Lenticular galaxy
Intermediate galaxy type with disk and bulge, lacking spiral arms.
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A lenticular galaxy, classified as S0, sits between elliptical (E) and spiral galaxies on the Hubble sequence. It has a large-scale disk but lacks large-scale spiral arms. These disk galaxies have consumed or lost most of their interstellar matter, so they have very little ongoing star formation, though they can still hold significant dust in their disks.
Consequently, like elliptical galaxies, they are composed mainly of aging stars. Despite their different shapes, lenticular and elliptical galaxies share similar spectral features and scaling relationships; both are considered early-type galaxies that evolve passively, at least in the local universe. Connecting ellipticals and lenticulars are ES galaxies, which have intermediate-scale disks.
Classification
In terms of structure, lenticular galaxies stand out because they have both a visible disk and a prominent bulge. Their bulge-to-disk ratios are much higher than those of typical spirals, and they lack the spiral arm structure of late-type galaxies, though they may contain a central bar. This bulge dominance shows up in the axis ratio distribution—the ratio of a disk galaxy’s observed minor to major axis. For lenticulars, this distribution rises steadily from 0.25 to 0.85, while for spirals it remains flat over the same range.
Larger axis ratios can come from viewing face-on disks or from a sample of bulge-dominated galaxies. For example, two edge-on disk galaxies—one with a bulge and one without—will show a larger axis ratio for the one with the bulge. So a sample of disk galaxies with prominent spheroids will have more galaxies at larger axis ratios. The rising distribution for lenticulars indicates they are dominated by a central bulge.
Lenticular galaxies are often seen as a poorly understood transition between spirals and ellipticals, which explains their intermediate spot on the Hubble sequence. This comes from having both a prominent disk and bulge. The disk is usually featureless, preventing a spiral-like classification, while the bulge is typically spherical, making elliptical classifications unsuitable.
Instead, lenticulars are divided into subclasses based on dust content or the presence of a central bar. Non-barred lenticulars are labeled S01, S02, and S03, with the subscript indicating the amount of dust absorption in the disk. Barred lenticulars are labeled SB01, SB02, and SB03.
Sérsic decomposition
The surface brightness profiles of lenticular galaxies fit well with a sum of a Sérsic model for the spheroidal component, an exponential model (Sérsic index near 1) for the disk, and often a third component for the bar. Sometimes a truncation appears in the profile at about four disk scalelengths.
These features match the general structure of spiral galaxies, but the bulge component of lenticulars is more like that of ellipticals in morphological classification. This spheroidal region dominates the inner structure and has a steeper surface brightness profile (Sérsic index typically from 1 to 4) than the disk. Lenticular samples can be distinguished from diskless elliptical galaxies (excluding small nuclear disks) by analyzing these profiles.
Bars
Like spirals, lenticulars can have a central bar. While normal lenticulars are classified by dust content, barred ones are classified by the bar’s prominence. SB01 galaxies have the least defined bar, only a slightly enhanced surface brightness along opposite sides of the bulge. The bar becomes more prominent with higher index numbers, so SB03 galaxies, such as NGC 1460, have very well-defined bars that can extend through the bulge-disk transition.
NGC 1460 has one of the largest bars seen among lenticulars. However, the properties of bars in lenticular galaxies have not been studied in great detail. Understanding these properties and the formation mechanism for bars would help clarify the formation or evolution history of lenticular galaxies.
Box-shaped bulges
Some lenticular galaxies, like NGC 1375 and NGC 1175, have box-shaped bulges. These are classified as SB0 pec. Box-shaped bulges are seen in edge-on galaxies, mostly spirals, but rarely in lenticulars.
In many ways, the composition of lenticular galaxies resembles that of ellipticals. Both consist mainly of older, redder stars, all thought to be older than about a billion years, which agrees with their offset from the Tully–Fisher relation.
Globular clusters are found more frequently in lenticulars than in spirals of similar mass and luminosity. Lenticulars have little to no molecular gas (hence no star formation) and no significant hydrogen alpha or 21-cm emission. Unlike ellipticals, though, they may still retain significant dust.
Quick Facts
- Type
- Galaxy morphological class
- Denoted
- S0
- Position on hubble sequence
- Intermediate between elliptical and spiral
- Key components
- Prominent bulge and disk, no large-scale spiral arms
Facts from the source article.
Lore & Background
Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component. They have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type galaxies, yet may exhibit a central bar. This bulge dominance can be seen in the axis ratio distribution of a lenticular galaxy sample, which rises steadily in the range 0.25 to 0.85, whereas the distribution for spirals is essentially flat in that same range. The fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component.
Lenticular galaxies are often considered a poorly understood transition state between spiral and elliptical galaxies, resulting in their intermediate placement on the Hubble sequence. They are divided into subclasses based upon either the amount of dust present or the prominence of a central bar. The classes of lenticular galaxies with no bar are S01, S02, and S03, where the subscripted numbers indicate the amount of dust absorption in the disk component; the corresponding classes for lenticulars with a central bar are SB01, SB02, and SB03.
Reader's Guide
Lenticular galaxies occupy a critical but poorly understood position in galaxy evolution, bridging spiral and elliptical types on the Hubble sequence. Their dual nature—possessing both a prominent bulge (like ellipticals) and a disk (like spirals)—makes them key to understanding how galaxies transform over time. They share spectral features and scaling relations with ellipticals, and both are considered early-type galaxies that are passively evolving in the local Universe. However, lenticulars retain significant dust and may host bars, box-shaped bulges, and globular clusters more frequently than spirals of similar mass.
Kinematically, they are dominated by a rotationally supported disk, yet their bulge is pressure-supported like an elliptical, leading to higher v/σ ratios than ellipticals. The offset Tully–Fisher relation suggests they may be an evolved stage of spiral galaxies. Because they lack cool gas, kinematic measurements rely on stellar absorption lines, which are less reliable than emission-line methods. Understanding lenticulars' formation and bar properties would help clarify their evolution history.
Frequently Asked Questions
What is a lenticular galaxy?
A lenticular galaxy (class S0) is a disc-shaped galaxy that occupies the middle ground between elliptical and spiral types on the Hubble sequence. It has a prominent central bulge and a flat disc, yet it lacks the large-scale spiral arms characteristic of true spiral galaxies.
Why do lenticular galaxies have so little star formation?
They have exhausted or shed most of their interstellar gas, leaving very little raw material to ignite new stars. As a result, their light is dominated by older, redder stars—much like the stellar populations found in elliptical galaxies.
What subclasses exist within the lenticular category?
Non-barred lenticulars are sorted into S01, S02, and S03 depending on how prominent the disc is relative to the bulge. Barred variants carry the SB prefix (SB01, SB02, SB03) and feature a straight central bar running through the nucleus.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Lenticular galaxy (CC BY-SA 4.0).
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