Double-peaked [O III]5007, profiles in active galactic nuclei (AGNs) may provide evidence for the existence of dual AGNs, but a good diagnostic for selecting them is currently lacking. Starting from $sim$ 7000 active galaxies in SDSS DR7, we assemble
a sample of 87 type 2 AGNs with double-peaked [O III]5007, profiles. The nuclear obscuration in the type 2 AGNs allows us to determine redshifts of host galaxies through stellar absorption lines. We typically find that one peak is redshifted and another is blueshifted relative to the host galaxy. We find a strong correlation between the ratios of the shifts and the double peak fluxes. The correlation can be naturally explained by the Keplerian relation predicted by models of co-rotating dual AGNs. The current sample statistically favors that most of the [O III] double-peaked sources are dual AGNs and disfavors other explanations, such as rotating disk and outflows. These dual AGNs have a separation distance at $sim 1$ kpc scale, showing an intermediate phase of merging systems. The appearance of dual AGNs is about $sim 10^{-2}$, impacting on the current observational deficit of binary supermassive black holes with a probability of $sim 10^{-4}$ (Boroson & Lauer).
Tracing the star formation history in circumnuclear regions (CNRs) is a key step towards understanding the starburst-AGN connection. However, bright nuclei outshining the entire host galaxy prevent the analysis of the stellar populations of CNRs arou
nd type-I AGNs. Obscuration of the nuclei by the central torus provides an unique opportunity to study the stellar populations of AGN host galaxies. We assemble a sample of 10, 848 type-II AGNs with a redshift range of $0.03le zle 0.08$ from the Sloan Digital Sky Surveys Data Release 4, and measure the mean specific star formation rates (SSFRs) over the past 100Myr in the central $sim1-2$ kpc . We find a tight correlation between the Eddington ratio ($lambda$) of the central black hole (BH) and the mean SSFR, strongly implying that supernova explosions (SNexp) play a role in the transportation of gas to galactic centers. We outline a model for this connection by accounting for the role of SNexp in the dynamics of CNRs. In our model, the viscosity of turbulence excited by SNexp is enhanced, and thus angular momentum can be efficiently transported, driving inflows towards galactic centers. Our model explains the observed relation $lambda propto rm SSFR^{1.5-2.0}$, suggesting that AGN are triggered by SNexp in CNRs.
Episodic activity of quasars is driving growth of supermassive black holes (SMBHs) via accretion of baryon gas. In this Letter, we develop a simple method to analyse the duty cycle of quasars up to redshift $zsim 6$ universe from luminosity functions
(LFs). We find that the duty cycle below redshift $zsim 2$ follows the cosmic history of star formation rate (SFR) density. Beyond $zsim 2$, the evolutionary trends of the duty cycle are just opposite to that of the cosmic SFR density history, implying the role of feedback from black hole activity. With the duty cycle, we get the net lifetime of quasars ($zle 5$) about $sim 10^9$yrs. Based on the local SMBHs, the mean mass of SMBHs is obtained at any redshifts and their seeds are of $10^5sunm$ at the reionization epoch ($z_{rm re}$) of the universe through the conservation of the black hole number density in comoving frame. We find that primordial black holes ($sim 10^3sunm$) are able to grow up to the seeds via a moderate super-Eddington accretion of $sim 30$ times of the critical rate from $z=24$ to $z_{rm re}$. Highly super-Eddington accretion onto the primordials is not necessary.
