Growth of massive black holes at their late stage

We derive the black hole mass density as a function of redshift with the bolometric luminosity function of AGN assuming that massive black holes grew via accreting the circumnuclear gases, in which the derived black hole mass density is required to match the measured local black hole mass density at z=0. ADAFs are supposed to present in low luminosity AGNs/normal galaxies, which are very hot and radiate mostly in the hard X-ray band. Most of the XRB is contributed by bright AGNs, and a variety of AGN population synthesis models were developed to model the observed XRB in the last two decades. Based on our derived black hole mass density, we calculate the contribution to the XRB from the ADAFs in faint AGNs/normal galaxies with a given Eddington ratio distribution, which is mostly in hard X-ray energy band with an energy peak at ~200 keV. The growth of massive black holes during ADAF phase can therefore be constrained with the observed XRB. Combining an AGN population synthesis model with our results, we find that the fitting on the observed XRB, especially at hard X-ray energy band with \ga 100 keV, is improved provided the contribution of the ADAFs in low luminosity AGNs/normal galaxies is properly included. It is found that less than ~15 per cent of local massive black hole mass density was accreted during ADAF phases. We suggest that more accurate measurements of the XRB in the energy band with \ga 100 keV in the future may help constrain the growth of massive black holes at their late stage. We also calculate their contribution to the extragalactic gamma-ray background, and find that less than ~1% of the observed EGRB is contributed by the ADAFs in these faint sources.Comment: 9 pages, 11 figures, accepted for publication in ApJ

The relation of optical/UV and X-ray emission in low-luminosity active galactic nuclei

We study the relation of optical/UV and X-ray emission in the low luminosity active galactic nuclei (LLAGNs), using a sample of 49 sources including 28 local Seyfert galaxies and 21 low-ionization nuclear emission-line regions (LINERs) with the optical/UV spectral luminosity at the wavelength \lambda=2500\AA, 23.0\leq \log L_{\nu(2500\AA)(erg/s/Hz)\leq 27.7, and the X-ray spectral luminosity at 2 keV, 20.5\leq \log L_{\nu(2 keV)}\leq 25.3. The strong correlations are found between the X-ray luminosity and the optical/UV to X-ray index, \alpha_{ox}, with the optical/UV luminosity, with the slopes very similar to the findings for the luminous AGNs in the previous works. The correlation between \alpha_{ox} and L_{\nu(2 keV) is very weak as that found for the luminous AGNs in the majority of previous similar works. We also study the relation between $\alpha_{\rm ox} and the Eddington ratio L_{bol}/L_{Edd} for our sample and find a significant anti-correlation for the sources with L_{bol}/L_{Edd}\lesssim 10^{-3}, which is opposite to the correlation between the two variables for the luminous AGNs. Using the advection dominated accretion flow (ADAF) model, we roughly reproduce this anti-correlationship for the two variables for the LLAGNs. This result strongly supports the ADAF as a candidate accretion mode in LLAGNs.Comment: 21 pages, 1 table, 5 figures. Accepted for publication in Ap

On the BL Lacertae objects/radio quasars and the FRI/II dichotomy

In the frame of unification schemes for radio-loud active galactic nuclei (AGNs), FR I radio galaxies are believed to be BL Lacertae (BL Lac) objects with the relativistic jet misaligned to our line of sight, and FR II radio galaxies correspond to misaligned radio quasars. The Ledlow-Owen dividing line for FR I/FR II dichotomy in the optical absolute magnitude of host galaxy-radio luminosity (M_R-L_Rad) plane can be translated to the line in the black hole mass-jet power (M_bh-Q_jet) plane by using two empirical relations: Q_jet-L_Rad and M_bh}-M_R. We use a sample of radio quasars and BL Lac objects with measured black hole masses to explore the relation of the jet power with black hole mass, in which the jet power is estimated from the extended radio emission. It is found that the BL Lac objects are clearly separated from radio quasars by the Ledlow & Owen FR I/II dividing line in the M_bh-Q_jet plane. This strongly supports the unification schemes for FR I/BL Lac object and FR II/radio quasar. We find that the Eddington ratios L_bol/L_Edd of BL Lac objects are systematically lower than those of radio quasars in the sample with a rough division at L_bol/L_Edd 0.01, and the distribution of Eddington ratios of BL Lac objects/quasars exhibits a bimodal nature, which imply that the accretion mode of BL Lac objects may be different from that of radio quasars.Comment: 4 pages, 2 figures, accepted by ApJ

The advection-dominated accretion flow+thin accretion disk model for two low-luminosity active galactic nuclei: M81 and NGC4579

It was found that the advection-dominated accretion flow (ADAF)+thin disk model calculations can reproduce the observed spectral energy distributions (SED) of the two low luminosity AGN, provided they are accreting at ~0.01-0.03 Eddington rates and the thin disks are truncated to ADAFs at ~100 R_s (Schwarzschild radii) for M81 and NGC4579 (Quataert et al., 1999). However, the black hole masses adopted in their work are about one order of magnitude lower than recent measurements on these two sources. Adopting the well estimated black hole masses, our ADAF+thin disk model calculations can reproduce the observed SEDs of these two LLAGN, if the black hole is accreting at 2.5e-4 Eddington rates with the thin disk truncated at 120 R_s for M81 (3.3e-3 and R_tr = 80R_s are required for NGC4579). The observed widths of the thermal X-ray iron lines at 6.8 keV are consistent with the Doppler broadening by the Keplerian motion of the gases in the transition zones at ~100R_s. The observed thermal X-ray lines provide a useful diagnosis on the physical properties of the transition zones. We calculate the thermal X-ray line emission from the transition zone between the ADAF and the thin disk with standard software package Astrophysical Plasma Emission Code (APEC), and the physical implications on the models of the transition zones are discussed.Comment: 10 pages, 2 figures, accepted by ChJA

Unification of Radio Galaxies and Their Accretion/Jet Properties

We investigate the relation between black hole mass, M_bh, and jet power, Q_jet, for a sample of BL Lacs and radio quasars. We find that BL Lacs are separated from radio quasars by the FR I/II dividing line in M_bh-Q_jet plane, which strongly supports the unification scheme of FR I/BL Lac and FR II/radio quasar. The Eddington ratio distribution of BL Lacs and radio quasars exhibits a bimodal nature with a rough division at L_bol/L_Edd~0.01, which imply that they may have different accretion modes. We calculate the jet power extracted from advection dominated accretion flow (ADAF), and find that it require dimensionless angular momentum of black hole j~0.9-0.99 to reproduce the dividing line between FR I/II or BL Lac/radio quasar if dimensionless accretion rate mdot=0.01 is adopted, which is required by above bimodal distribution of Eddington ratios. Our results suggest that black holes in radio galaxies are rapidly spinning.Comment: To appear JAA in Jun