Universal transition diagram from dormant to actively accreting supermassive black holes

The vast majority of supermassive black holes (SMBHs) in the local universe exhibit levels of activity much lower than those expected from gas supplying rates onto the galactic nuclei, and only a small fraction of silent SMBHs can turn into active galactic nuclei. Revisiting observational data of very nearby SMBHs whose gravitational spheres of influence are spatially reached by the Chandra X-ray satellite, we find that the level of BH activity drastically increases from the quiescent phase when the inflow rate outside of the BH influence radius is higher than 0.1% of the Eddington accretion rate. We also show that the relation between the nuclear luminosity and gas accretion rate from the BH influence radius measured from X-ray observations is well described by the universal state transition of accreting SMBHs, as predicted by recent hydrodynamical simulations with radiative cooling and BH feedback. After the state transition, young massive stars should form naturally in the nucleus, as observed in the case of the nearest SMBH, Sagittarius A$^\ast$, which is currently quiescent but was recently active.Comment: 9 pages (main text), 2 figures, 1 table, accepted for publication in Ap

Wino Dark Matter and Future dSph Observations

We discuss the indirect detection of the wino dark matter utilizing gamma-ray observations of dwarf spheroidal galaxies (dSphs). After carefully reviewing current limits with particular attention to astrophysical uncertainties, we show prospects of the wino mass limit in future gamma-ray observation by the Fermi-LAT and the GAMMA-400 telescopes. We find that the improvement of the so-called $J$-factor of both the classical and the ultra-faint dSphs will play a crucial role to cover whole mass range of the wino dark matter. For example, with $\delta (\log_{10}J) = 0.1$ for both the classical and the ultra-faint dSphs, whole wino dark matter mass range can be covered by 15 years and 10 years data at the Fermi-LAT and GAMMA-400 telescopes, respectively.Comment: 32 pages, 13 figure

Infrared 3-4 Micron Spectroscopy of Nearby PG QSOs and AGN-Nuclear Starburst Connections in High-luminosity AGN Populations

We present the results of infrared L-band (3-4 micron) slit spectroscopy of 30 PG QSOs at z < 0.17, the representative sample of local high-luminosity, optically selected AGNs. The 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission feature is used to probe nuclear (< a few kpc) starburst activity and to investigate the connections between AGNs and nuclear starbursts in PG QSOs. The 3.3 micron PAH emission is detected in the individual spectra of 5/30 of the observed PG QSOs. We construct a composite spectrum of PAH-undetected PG QSOs and discern the presence of the 3.3 micron PAH emission therein. We estimate the nuclear-starburst and AGN luminosities from the observed 3.3 micron PAH emission and 3.35 micron continuum luminosities, respectively, and find that the nuclear-starburst-to-AGN luminosity ratios in PG QSOs are similar to those of previously studied AGN populations with lower luminosities, suggesting that AGN-nuclear starburst connections are valid over the wide luminosity range of AGNs in the local universe. The observed nuclear-starburst-to-AGN luminosity ratios in PG QSOs with available supermassive black hole masses are comparable to a theoretical prediction based on the assumption that the growth of a supermassive black hole is controlled by starburst-induced turbulence.Comment: 10 pages, 5 figures, accepted for publication in PASJ (No. 63, 2011 March, Subaru special issue

Obscuring fraction of active galactic nuclei implied by supernova and radiative feedbacks

We study the obscuring structure of circumnuclear disks (CNDs) by considering supernova (SN) feedbacks from nuclear starburst and the effect of anisotropic radiative pressure from AGNs. We suppose that the mass accretion onto a central supermassive black hole (SMBH) is triggered by SN-driven turbulence within CNDs, and we explore how the structures of CNDs depend on the BH mass ($M_{\rm BH}$) and AGN luminosity ($L_{\rm AGN}$). We find that the obscuring fraction ($f_{\rm obs}$) peaks at $\sim10\%$ of the Eddington luminosity ($L_{\rm Edd}$), and its maximal value is $f_\mathrm{obs} \sim 0.6$ for less massive SMBHs (e.g., $M_{\rm BH} < 10^{8}M_{\odot}$). This is because the scale height of CNDs is determined by the SN-driven accretion for a smaller $L_{\rm AGN}$, while the dusty molecular gas in CNDs is blown away by the radiation pressure from AGNs beyond the critical luminosity. On the other hand, for massive SMBHs (e.g., $M_{\rm BH} > 10^{8}M_{\odot}$), $f_{\rm obs}$ is always smaller than $0.2$, and it is almost independent of $L_{\rm AGN}$ because the scale height of CNDs is mainly controlled by the maximal star-formation efficiency ($C_{\rm *, max}$) in CNDs. By comparison with the obscuring fractions suggested from the mid-infrared observations of nearby AGNs, the SN plus radiative feedback model with $C_{\rm *, max} = 10^{-7}\, {\rm yr}^{-1}$ well reproduces the observations for $M_\mathrm{BH} = 10^8 M_\odot$. We also find that the intense starburst or the existence of dust-free absorbers inside CNDs are necessary, to explain X-ray observations.Comment: 17 pages, 11 figures, Accepted for publication in Ap

Dust Destruction by Charging: A Possible Origin of Grey Extinction Curves of Active Galactic Nuclei

Observed extinction curves of active galactic nuclei (AGNs) are significantly different from those observed in the Milky Way. The observations require preferential removal of small grains at the AGN environment; however, the physics for this remains unclear. In this paper, we propose that dust destruction by charging, or Coulomb explosion, may be responsible for AGN extinction curves. Harsh AGN radiation makes a dust grain highly charged through photoelectric emission, and grain fission via the Coulomb explosion occurs when the electrostatic tensile stress of a charge grain exceeds its tensile strength. We show that the Coulomb explosion can preferentially remove both small silicate and graphite grains and successfully reproduce both flat extinction curves and the absence of 2175\AA~bump.Comment: 8 pages, 6 Figures; Accepted for publication in Ap