Editing Lenticular galaxy (section)

==Morphology and structure==
===Classification===
[[File:NGC 2787.jpg|thumb|left|[[NGC 2787]] is an example of a lenticular galaxy with visible dust absorption.  While this galaxy has been classified as an S0 galaxy, one can see the difficulty in differentiating among spirals, ellipticals, and lenticulars.  Credit: [[Hubble Space Telescope|HST]]]]
[[File:NGC1387 - hst 10217R850GB475.png|thumb|left|[[NGC 1387]] has a large nuclear ring. This galaxy is a member of the [[Fornax Cluster]].]]
[[File:Hubble-Grid.jpg|thumb|300px|Grid showing the location of early-type galaxies (including the lenticular S0 galaxies) relative to the late-type spiral galaxies. The horizontal axis shows the morphological type, primarily dictated by the nature of the spiral arms.]]
[[File:wikifigure.png|thumb|300px|The percentage of galaxies with a particular axis ratio (minor/major) for a sample of lenticular and spiral galaxies. The inset is a visual representation of the profile of either at the specified minor (b) to major (a) axis ratios.<ref name=bmbook />]]
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<ref group=note name=latevsearly>Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while those to the right are "late-type".</ref> galaxies, yet may exhibit a central bar.<ref name=bmbook /> This bulge dominance can be seen in the axis ratio (i.e. the ratio between the observed minor and major axial of a disk galaxy) distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises steadily in the range 0.25 to 0.85 whereas the distribution for spirals is essentially flat in that same range.<ref name="Shapes">{{cite journal|last=Lambas|first=D.G.|author2=S.J.Maddox and J. Loveday |journal=MNRAS|date=1992|volume=258|issue=2|pages=404–414|bibcode = 1992MNRAS.258..404L|title=On the true shapes of galaxies|doi=10.1093/mnras/258.2.404|doi-access=free}}</ref> Larger axial ratios can be explained by observing face-on disk galaxies ''or'' by having a sample of spheroidal (bulge-dominated) galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge. The galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of disk galaxies with prominent spheroidal components will have more galaxies at larger axial ratios. The fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component.<ref name=bmbook />

Lenticular galaxies are often considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their intermediate placement on the [[Hubble sequence]]. This results from lenticulars having both prominent disk and bulge components. The disk component is usually featureless, which precludes a classification system similar to spiral galaxies. As the bulge component is usually spherical, elliptical galaxy classifications are also unsuitable. Lenticular galaxies are thus 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 S0<sub>1</sub>, S0<sub>2</sub>, and S0<sub>3</sub> where the subscripted numbers indicate the amount of dust absorption in the disk component; the corresponding classes for lenticulars with a central bar are  SB0<sub>1</sub>, SB0<sub>2</sub>, and SB0<sub>3</sub>.<ref name=bmbook />

===Sérsic decomposition===

The [[surface brightness]] profiles of lenticular galaxies are well described by the sum of a [[Sersic profile|Sérsic model]] for the spheroidal component plus an exponentially declining model (Sérsic index of n ≈ 1) for the disk, and often a third component for the bar.<ref>Laurikainen, Eija; Salo, Heikki; Buta, Ronald (2005), [http://adsabs.harvard.edu/abs/2005MNRAS.362.1319L Multicomponent decompositions for a sample of S0 galaxies]</ref>  Sometimes there is an observed truncation in the surface brightness profiles of lenticular galaxies at ~ 4 disk scalelengths.<ref name=BMReview /> These features are consistent with the general structure of spiral galaxies. However, the bulge component of lenticulars is more closely related to elliptical galaxies in terms of morphological classification. This spheroidal region, which dominates the inner structure of lenticular galaxies, has a steeper surface brightness profile (Sérsic index typically ranging from n = 1 to 4)<ref>Andredakis, Y. C.; Peletier, R. F.; Balcells, M. (2016), [http://adsabs.harvard.edu/abs/1995MNRAS.275..874A The Shape of the Luminosity Profiles of Bulges of Spiral Galaxies]</ref><ref>Alister W. Graham and Clare C. Worley(2016), [http://adsabs.harvard.edu/abs/2008MNRAS.388.1708G Inclination- and dust-corrected galaxy parameters: bulge-to-disc ratios and size-luminosity relations]</ref> than the disk component.  Lenticular galaxy samples are distinguishable from the diskless (excluding small nuclear disks) elliptical galaxy population through analysis of their surface brightness profiles.<ref>G. A. D. Savorgnan and G. W. Graham (2016), [http://adsabs.harvard.edu/abs/2016ApJS..222...10S Supermassive Black Holes and Their Host Spheroids. I. Disassembling Galaxies]</ref>

===Bars===
Like spiral galaxies, lenticular galaxies can possess a central bar structure. While the classification system for normal lenticulars depends on dust content, barred lenticular galaxies are classified by the prominence of the central bar. SB0<sub>1</sub> galaxies have the least defined bar structure and are only classified as having slightly enhanced surface brightness along opposite sides of the central bulge. The prominence of the bar increases with index number, thus SB0<sub>3</sub> galaxies, like the [[NGC 1460]] have very well defined bars that can extend through the transition region between the bulge and disk.<ref name=bmbook /> [[NGC 1460]] is actually the galaxy with one of the largest bars seen among lenticular galaxies. Unfortunately, the properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties, as well as understanding the formation mechanism for bars, would help clarify the formation or evolution history of lenticular galaxies.<ref name=BMReview />

{{Multiple image
| align             = center
| direction         =
| width             =
| image1            = NGC 2787.jpg
| width1            = 201
| caption1          = '''SB0<sub>1</sub>''' ([[NGC 2787]])
| image2            = NGC 1533 .jpg
| width2            = 339
| caption2          = '''SB0<sub>2</sub>''' ([[NGC 1533]])
| image3            = NGC 1460 -HST10217 18-R850GB475.png
| width3            = 235
| caption3          = '''SB0<sub>3</sub>''' ([[NGC 1460]])
| footer            = Barred lenticular galaxies by classification
| total_width       =
| alt1              =
}}

=== Box-shaped bulges ===
[[NGC 1375]] and [[NGC 1175]] are examples of lenticular galaxies that have so-called box-shaped bulges. They are classified as SB0 pec. Box-shaped bulges are seen in edge-on galaxies, mostly spiral, but rarely lenticular.<ref>{{Cite journal |last=Shaw |first=M. A. |date=1987-12-01 |title=The nature of 'box' and 'peanut' shaped galactic bulges |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=229 |issue=4 |pages=691–706 |doi=10.1093/mnras/229.4.691 |doi-access=free |issn=0035-8711}}</ref>

===Content===
[[File:Hubble image of ESO 381-12.jpg|thumb|Hubble image of [[ESO 381-12]]<ref>{{cite web|title=A galaxy in bloom|url=http://www.spacetelescope.org/news/heic1516/|access-date=13 July 2015}}</ref>]]

In many respects the composition of lenticular galaxies is like that of [[elliptical galaxies|ellipticals]]. For example, they both consist of predominately older, hence redder, stars. All of their stars are thought to be older than about a billion years, in agreement with their offset from the [[Tully–Fisher relation]] (see below). In addition to these general stellar attributes, [[globular clusters]] are found more frequently in lenticular galaxies than in spiral galaxies of similar mass and luminosity. They also have little to no molecular gas (hence the lack of star formation) and no significant hydrogen α or 21-cm emission. Finally, unlike ellipticals, they may still possess significant dust.<ref name="bmbook">{{cite book|last=Binney & Merrifield|title=Galactic Astronomy|date=1998|publisher=Princeton University Press |isbn=0-691-02565-7}}</ref>