arXiv subjects

Ting-Gui Wang

Publications and source records attributed to Ting-Gui Wang.

At least 37 records · Page 2Linked to original sources

arXiv2012

A Uniformly Selected Sample of Low-mass Black Holes in Seyfert 1 Galaxies

We have conducted a systematic search of low-mass black holes (BHs) in active galactic nuclei (AGNs) with broad Halpha emission lines, aiming at building a homogeneous sample that is more complete than previous ones for fainter, less highly accreting sources. For this purpose, we developed a set of elaborate, automated selection procedures and applied it uniformly to the Fourth Data Release of the Sloan Digital Sky Survey. Special attention is given to AGN--galaxy spectral decomposition and emission-line deblending. We define a sample of 309 type 1 AGNs with BH masses in the range $8 \times 10^4$--$2 \times 10^6$ \msun (with a median of $1.2 \times 10^6$ solar mass), using the virial mass estimator based on the broad Halpha line. About half of our sample of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered here for the first time. Our new sample picks up more AGNs with low accretion rates: the Eddington ratios of the present sample range from $<~0.01$ to ~1, with 30% below 0.1. This suggests that a significant fraction of low-mass BHs in the local Universe are accreting at low rates. The host galaxies of the low-mass BHs have luminosities similar to those of $L^*$ field galaxies, optical colors of Sbc spirals, and stellar spectral features consistent with a continuous star formation history with a mean stellar age of less than 1 Gyr.

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arXiv2011

Transient Super-strong Coronal Lines and Broad Bumps in the Galaxy SDSS J074820.67+471214.3

Variable super-strong coronal emission lines were observed in one galaxy, SDSS J095209.56+214313.3, and their origin remains controversy. In this paper, we report the detection of variable broad spectral bumps, reminiscent of supernova (SN) II-Plateau (II-P) spectra taken a few days after the shock breakout, in the second galaxy with variable super-strong coronal lines, SDSS J074820.67+471214.3. The coronal line spectrum shows unprecedented high ionization with strong [Fe X], [Fe XI], [Fe XIV], [S XII] and [Ar XIV], but without detectable optical [Fe VII] lines. The coronal line luminosities are similar to that observed in bright Seyfert galaxies, and 20 times more luminous than that reported in the hottest Type IIn SN 2005ip. The coronal lines ($σ~120-240$ km s-1) are much broader than the narrow lines ($σ\sim 40$ km/s) from the star forming regions in the galaxy, but at nearly the same systematic redshift. We also detected a variable non-stellar continuum in optical and UV. In the follow-up spectra taken 4-5 years later, the coronal lines, SN-like feature, and non-stellar continuum disappeared, while the [O III] intensity increased by about a factor of ten. Our analysis suggests that the coronal line region should be at least ten light days in size, and be powered either by a quasi-steady ionizing source with a soft X-ray luminosity at least a few 10^{42} erg s-1 or by a very luminous soft X-ray outburst. These findings can be more naturally explained by a star tidally disrupted by the central black hole than by an SN explosion.

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arXiv2011

Interplay between Stellar Spirals and the ISM in Galactic Disks

We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a `live' stellar disk. We focus especially on spiral structures excited in a isolated galactic disk without a stellar bar. Using high-resolution, 3-dimensional N-body/SPH simulations, we found that the spiral features of the gas in galactic disks are formed by essentially different mechanisms from the galactic shock in stellar density waves. The stellar spiral arms and the interstellar matter on average corotate in a galactic potential at any radii. Unlike the stream motions in the galactic shock, the interstellar matter flows into the local potential minima with irregular motions. The flows converge to form dense gas clouds/filaments near the bottom of the stellar spirals, whose global structures resemble dust-lanes seen in late-type spiral galaxies. The stellar arms are non-steady; they are wound and stretched by the galactic shear, and thus local densities of the arm change on a time scale of ~ 100 Myrs, due to bifurcating or merging with other arms. This makes the gas spirals associated with the stellar arms non-steady. The association of dense gas clouds are eventually dissolved into inter-arm regions with non-cirucular motions. Star clusters are formed from the cold, dense gases, whose ages are less than ~30 Myrs, and they are roughly associated with the background stellar arms without a clear spatial offset between gas spiral arms and distribution of young stars.

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arXiv2010

The Prevalence of Narrow Optical Fe II Emission Lines in Type 1 Active Galactic Nuclei

From detailed spectral analysis of a large sample of low-redshift active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey, we demonstrate---statistically for the first time---that narrow optical Fe II emission lines, both permitted and forbidden, are prevalent in type 1 AGNs. Remarkably, these optical lines are completely absent in type 2 AGNs, across a wide luminosity range, from Seyfert 2 galaxies to type 2 quasars. We suggest that the narrow FeII-emitting gas is confined to a disk-like geometry in the innermost regions of the narrow-line region on physical scales smaller than the obscuring torus.

