2007ApJ...663...61L

Recent observations reveal that a majority of galaxies contain a central massive object (CMO), either a supermassive black hole (SMBH) or a compact stellar nucleus, regardless of the galaxy mass or morphological type. The masses of these CMOs correlate tightly with those of the host galaxies, M_CMO~0.002M_gal. Several recent studies argue that feedback from black holes can successfully explain the M_BH-σ correlation in massive elliptical galaxies that contain SMBHs. However, puzzles remain in spirals or dwarf spheroids that do not appear to have black holes but instead harbor a compact central stellar cluster. Here we use three-dimensional, smoothed particle hydrodynamics simulations of both isolated and merging galaxies to study the formation and evolution of CMOs in bulgeless disk galaxies, and the transition of the CMO-host relations from late-type bulgeless spirals to early-type ellipticals. Our simulations show that the mass of the CMO correlates with that of the host galaxy in both isolated bulgeless spirals and in ellipticals formed through mergers, and that M_CMO correlates with the global star formation efficiency in the galaxy. We find that the final mass of the CMO is dominated by the accreted mass, rather than the initial fragment mass, and that both the final accreted CMO mass and the recently formed stellar mass increase monotonically with the total mass of the galaxy. Our results suggest that the observed correlations may be established primarily by the depletion of gas in the central region by accretion and star formation, and may hold for all galaxy types. A systematic search for CMOs in the nuclei of bulgeless disk galaxies would offer a test of this conclusion.