formed their rings through a process called galactic tidal disruption. This occurs when the gravitational pull from a nearby large galaxy tears away stars and material from the outer part of a smaller galaxy, leaving a ring behind.
Famous Ring Galaxies
- Cartwheel Galaxy
- Hoag’s Object
- The Lindsay-Shapley Ring
- AM 0644-741
Collisional Ring Galaxies
A collisional ring galaxy is formed when two galaxies collide in a more-or-less head-on bulls-eye collision. The resulting gravitational disruption causes expanding density waves that sweep up dust and gas into a ring structure around the central nucleus. It is estimated that only around 0.1% of galaxies interact in this specific way to form a collisional ring system. One of the most famous examples is The Ring, which lies in the constellation Leo Minor about 600 million light years from Earth.
Famous Collisional Ring Galaxies
- The Ring (PGC 1000714)
- II Zw 28
- AM0644-741
Polar Ring Galaxies
Polar ring galaxies are extremely rare systems where a ring of stars, gas, and dust orbits a central galaxy perpendicular to the disk. It is believed collisions or the accretion of material from another galaxy can create these unusual right-angle configurations. Only around 0.5% of galaxies are estimated to have polar rings. A well known case is NGC 4650A, located about 130 million light years away in the constellation Centaurus. Models suggest its polar ring was formed when two galaxies collided and merged together billions of years ago.
Famous Polar Ring Galaxies
- NGC 4650A
- NGC 660
- ESO 415- G 026
Peculiar Galaxies
“Peculiar galaxies” is a catch-all term for galaxies with irregular, asymmetric, or chaotic shapes that don’t fit into standard classifications. They may make up 10% or less of observed galaxies. Peculiar shapes arise when galaxies gravitationally interact and warp each other through mergers or close encounters. The intense gravitational forces distort their structures into odd forms. Famous examples include the colliding Antennae Galaxies and the Medusa Merger.
Famous Peculiar Galaxies
- Antennae Galaxies
- Medusa Merger
- NGC 7252 (“Atoms for Peace” Galaxy)
- Mayall’s Object
Protogalaxies
In the early universe billions of years ago, primordial galaxies were just beginning to form and coalesce out of clouds of hydrogen gas. Astronomers have detected a small number of candidates for these proto-galaxies existing when the universe was less than a billion years old. While extremely distant and faint, they provide a unique window into the conditions of the early universe. These young protogalaxies appear to be small, compact, and highly irregular before settling into more structured shapes like spirals and ellipticals. The most distant recognized is GN-z11, seen as it existed just 400 million years after the Big Bang.
Famous Protogalaxies
- GN-z11
- A1689-zD1
- MACS1149-JD1
Conclusion
While spiral, elliptical, and irregular galaxies dominate the cosmic population in our local universe, a number of unique, rare galaxy types reveal exceptional conditions of formation and evolution. Collisional interactions between galaxies can distort their shapes into magnificent rings, discs, and swirling antennae. And peeking back to the early universe, protogalaxies show the beginnings of galactic structure emerging just a billion years after the Big Bang. Continued observations with new telescopes will likely uncover more of these cosmic oddities that challenge conventional theories and illuminate our understanding of how galaxies form and grow across cosmic time.