arXiv subjects

J. -M. Wang

Publications and source records attributed to J. -M. Wang.

25 records · Page 2Linked to original sources

arXiv2009

Episodic Random Accretion and the Cosmological Evolution of Supermassive Black Hole Spins

The growth of supermassive black holes (BHs) located at the centers of their host galaxies comes mainly from accretion of gas, but how to fuel them remains an outstanding unsolved problem in quasar evolution. This issue can be elucidated by quantifying the radiative efficiency parameter ($η$) as a function of redshift, which also provides constraints on the average spin of the BHs and its possible evolution with time. We derive a formalism to link $η$ with the luminosity density, BH mass density, and duty cycle of quasars, quantities we can estimate from existing quasar and galaxy survey data. We find that $η$ has a strong cosmological evolution: at z~2, $η\approx 0.3$, and by $z\approx 0$ it has decreased by an order of magnitude, to $η\approx 0.03$. We interpret this trend as evolution in BH spin, and we appeal to episodic, random accretion as the mechanism for reducing the spin. The observation that the fraction of radio-loud quasars decreases with increasing redshift is inconsistent with the popular notion that BH spin is a critical factor for generating strong radio jets. In agreement with previous studies, we show that the derived history of BH accretion closely follows the cosmic history of star formation, consistent with other evidence that BHs and their host galaxies coevolve.

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arXiv2009

The starburst-AGN connection: the role of young stellar populations in fueling supermassive black holes

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 around 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 $0.03\le z\le 0.08$ from the Sloan Digital Sky Survey's 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 ($λ$) 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 $λ\propto \rm SSFR^{1.5-2.0}$, suggesting that AGN are triggered by SNexp in CNRs.

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arXiv2007

Suppressed star formation in circumnuclear regions in Seyfert galaxies

Feedback from black hole activity is widely believed to play a key role in regulating star formation and black hole growth. A long-standing issue is the relation between the star formation and fueling the supermassive black holes in active galactic nuclei (AGNs). We compile a sample of 57 Seyfert galaxies to tackle this issue. We estimate the surface densities of gas and star formation rates in circumnuclear regions (CNRs). Comparing with the well-known Kennicutt-Schmidt (K-S) law, we find that the star formation rates in CNRs of most Seyfert galaxies are suppressed in this sample. Feedback is suggested to explain the suppressed star formation rates.

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arXiv2006

The Unified Model of Active Galactic Nuclei: I. Non-hidden Broad Line Region Seyfert 2 and Narrow Line Seyfert 1 Galaxies

The unified model of Seyfert galaxies suggests that there are hidden broad-line regions (HBLRs) in Seyfert 2 galaxies (S2s). However, there is increasing evidence for the appearance of a subclass of S2s lacking of HBLR (non-HBLR S2s). An interesting issue arises as to relations of non-HBLR S2s with other types of Seyfert galaxies and whether or not they can be included in the unified model. We assemble two sub-samples consisting of 42 non-HBLR S2s and 44 narrow-line Seyfert 1s (NLS1s) with redshift $z\le 0.05$ from published literatures to explore this issue. We compare black hole masses in the galactic centers, accretion rates, infrared color ratio ($f_{60 μ\rm m}/f_{25 μ\rm m}$) as a potential indicator of the dusty torus orientation, \oiii $λ5007$, radio and far infrared luminosities. We find that non-HBLR S2s and NLS1s have: 1) similar distributions of the black hole masses ($10^6-3\times 10^7\sunm$) and the Eddington ratios ($L_{\rm Bol}/L_{\rm Edd}\sim 1$); 2) significantly different distributions of $f_{60 μ\rm m}/f_{25 μ\rm m}$ ratios; 3) similar distributions of bulge magnitudes and luminosities of [O {\sc iii}], radio, far infrared emission. The similarities and differences can be understood naturally if they are intrinsically same but non-HBLR S2s are viewed at larger angles of observer's sight than NLS1s. We thus suggest that non-HBLR S2s only have "narrower" broad line regions and they are the counterparts of NLS1s viewed at high inclination angles. The absence of the polarized emission line in non-HBLR S2s is caused by the less massive black holes and high accretion rate similar to NLS1s. The implications of the unification scheme of non-HBLR S2s and NLS1s are discussed.

