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The nuclear gas kinematics probed by water vapor maser emission has shownthat two nearby active galaxies, NGC 1068 and NGC 4258, have a supermassiveobject in their nuclei and their masses are nearly comparable; a few 10^7 solarmasses. Nevertheless, the activity of the central engine of NGC 1068 is morepowerful by two orders of magnitude than that of NGC 4258. Since it isgenerally considered that the huge luminosities of active galactic nuclei areattributed to the mass accretion onto a supermassive black hole, the aboveobservational results suggest that the accretion rate in NGC 1068 is muchhigher than that in NGC 4258. Comparing the kinematical properties of theaccreting molecular clouds between NGC 1068 and NGC 4258, we find possibleevidence for dynamical gas accretion in NGC 1068, which may be responsible forthe more powerful central engine in this galaxy.Comment: 9 Latex (AASTeX) pages, 2 PostScript figures. To appear in  Publications of the Astronomical Society of Japa

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