Evolution of massive black hole in galactic nucleus

Abstract: We propose a scenario for mass evolution of massive black holes (MBH) in galactic nuclei, to explain both the mass correlation of the supermassive black hole (SMBH) with the bulge and the down-sizing behavior of the active galactic nuclei. Primordial gas structures to evolve galactic bulges are supposed to be formed at $z \sim$ 10 and the core region, called the nuclear region (NR) here, is considered to be a place for a MBH to grow to the SMBH. The down-sizing behavior requires the MBH to significantly increase the mass in a time $\sim$ 1 Gyr. The rapid mass increase is discussed to be realized only when the MBH stays in a very high density region such as a core of a molecular cloud throughout the period $\sim$ 1 Gyr. According to these arguments, the MBHs formed from the population III stars born in the mini halos at $z \sim$ 20 - 30 are excluded from the candidates for the seed black hole to the SMBH and only the MBHs from the population II stars born in the core of the central molecular cloud (CMC) in the NR remain as them. The MBHs in the dense core of the CMC started increasing the mass through mass-accretion and the most massive black hole (MMBH) got the most rapid evolution, possibly restraining relatively slow evolutions of the less massive black holes. Dynamical interactions of the MMBH with the ambient MCs induced the wandering motion and the further mass-increase. However, when the MMBH mass exceeded a boundary mass, the dynamical friction with the field stars brakes the MMBH wandering and the mass accretion. This scenario can semi-quantitatively reproduce both the down-sizing behavior and the SMBH mass - bulge mass correlation with reasonable parameter values.