Evolution of massive black holes in galactic nuclei

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