The Spindle Galaxy NGC 5866 is a lenticular galaxy located in the constellation Draco that serves as the archetype for the S0 galaxy class. It possesses the flat rotating structure of a spiral galaxy but lacks the defining spiral arms that give such systems their name. This object appears as a smooth spindle-shaped lens of light containing a large-scale disc but no large-scale spiral arms.
Astronomers measure the structural identity of a lenticular galaxy through the axial ratio which quantifies the relationship between the observed minor and major axes of the galaxy. The distribution of axial ratios for these systems rises steadily in the range of 0.25 to 0.85 indicating that the spheroidal component is the primary driver of their shape. This measurement confirms that lenticular galaxies exhibit much higher bulge-to-disk ratios than typical spiral galaxies.
The v/σ ratio measures the relationship between rotational velocity and velocity dispersion to determine the internal dynamics of a galaxy. For a lenticular galaxy this ratio is significantly larger than that of an elliptical galaxy which typically has a v/σ ratio below 0.5 for a given ellipticity. This distinction proves that lenticular galaxies possess a disc structure that elliptical galaxies lack.
Astronomers believe lenticular galaxies are spiral galaxies that have run out of fuel because they lack gas and show no recent star formation while retaining dust and spiral-like properties. The Tully-Fisher relation links the luminosity of a galaxy to its rotation speed and shows that lenticular galaxies follow the same slope as spiral galaxies but are offset by approximately 1.5 magnitudes on the luminosity axis. This offset implies that they were once spiral galaxies but are now dominated by old red stars.
Barred lenticular galaxies are classified by the prominence of their central bar ranging from SB01 which has the least defined bar structure to SB03 which features very well-defined bars. The galaxy NGC 1460 is a prime example of an SB03 galaxy boasting one of the largest bars seen among lenticular galaxies. Understanding these properties and the formation mechanisms for bars would help clarify the formation or evolution history of lenticular galaxies.
A box-shaped bulge is a rare and peculiar feature found in some lenticular galaxies that defies the typical spherical expectations of galaxy cores. Examples such as NGC 1375 and NGC 1175 are classified as SB0 pec indicating a peculiar morphology. The presence of such a structure suggests that the internal dynamics of these galaxies are more complex than simple rotation or pressure support.