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15.16 Lenticular Galaxies

Lenticular galaxies earned their name from their edge-on lens-like appearance. Often referred to as armless-spirals, or armless disks, lenticular galaxies define the forking point in the Hubble galaxy classification scheme, marking the transition from elliptical to spiral galaxies. Viewed edge on, it is impossible to distinguish a lenticular galaxy from a spiral galaxy, and viewed face-on, some lenticulars may be mistaken for elliptical galaxies, especially E0 galaxies. They are also sometimes called "arm-less" spirals.

Like spiral galaxies, lenticular systems possess a disk and may have a central nucleus and a galactic halo. Some lenticulars also have galactic bars and even rings of star formation. What they don't have is spiral arms. This may be due to their lack of star formation (outside of any bars or rings). Most lenticular systems are poor in gas and dust and possess older stellar populations similar to those found in elliptical systems.
There are two different explanations for the formation of a lenticular galaxy. One is that they are spirals that have faded over time. The absence of gas, lack of recent star formation, and rotational support are all consistent with this idea. Astronomers have identified a related class of galaxies called anemic spirals, where, if time passed and the spiral arms faded, they would appear indistinguishable from lenticular galaxies. The other explanation is mergers, which would account for the fact that lenticulars have larger bulges than most spirals, because after a merger many stars reorganize into the central region where they would appear as a bulge.