Star Formation in Ring and Polar-Ring Galaxies
Summary
Ring galaxies and polar-ring galaxies offer natural laboratories for investigating how dynamical processes shape star formation. In collisional ring systems, a near head-on encounter launches a radially expanding density wave through the disc, compressing gas and igniting a transient burst of star formation that outlines one or more concentric rings. These rings often exhibit blue, UV-bright stellar populations and elevated star formation rates compared with undisturbed disc regions. By contrast, polar-ring galaxies consist of a central host—typically lenticular or early-type—encircled by an orthogonal ring or disc of gas, dust and stars. The polar structure forms via accretion or minor merger events, and its kinematic decoupling from the host reveals the external origin of fresh gas that can sustain prolonged, though often less intense, star formation. Across both classes, multiwavelength surveys have mapped variations in metallicity, gas pressure and molecular content to probe how interaction geometry, gas accretion, and secular evolution govern the efficiency and longevity of ring-driven star formation.
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Star Formation in Ring and Polar-Ring Galaxies publication trend
The graph below shows the total number of articles in star formation in ring and polar-ring galaxies across all publications each year (not limited to Nature Index journals).
Technical terms
Collisional ring galaxy: A disc galaxy in which a near head-on collision with a companion induces an expanding ring of enhanced density and star formation.
Polar-ring galaxy: A galaxy with a primary stellar component surrounded by a ring or disc of stars and gas oriented roughly perpendicular to the host plane, signifying external accretion.
Star formation rate: The mass of gas converted into stars per unit time, typically expressed in solar masses per year.
Hydrodynamical simulation: A computational model that solves the fluid equations governing gas dynamics, often coupled with dark matter and star formation prescriptions, to study galaxy formation.
Lenticular galaxy (S0): A galaxy type intermediate between elliptical and spiral, characterised by a central bulge and a disc but lacking prominent spiral arms and with low ongoing star formation.
References
- Polar-ring galaxies in the Illustris TNG50 simulation. Monthly Notices of the Royal Astronomical Society (2023).
- Star formation in outer rings of S0 galaxies. Astronomy & Astrophysics (2023).
- The occurrence rate of galaxies with polar structures may be significantly underestimated. Astronomy & Astrophysics (2024).
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