Editing Lenticular galaxy (section)

==Kinematics==

===Measurement difficulties and techniques===

[[File:Potw1328a.tif|thumb|[[NGC 4866]] is a lenticular galaxy located in the constellation of Virgo.<ref>{{cite news|title=A stranger in the crowd|url=http://www.spacetelescope.org/images/potw1328a/|access-date=21 July 2013|newspaper=ESA/Hubble Picture of the Week}}</ref> ]]

Lenticular galaxies share kinematic properties with both spiral and elliptical galaxies.<ref name="MoranDynamicDistinction">{{cite journal|last=Moran|first=Sean M.|author2=Boon Liang Loh |author3=Richard S. Ellis |author4=Tommaso Treu |author5=Kevin Bundy |author6=Lauren MacArthur |title=The Dynamical Distinction Between Elliptical and Lenticular Galaxies in Distant Clusters: Further Evidence for the Recent Origin of S0 Galaxies|journal=The Astrophysical Journal|date=20 August 2007|volume=665|issue=2|pages=1067–1073|doi=10.1086/519550|bibcode=2007ApJ...665.1067M|arxiv = astro-ph/0701114 |s2cid=8602518}}</ref> This is due to the significant bulge and disk nature of lenticulars. The bulge component is similar to elliptical galaxies in that it is pressure supported by a central [[velocity dispersion]]. This situation is analogous to a balloon, where the motions of the air particles (stars in a bulge's case) are dominated by random motions. However, the kinematics of lenticular galaxies are dominated by the rotationally supported disk. Rotation support implies the average circular motion of stars in the disk is responsible for the stability of the galaxy. Thus, kinematics are often used to distinguish lenticular galaxies from elliptical galaxies.  Determining the distinction between elliptical galaxies and lenticular galaxies often relies on the measurements of velocity dispersion (σ), rotational velocity (v), and ellipticity (ε).<ref name=MoranDynamicDistinction /> In order to differentiate between lenticulars and ellipticals, one typically looks at the v/σ ratio for a fixed ε. For example, a rough criterion for distinguishing between lenticular and elliptical galaxies is that elliptical galaxies have v/σ < 0.5 for ε = 0.3.<ref name=MoranDynamicDistinction /> The motivation behind this criterion is that lenticular galaxies do have prominent bulge and disk components whereas elliptical galaxies have no disk structure. Thus, lenticulars have much larger v/σ ratios than ellipticals due to their non-negligible rotational velocities (due to the disk component) in addition to not having as prominent of a bulge component compared to elliptical galaxies.  However, this approach using a single ratio for each galaxy is problematic due to the dependence of the v/σ ratio on the radius out to which it is measured in some early-type galaxies.  For example, the ES galaxies that bridge the E and S0 galaxies, with their intermediate-scale disks, have a high v/σ ratio at intermediate radii that then drops to a low ratio at large radii.<ref>Alister W. Graham et al. (2017), [http://adsabs.harvard.edu/abs/2017ApJ...840...68G Implications for the Origin of Early-type Dwarf Galaxies: A Detailed Look at the Isolated Rotating Early-type Dwarf Galaxy LEDA 2108986 (CG 611), Ramifications for the Fundamental Plane's S<sub>K</sub><sup>2</sup> Kinematic Scaling, and the Spin-Ellipticity Diagram]</ref><ref>Sabine Bellstedt et al. (2017), [http://adsabs.harvard.edu/abs/2017MNRAS.470.1321B The SLUGGS Survey: trails of SLUGGS galaxies in a modified spin-ellipticity diagram]</ref>

The kinematics of disk galaxies are usually determined by [[H-alpha|Hα]] or [[Hydrogen line|21-cm]] emission lines, which are typically not present in lenticular galaxies due to their general lack of cool gas.<ref name=BMReview /> Thus kinematic information and rough mass estimates for lenticular galaxies often comes from stellar absorption lines, which are less reliable than emission line measurements. There is also a considerable amount of difficulty in deriving accurate rotational velocities for lenticular galaxies. This is a combined effect from lenticulars having difficult inclination measurements, projection effects in the bulge-disk interface region, and the random motions of stars affecting the true rotational velocities.<ref name="FornaxKin">{{cite journal|last=Bedregal|first=A.G. |author2=A. Aragon-Salamanca |author3=M.R. Merrifield |author4=B. Milvang-Jensen |title=S0 Galaxies in Fornax: data and kinematics|journal=[[Monthly Notices of the Royal Astronomical Society]]|date=October 2006|volume=371|issue=4|pages=1912–1924|doi=10.1111/j.1365-2966.2006.10829.x|doi-access=free |bibcode=2006MNRAS.371.1912B|arxiv = astro-ph/0607434 |s2cid=6872442 }}</ref> These effects make kinematic measurements of lenticular galaxies considerably more difficult compared to normal disk galaxies.

===Offset Tully–Fisher relation===
[[File:OffsetTF.png|thumb|300px|This plot illustrates the Tully–Fisher relation for a spiral galaxy sample (black) as well as a lenticular galaxy sample (blue).<ref name="TFS0galaxies">{{cite journal|last=Bedregal|first=A. G. |author2=A. Aragon-Salamanca |author3=M. R. Merrifield|title=The Tully-Fisher relation for S0 galaxies|journal=Monthly Notices of the Royal Astronomical Society|date=December 2006|volume=373|issue=3|pages=1125–1140|doi=10.1111/j.1365-2966.2006.11031.x|doi-access=free |bibcode=2006MNRAS.373.1125B|arxiv = astro-ph/0609076 |s2cid=9274153 }}</ref> One can see how the best-fit line for spiral galaxies differs from the best-fit line for lenticular galaxies.<ref name="SpiralScalingRelations">{{cite journal|last=Courteau|first=Stephane |author2=Aaron A. Dutton |author3=Frank C. van den Bosch |author4=Lauren A. MacArthur |author5=Avishai Dekel |author6=Daniel H. McIntosh |author7=Daniel A. Dale |title=Scaling Relations of Spiral Galaxies|journal=The Astrophysical Journal|date=10 December 2007|volume=671|issue=1|pages=203–225|doi=10.1086/522193|bibcode=2007ApJ...671..203C|arxiv = 0708.0422 |s2cid=15229921 }}</ref>]]

The kinematic connection between spiral and lenticular galaxies is most clear when analyzing the Tully–Fisher relation for spiral and lenticular samples. If lenticular galaxies are an evolved stage of spiral galaxies then they should have a similar Tully–Fisher relation with spirals, but with an offset in the luminosity / absolute magnitude axis. This would result from brighter, redder stars dominating the stellar populations of lenticulars. An example of this effect can be seen in the adjacent plot.<ref name=BMReview /> One can clearly see that the best-fit lines for the spiral galaxy data and the lenticular galaxy have the same slope (and thus follow the same Tully–Fisher relation), but are offset by ΔI ≈ 1.5. This implies that lenticular galaxies were once spiral galaxies but are now dominated by old, red stars.