Estimation of Central Black Hole Masses in Low-Luminosity Active Galactic Nuclei

We analyzed six low-luminosity active galactic nuclei (hereafter LLAGNs), which consist of two bright LINERs (low-ionization nuclear emission line region) with broad H$ \alpha$, and of four bright low-luminosity Seyferts observed by the X-ray satellite ASCA. We examined the time variabilities of these targets, and found that most targets show no significant time variabilities, although these galaxies belong to LLAGNs. By applying Fourier analysis to their light curves in the 2–10keV band, we obtained the lower limits of their time variability scales, which correspond to their central black hole masses greater than several times $10^{6}\,{{{M}_{\odot}}}$. Our result suggests that these LLAGNs are harbors of super-massive black holes, but the emitting powers are only $ < 1 \%$ of the Eddington luminosities of the central engines in the LLAGNs due to their small accretion rates. Since a black hole would grow by mass accretion, it is hard to create such a super-massive black hole under a small accretion rate. Therefore, the mass-accretion rates in the past must have been larger than those at present, and the galaxies have probably been as luminous as QSOs. This will be a hint to find out the cause of the lack of QSOs in the local universe.