Lenticular galaxies (S0 in the Hubble Sequence) are an intermediate class of stellar systems. They occupy a unique space between featureless elliptical galaxies and star-forming spiral galaxies. Lenticulars possess qualities of both types, making them a transitional form in galaxy classification. Studying them provides clues about how galaxies change their morphology and activity over cosmic timescales.
Defining Physical Structure
Lenticular galaxies are defined by a flattened, rotating disk, a trait shared with spiral galaxies, but they lack winding spiral arms. The term “lenticular” refers to their lens-like shape when viewed edge-on. This disk component is smooth and relatively featureless, often appearing faint compared to the prominent central bulge.
The central bulge in an S0 galaxy is larger and brighter relative to the disk than in most spiral galaxies. This high bulge-to-disk luminosity ratio contributes to the galaxy’s overall light profile. The disk clearly distinguishes lenticulars from elliptical galaxies, which are spheroidal and lack a flattened disk structure. Some lenticulars also exhibit a central bar structure, similar to barred spiral galaxies.
Stellar Population and Interstellar Content
The material content of lenticular galaxies is similar to elliptical galaxies. Lenticulars are populated by older, redder stars, known as an aging stellar population. Most stars in these systems are older than a billion years. This composition leads to a predominantly red color, contrasting with the blue glow of star-forming spiral galaxies.
A lack of significant cold gas and dust, the interstellar medium (ISM), is a defining characteristic of S0 galaxies. This gas and dust is the fuel necessary for new star formation, and its depletion results in near-zero rates of current star birth. While some lenticulars retain dust, it is not enough to sustain widespread star formation. The resulting quiescence and aged stellar composition explain why they are classified as “early-type” galaxies along with ellipticals, despite possessing a disk.
Theories of Galactic Evolution
The prevailing hypothesis is that lenticular galaxies are “faded” spiral galaxies that have undergone a transformation. This occurs when a spiral galaxy exhausts or loses the gas and dust necessary to maintain its spiral arms and star formation. The spiral structure then dissipates, leaving behind the smooth, armless disk of an S0 galaxy.
Environmental interactions play a substantial role in this transformation, especially for lenticulars found in dense galaxy clusters. Mechanisms such as ram-pressure stripping occur when a galaxy moves rapidly through hot, dense intergalactic gas, stripping away its cold gas reservoir. This process removes the star-forming fuel, leading to a cessation of new star formation.
Tidal interactions, caused by the gravitational forces of nearby massive galaxies, can also remove a galaxy’s outer gas and disrupt its structure. The merger scenario proposes that minor or “dry” mergers, which involve little gas, can smooth out spiral arms and contribute to the growth of the central bulge. Major mergers between gas-rich spiral galaxies can also create massive lenticulars by driving gas into the center and triggering a burst of star formation before the remaining gas is consumed or expelled.