Lenticular galaxy

Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations.

The disk component is usually featureless, which precludes a classification system similar to spiral galaxies.

Lenticular galaxies are thus divided into subclasses based upon either the amount of dust present or the prominence of a central bar.

[6] Sometimes there is an observed truncation in the surface brightness profiles of lenticular galaxies at ~ 4 disk scalelengths.

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)[8][9] than the disk component.

SB01 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 bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion.

Rotation support implies the average circular motion of stars in the disk is responsible for the stability of the galaxy.

There is also a considerable amount of difficulty in deriving accurate rotational velocities for lenticular galaxies.

Their disk-like, possibly dusty, appearance suggests they come from faded spiral galaxies, whose arm features disappeared.

[7] This possibility is further enhanced by the existence of gas poor, or "anemic", spiral galaxies.

[23] Moore et al. also document that tidal harassment – the gravitational effects from other, near-by galaxies – could aid this process in dense regions.

[24] The clearest support for this theory, however, is their adherence to slightly shifted version of Tully–Fisher relation, discussed above.

A 2012 paper that suggests a new classification system, first proposed by the Canadian astronomer Sidney van den Bergh, for lenticular and dwarf spheroidal galaxies (S0a-S0b-S0c-dSph) that parallels the Hubble sequence for spirals and irregulars (Sa-Sb-Sc-Im) reinforces this idea showing how the spiral–irregular sequence is very similar to this new one for lenticulars and dwarf ellipticals.

[25] The analyses of Burstein[26] and Sandage[27] showed that lenticular galaxies typically have surface brightness much greater than other spiral classes.

[28][29] If S0s were formed by mergers of other spirals these observations would be fitting and it would also account for the increased frequency of globular clusters.

Mergers are also unable to account for the offset from the Tully–Fisher relation without assuming that the merged galaxies were quite different from those we see today.

The Spindle Galaxy (NGC 5866), a lenticular galaxy in the constellation Draco . This image shows that lenticular galaxies may retain a considerable amount of dust in their disk. However, there is little to no gas, and thus they are considered deficient in interstellar matter .
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: HST
NGC 1387 has a large nuclear ring. This galaxy is a member of the Fornax Cluster .
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.
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. [ 4 ]
Hubble image of ESO 381-12 [ 12 ]
NGC 4866 is a lenticular galaxy located in the constellation of Virgo. [ 13 ]
This plot illustrates the Tully–Fisher relation for a spiral galaxy sample (black) as well as a lenticular galaxy sample (blue). [ 18 ] One can see how the best-fit line for spiral galaxies differs from the best-fit line for lenticular galaxies. [ 19 ]
Messier 85 is a merged galaxy.