Why Does NGC 1068 Have a More Powerful Active Galactic Nucleus than NGC 4258

Nuclear gas kinematics probed by water-vapor maser emission has shown that two nearby active galaxies, NGC 1068 and NGC 4258, have a supermassive object in their nuclei, and that their masses are nearly comparable, a few 107 solar masses. Nevertheless, the activity of the central engine of NGC 1068 is more powerful by two orders of magnitude than that of NGC 4258. Since it is generally considered that the huge luminosities of active galactic nuclei can be attributed to mass accretion onto a supermassive black hole, the above observational results suggest that accretion rate in NGC 1068 is much higher than that in NGC 4258. Comparing the kinematical properties of the accreting molecular clouds between NGC 1068 and NGC 4258, we find possible evidence for dynamical gas accretion in NGC 1068, which may be responsible for the more powerful central engine in this galaxy.