Last updated: July-7-99

Lenticular and S0 Galaxies

Hubble [53] introduced S0 galaxies as a morphological transition case between ellipticals and early-type spirals. A detailed description of these lenticular galaxies and a possible sub-classification scheme is given in [157]; see the table below. Others [9, 196] have suggested that S0's actually run parallel to the ordinary spiral stages but form one extreme of gas-depleted disk systems (S0a, S0b, S0c) coupled to the gas-rich spirals by yet another intermediate sequence of ``anaemic'' spirals (Aa, Ab, Ac). de Vaucouleurs [27] suggests that lenticulars may be subdivided as well, and his ``early-type'' and ``late-type'' classification is also outlined below.

**Sub-Divisions of S0/Lenticular Galaxies**
Type	Definition	Examples
(a) de Vaucouleurs [27]
S0^-	Traces of structure can be found in the smooth lens and envelope; a small nucleus may be present.	NGC 5273 NGC 7166
S0°	A weak trace of a ring appears at the edge of the lens; a distinct nucleus and envelope may be present.	NGC 1553 NGC 4459
S0⁺	A well defined ring is present separating the inner nuclear bulge from outer incipient spiral structure.	NGC 2855 NGC 7702
(b) Sandage [157]
S0₁	The existence of an outer envelope, flattened to a fundamental plane, defines this sub-type; the ellipticals of the central section of such galaxies are flatter than E7.	NGC 1201 NGC 3245 NGC 4762
S0₂	The first appearance of a circular absorption pattern or a true depletion of material in the envelope defines this intermediate sub-type.	NGC 0542 NGC 3065 NGC 4111
S0₃	A strong internal circular ring defines this extreme of the S0 class.	NGC 5866 NGC 3032 NGC 4459

Note: There are in fact many non-ringed S0° and S0⁺ galaxies also, see Table 3 (pg.35) in [29].