6. COMPACT MASSIVE BULGES

Fig. 7 reveals that bulges are compact, with the smaller bulges being similar to low-mass compact elliptical galaxies in the local universe and the larger bulges being equivalent to the massive compact galaxies in the distant universe (see Dullo & Graham 2013). Gas accretion from cold streams (Bouquin 2013; Combes 2013) is expected to build disks around the high-z compact galaxies. This feeding is ultimately coplanar rather than random and thereby establishes the disk (Pichon et al. 2011; Stewart et al. 2013; Prieto et al. 2013). As suggested in Graham (2013), and see Debattista et al. (2013) and Driver et al. (2013), the high-z compact galaxies may indeed now be the old, massive bulges in today's large disk galaxies (Dullo 2013), while the local (z = 0) compact elliptical galaxies may be the bulges of stripped disk galaxies, and/or some may be too small to have ever acquired a disk.

Figure 7. Size-mass and density-mass diagrams, adapted from Graham (2013). The single continuous, but curved, dwarf elliptical (dE) and elliptical (E) galaxy sequence is shown by the open circles. The denser bulges are shown by the filled circles. The dashed area shows the location of the compact, massive high-z galaxies (Damjanov et al. 2009). The solid rhomboidal shape denotes the location of compact elliptical (cE) galaxies observed in the local universe.