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arXiv2005

Feedback Limits Rapid Growth of Seed Black Holes at High Redshift

Seed black holes formed in the collapse of population III stars have been invoked to explain the presence of supermassive black holes at high redshift. It has been suggested that a seed black hole can grow up to $10^{5\sim 6}\sunm$ through highly super-Eddington accretion for a period of $\sim 10^{6\sim 7}$ yr between redshift $z=20\sim 24$. We studied the feedback of radiation pressure, Compton heating and outflow during the seed black hole growth. It is found that its surrounding medium fueled to the seed hole is greatly heated by Compton heating. For a super-critical accretion onto a $10^3\sunm$ seed hole, a Compton sphere (with a temperature $\sim 10^6$K) forms in a timescale of $1.6\times 10^3$yr so that the hole is only supplied by a rate of $10^{-3}$ Eddington limit from the Compton sphere. Beyond the Compton sphere, the kinetic feedback of the strong outflow heats the medium at large distance, this leads to a dramatical decrease of the outer Bondi accretion onto the black hole and avoid the accumulation of the matter. The highly super-critical accretion will be rapidly halted by the strong feedback. The seed black holes hardly grow up at the very early universe unless the strong feedback can be avoided.

astro-ph↗

arXiv2005

Captures of Red Giant Stars by Black Holes in Elliptical Galaxies: Feedback to the Hot Gas

The highly disturbed hot gas in elliptical galaxies, as revealed in many {\em Chandra} X-ray images, implies a source of energy in the galactic nucleus. In some elliptical galaxies faint X-ray ``ghost'' cavities appear without corresponding radio lobes. It has been suggested that ghost cavities are caused by short-lived activity with a timescale of $\sim 10^3-10^4$ years, but this is difficult to understand within the popular paradigm of active galactic nuclei. We suggest an episode model for ghost cavities, invoking captures of red giant stars by the black hole located at the center of the elliptical galaxies at a typical rate of $10^{-5}$yr$^{-1}$ per galaxy. The accretion of tidally disrupted red giant stars onto the black hole powers activity in a timescale of a few years. The total energy channeled into the jet/outflow during the cooling time of the hot gas is $\sim 10^{56}$ erg, which is the typical work required to form the observed cavities. In this scenario, the faint cavities are produced by the feedback following accretion of the debris of the captured red giant stars onto the black holes. We apply the present model to several elliptical galaxies and find that it can explain the formation of the ghost cavities. This model can be tested in the future by comparisons between radio and X-ray observations.

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arXiv2005

Evolutionary Consequences of Dusty Tori in Active Galactic Nuclei

Deep surveys of {\em Chandra} and {\em HST} (Hubble Space Telescope) show that active galactic nucleus (AGN) populations are changing with hard X-ray luminosities. This arises an interesting question whether the dusty torus is evolving with the central engines. We assemble a sample of 50 radio-quiet PG quasars to tackle this problem. The covering factors of the dusty tori can be estimated from the multiwavelength continuum. We find they are strongly correlated with the hard X-ray luminosity. Interestingly this correlation agrees with the fraction of type II AGNs discovered by {\em Chandra} and {\em HST}, implying strong evidence for that the AGN population changing results from the evolution of the tori. We also find that the frequencies of the dips around 1$μ$m in the continuum correlate with the covering factors in the present sample, indicating the dip frequencies are adjusted by the covering factors. In the scenario of fueling black hole from the torus, the covering factor is a good and the dip frequency is a potential indicator of the torus evolution.

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