A Magellanic spiral galaxy is a spiral galaxy with only one spiral arm. Magellanic spiral galaxies are classified as the type Sm (with sub-categories SAm, SBm, SABm); the prototype galaxy and namesake for Magellanic spirals is the Large Magellanic Cloud, an SBm galaxy. They are usually smaller dwarf galaxies and can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies. They are found in proximity to larger spiral galaxies such as the Milky Way, as is the case with the LMC and the Small Magellanic Cloud (SMC).
Magellanic spiral galaxies also have a stratified stellar structure; main sequence stars are found in their spiral arm, and supergiants are clustered in a thick rectangular bar across the middle.
Magellanic spirals SAm galaxies are a type of unbarred spiral galaxy, while SBm are a type of barred spiral galaxy.Linda S. Sparke, John Sill Gallagher, Galaxies in the Universe: An Introduction, 2ed., Cambridge University Press, 2007, SABm are a type of intermediate spiral galaxy.
Type Sm and Im galaxies have also been categorized as irregular galaxies with some structure (type Irr-1). Sm galaxies are typically disrupted and asymmetric.citeBase; Neutral Hydrogen in the Interacting Magellanic Spirals NGC 4618/4625 ; Stephanie J. Bush; Eric M. Wilcots; (accessed 1 March 2009) dSm galaxies are dwarf spiral galaxies or dwarf irregular galaxies, depending on categorization scheme.
The Magellanic spiral classification was introduced by Gerard de Vaucouleurs, along with Magellanic irregular (Im), when he revamped the Hubble classification of galaxies.
Grades of Magellanic spiral galaxies
Example Type Image Information and NotesMagellanic Spirals SAdmNGC 5204SAmSatellite of Pinwheel GalaxyNGC 4676A SABdmPart of a galaxy merger, to the left of it is the galaxy NGC 4676B.NGC 4625SABmIt has only one spiral arm. In ultraviolet it extends four times farther than in visible light.NGC 4236SBdmIn M81 GroupLarge Magellanic CloudSBmPrototype galaxy for this galaxy class, and satellite of Milky Way Sdm SmDwarf Magellanic Spirals dSAdm dSAm dSABdm dSABm dSBdm dSBm dSdmNGC 5474dSmSatellite of Pinwheel Galaxy
List of Magellanic spirals
Barred (SBm) Large Magellanic Cloud (LMC; prototype galaxy) Small Magellanic Cloud (SMC) NGC 1311Paul B. Eskridge; "Recent Star and Cluster Formation in the Nearby Magellanic Spiral NGC 1311"; 'American Astronomical Society Meeting' 208, #14.04; Bulletin of the American Astronomical Society, Vol. 38, p.93; (accessed 1 March 2009) NGC 4618University of Wisconsin, Barred Magellanic Spirals (accessed 1 March 2009) NGC 4236 NGC 55 NGC 4214 NGC 3109 IC 4710
Intermediate (SABm) NGC 4625 NGC 5713
Unbarred (SAm) NGC 5204NASA/IPAC Extragalactic Database NGC 2552
See also Galaxy morphological classification
References