Estimation of central black hole masses in low-luminosity active galactic nuclei

We analyzed six low-luminosity active galactic nuclei (hereafter LLAGNs), w hich consist of two bright LINERS (low-ionization nuclear emission line reg ion) with broad Hoe, and of four bright low-luminosity Seyferts observed by the X-ray satellite ASCA. We examined the time variabilities of these targ ets, and found that most targets show no significant time variabilities, al though these galaxies belong to LLAGNs. By applying Fourier analysis to the ir light curves in the 2-10 keV band, we obtained the lower limits of their time variability scales, which correspond to their central black hole mass es greater than several times 10(6) M-circle dot. Our result suggests that these LLAGNs are harbors of super-massive black holes. but the emitting pow ers are only < 1% of the Eddington luminosities of the central engines in t he 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 un der a small accretion rate. Therefore, the mass-accretion rates in the past must have been larger than those at present, and the galaxies have probabl y been as luminous as QSOs. This will be a hint to find out the cause of th e lack of QSOs in the local universe.