|Annu. Rev. Astron. Astrophys. 2009. 47:
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The lenticular galaxies, or S0s, are classified in the Hubble sequence in between the spiral population and the ellipticals. They are disk galaxies, but like ellipticals are smooth, concentrated, and have low specific star-formation rates (Caldwell et al. 1993). They are distinct from the anemic spirals (Section 3.5), in that they have very little molecular gas or spiral structure. However, some S0 galaxies might form from anemic spirals whose spiral wave pattern has disappeared due to its short-lived nature (Elmegreen et al. 2002), possibly aided by tidal "harassment" in dense regions (Moore, Lake & Katz 1998).
Figure 11 shows some images of typical lenticular galaxies as classified by NED. For this figure, we rejected through visual inspection about one-quarter of the NED classifications as being clearly incorrect (ambiguous cases were kept). The broad-band properties of S0s are shown as the orange points in Figure 12. Evidently, in these properties they are practically inseparable from ellipticals.
Figure 11. SDSS images of lenticular (S0) galaxies, selected according to classifications in NED. The images are sorted by absolute magnitude in the horizontal direction, ranging between Mr - 5log10 h ~ -18.5 and -22 from left to right, and concentration (r90 / r50) in the vertical direction, ranging between 2.2 and 3.8 from the bottom to the top. Thus, the brightest, most concentrated S0s are in the upper right. The galaxies shown were selected randomly, but roughly one-quarter were replaced because their NED classifications were clearly incorrect. We left cases that could be considered ambiguous in the figure.
Consequently, though there is a clear physical distinction between Es and S0s, few researchers have implemented objective measurements on large data sets that neatly separate the two populations. Visual classification schemes usually rely on the axis ratio, which for inclined S0s makes their disk-like nature clear, or the strong break in surface brightness at their disk edge (similar to that of spirals; Section 3.8). However, S0s with a ring or multiple rings are also obvious visually, and barred S0s can exhibit the upturn in radial profile discussed in Section 3.8. S0s are of particular interest because they appear to be structurally similar to spirals, but to have ended their star-formation.
Given their observed properties, the most compelling question about S0s is whether they are spirals that ran out of gas and faded onto the red sequence. One simple way of testing this hypothesis is to ask whether they scatter to low luminosities in the Tully-Fisher relation, as we would expect for a faded population. Our understanding of the Tully-Fisher relation is poorer for S0s than spirals because S0s lack significant H or 21-cm emission, making dynamical mass estimates challenging. Nevertheless, beginning with Dressler & Sandage (1983), a number of investigators have tried to measure S0 dynamics using stellar absorption lines, which generally yield lower signal-to-noise velocities. Bedregal, Aragón-Salamanca & Merrifield (2006) compile a set of dynamically analyzed S0 measurements and find that the S0 Tully-Fisher relation is offset in the K-band from that of spirals by about 1 mag (see also Hinz, Rieke & Caldwell 2003). In Figure 10, we show these S0 galaxies in the I band, where the offset is closer to about 1.5 mag relative to the spirals in Courteau et al. (2007); naturally, the actual offset depends on the Tully-Fisher zeropoint and is somewhat uncertain.
This offset is about what one would expect if S0s were simply "faded" versions of spiral galaxies, whose star-formation had shut-off several billion years ago, but were otherwise well represented by the early-type spiral population today. In this manner, the S0s may form a continuum with the Sa galaxies or other red spirals, which as noted in Section 3.9 are also offset from Tully-Fisher by about 0.5 mag (at high Vc at least). Similar conclusions result from the study of globular cluster populations, which are substantially more frequent per unit luminosity in S0s than in spirals of similar mass; this trend would result if the stellar population faded while the globular cluster population remained unchanged (Barr et al. 2007).
However, one might naïvely expect that if S0s are merely dead spirals then the luminosity and surface brightness distributions of S0s would be systematically fainter than that of spirals, at least within a common environment. A recent analysis of Burstein et al. (2005) instead showed that S0s tend to be brighter than any other spiral type even at fixed environment. Similarly, Sandage (2005) shows that the typical surface brightness of S0s is larger than that of spirals. Similar trends can be seen in Figures 8 and 12. In more detail, S0s have larger bulge-to-disk ratios than could result from fading disks in early-type spirals (Dressler 1980, Christlein & Zabludoff 2004). As a cautionary note, analyses that account for the bar and a general Sérsic profile for the bulge indicate much smaller bulges for S0s and might change this conclusion (Laurikainen, Salo & Buta 2005).
These latter facts support a hypothesis that mergers (which would increase the overall stellar mass) are responsible for the transformation of S0s. If mergers sparked globular cluster formation they might also account for the increased globular cluster frequency. However, merger scenarios have a hard time explaining the S0 Tully-Fisher relation unless the progenitor population was quite different from any that exists in abundance today.
Interestingly, the dependence of S0 fraction and S0 properties on environment has not been studied with the new, large samples available, in large part due to the difficulty in identifying them automatically or unambiguously. Thus, while Dressler et al. (1997) show that S0s become relatively more frequent as one approaches the centers of clusters, no significant improvement or refinement of that measurement has been undertaken for the nearby Universe.