4.4. Many "Bulges" Look as Flat as Disks

If some "bulges" are really disks, then this should be evident in the distribution of bulge ellipticities. Figure 8 shows this effect in bulge-disk decompositions computed by Kent (1985, 1987, 1988). I use only the decompositions in which the ellipticity of the bulge was a free parameter. Also, I include only objects with disk ellipticities _disk 0.14, since face-on objects have no leverage on the problem. Bulge-disk decompositions are uncertain, so interpretation should be cautious. However, in agreement with Kent, I conclude that many bulges look as flattened as their associated disks. Some look more flattened; these may be triaxial.

Figure 8. Ratio bulge / disk of bulge ellipticity to disk ellipticity, from bulge-disk photometric decompositions by Kent (1985, 1987, 1988). Different symbols encode bulge-to-total luminosity ratio B/T and the distinction between barred and unbarred galaxies (see key). Horizontal tics are drawn at the median bulge / disk for each Hubble type.

The median ratio _bulge / _disk is smallest for Sa galaxies and increases toward later Hubble types. This agrees with other evidence which suggests that disklike "bulges" are more common at later Hubble types. Finally, it is interesting that the median _bulge / _disk for S0 galaxies is similar to that for Scs, not Sas. Kinematically disklike bulges also are more common in S0s than in Sas. Similar effects led van den Bergh (1976b) to develop his "parallel sequence" classification.