Bulges and disks are major structural components that define galaxy morphology. The mass ratios of bulges and disks increase statistically with the galaxy mass, with the high-mass end occupied by elliptical galaxies. Although previous theoretical studies have succeeded in reproducing this morphological sequence, it is not yet fully understood why and how this morphological sequence emerged. Galaxy mergers accompanying dark matter halo mergers have been proposed as the major route for bulge formation. On the other hand, it is observationally known that the mass fraction of galaxies (stars plus cold gas) in dark matter halos attains the peak value at Mₕalo ~ 10¹2Mₛol throughout the cosmic time. Using a simple galaxy evolution model including mergers, we show that this feature is the fundamental cause of the morphological sequence. Halos hosting massive galaxies, which stay more massive than this peak mass for long periods during their growth, merge mostly with satellite halos having larger galaxy mass fractions than themselves. Such mergers increase the bulge mass fraction efficiently. In contrast, host halos of low-mass galaxies evolve under unfavorable condition for bulge growth because they stay below the peak mass and merge with satellite halos with smaller galaxy mass fractions. Previous studies suggest that the peak in galaxy mass fraction is created by feed-back processes from active galactic nuclei (AGN) and young massive stars including supernovae (SN), which are considered to suppress star formation in high-mass and low-mass galaxies, respectively. This study thus points to a close relationship between the galaxy morphology and feedback processes which have hitherto been considered unrelated and suggests the importance of further investigation into their causal relationship.
Masafumi Noguchi (Tue,) studied this question.