This paper combines visible-galaxy scaling relations from Kormendy & Bender (2012) with dark matter scaling relations from Kormendy & Freeman (2014). Results on visible galaxies are conveniently encoded in a parallel sequence galaxy classification scheme shown in Fig. 1.
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Figure 1. Parallel sequence galaxy classification including spheroidal (Sph) galaxies as bulgeless S0 galaxies juxtaposed with irregular (Im) galaxies. From Kormendy & Bender (2012). |
This updates a parallel sequence classification proposed by van den Bergh (1976). Kormendy & Bender add Sph galaxies as S0s that have no bulge component. This is motivated by the observation that S0 galaxies have bulge-to-total luminosity ratios that range from almost 1 to almost 0 (see also Laurikainen et al. 2010). Kormendy & Bender find a continuous transition in properties - including density profiles but also kinematic properties - between S0 disks and spheroidal galaxies. Figure 2 shows the parameter correlations that illustrate this continuity. S0-like galaxies are given a diffferent name when they have no bulges: They are called Sphs.
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Figure 2. Parameter correlations for ellipticals, bulges, Sph galaxies, and S0 disks. Bulges and disks of S0s are plotted separately. Plotted parameters are the major-axis effective radius re that encloses half of the light of the galaxy component, the effective brightness μe at re, and the total V-band absolute magnitude of the galaxy or galaxy component. The middle panel shows the Freeman (1970) result that disks of giant galaxies tend to have the same central surface brightness μ0. Here, μe ≃ 22.0 V mag arcsec-2 corresponds (for an exponential) to μ0 = μe - 1.82 ≃ 20.2 V mag arcsec-2 ≃ 21 B mag arcsec-2, brighter than Freeman's value 21.65 B mag arcsec-2 because μe is not corrected to face-on orientation. From Kormendy & Bender (2012), who conclude: (1) Sphs are continuous with the disks of S0 galaxies. (2) The kink in the μe - MV relation at MV ~ -18, where bulges disappear in Fig. 4, marks the transition to a baryon retention sequence: tinier dwarf galaxies retain fewer baryons (Fig. 6). Continuity between Sphs and S0 disks is one reason why Fig. 1 shows spheroidal galaxies as bulgeless S0s. Note that Sphs overlap in MV with but are distinct from bulges and elliptical galaxies. They are not "dwarf ellipticals;" they are related to disks (Kormendy 1985, 1987). |
Cappellari et al. (2011) and Krajnovic et al. (2011) propose a similar parallel sequence classification (without Sphs).