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arXiv2010

Low-z MgII Broad Absorption-Line Quasars from the Sloan Digital Sky Survey

We present a sample of 68 low-z MgII low-ionization broad absorption-line (loBAL) quasars. The sample is uniformly selected from the SDSS5 according to the following criteria: (1) 0.4 7, and (3) MgII absorption-line width > 1600 \kms. The last criterion is a trade-off between the completeness and consistency with respect to the canonical definition of BAL quasars that have the `balnicity index' BI>0 in CIV BAL. We adopted such a criterion to ensure that ~90% of our sample are classical BAL quasars and the completeness is ~80%, based on extensive tests using high-z quasar samples with measurements of both CIV and MgII BALs. We found (1) MgII BAL is more frequently detected in quasars with narrower Hbeta emission-line, weaker [OIII] emission-line, stronger optical FeII multiplets and higher luminosity. In term of fundamental physical parameters of a black hole accretion system, loBAL fraction is significantly higher in quasars with a higher Eddington ratio than those with a lower Eddington ratio. The fraction is not dependent on the black hole mass in the range concerned. The overall fraction distribution is broad, suggesting a large range of covering factor of the absorption material. (2) [OIII]-weak loBAL quasars averagely show undetected [NeV] emission line and a very small line ratio of [NeV] to [OIII]. However, the line ratio in non-BAL quasars, which is much larger than that in [OIII]-weak loBAL quasars, is independent of the strength of the [OIII] line. (3) loBAL and non-loBAL quasars have similar colors in near-infrared to optical band but different colors in ultraviolet. (4) Quasars with MgII absorption lines of intermediate width are indistinguishable from the non-loBAL quasars in optical emission line properties but their colors are similar to loBAL quasars, redder than non-BAL quasars. We also discuss the implication of these results.

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arXiv2010

Type 1 Active Galactic Nucleus Fraction in SDSS/FIRST Survey

In the unification scheme, narrow-lined (type 2) active galactic nuclei (AGN) are intrinsically similar to broad-lined (type 1) AGN with the exception that the line of sight to the broad emission line region and accretion disk is blocked by a dusty torus. The fraction of type 1 AGN measures the average covering factor of the torus. In this paper, we explore the dependence of this fraction on nuclear properties for a sample of low redshift (z <0.35) radio strong (P_{1.4GHz} >10^{23}W/Hz) AGN selected by matching the spectroscopic catalog of Sloan Digital Sky Survey and the radio source catalog of Faint Image of Radio Sky at Twenty cm. After correcting for several selection effects, we find that : (1) type 1 fraction $f_1$ keeps at a constant of ~20 per cent in the [O III] 5007 luminosity range of 40.7< log(L_{[O III]}/ erg/s) <43.5 . This result is significantly different from previous studies, and the difference can be explained by extinction correction and different treatment of selection effects. (2) $f_1$ rises with black hole mass from ~20 per cent (M_bh below 10^8 Msun) to ~30 per cent (M_bh above that). This coincides with the decrease of the fraction of highly-inclined disk galaxies with black hole mass, implying a population of Seyfert galaxies seen as type 2 due to galaxy-scale obscuration in disk when the host galaxy type transfer from bulge-dominant to disk-dominant. (3) $f_1$ is independent of the Eddington ratio for its value between 0.01 and 1; (4) $f_1$ ascends from 15 per cent to 30 per cent in the radio power range of 23< log(P_{1.4GHz}/ W/Hz) <24, then remain a constant at ~30 per cent up to 10^{26} W/Hz.

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arXiv2009

Estimating Black Hole Masses in Active Galactic Nuclei Using the MgII 2800 Emission Line

We investigate the relationship between the linewidths of broad Mg II \lambda2800 and Hbeta in active galactic nuclei (AGNs) to refine them as tools to estimate black hole (BH) masses. We perform a detailed spectral analysis of a large sample of AGNs at intermediate redshifts selected from the Sloan Digital Sky Survey, along with a smaller sample of archival ultraviolet spectra for nearby sources monitored with reverberation mapping. Careful attention is devoted to accurate spectral decomposition, especially in the treatment of narrow-line blending and Fe II contamination. We show that, contrary to popular belief, the velocity width of Mg II tends to be smaller than that of Hbeta, suggesting that the two species are not cospatial in the broad-line region. Using these findings and recently updated BH mass measurements from reverberation mapping, we present a new calibration of the empirical prescriptions for estimating virial BH masses for AGNs using the broad Mg II and Hbeta lines. We show that the BH masses derived from our new formalisms show subtle but important differences compared to some of the mass estimators currently used in the literature.

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arXiv2009

Eddington ratio governs the equivalent width of MgII emission line in active galactic nuclei

We have investigated the ensemble regularities of the equivalent widths (EWs) of MgII 2800 emission line of active galactic nuclei (AGNs), using a uniformly selected sample of 2092 Seyfert 1 galaxies and quasars at 0.45 <= z <= 0.8 in the spectroscopic data set of Sloan Digital Sky Survey Fourth Data Release. We find a strong correlation between the EW of MgII and the AGN Eddington ratio (L/L_Edd): EW(MgII) \propto (L/L_Edd)^{-0.4}. Furthermore, for AGNs with the same L/L_Edd, their EWs of MgII show no correlation with luminosity, black hole mass or line width, and the MgII line luminosity is proportional to continuum luminosity, as expected by photoionization theory. Our result shows that MgII EW is not dependent on luminosity, but is solely governed by L/L_Edd.

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arXiv2009

partly obscured accretion disk model to explain Shifted broad Balmer emission lines of Active Galactic Nuclei

We present a new model to explain the appearance of red/blue-shifted broad low-ionization emission lines, especially emission lines in optical band, which is commonly considered as an indicator of radial motion of the line emitting gas in broad emission line regions (BLRs) of Active Galactic Nuclei (AGN). We show that partly obscured disk-like BLRs of dbp emitters (AGN with double-peak broad low-ionization emission lines) can also successfully produce shifted standard Gaussian broad balmer emission lines. Then we calculate two kinds of BH masses for AGN with shifted broad balmer emission lines selected from SDSS. We find that the BH masses calculated from M-sigma relation are systematically larger than virial BH masses for the selected objects, even after the correction of internal reddening effects in BLRs. The smaller virial BH masses than BH masses from M-sigma relation for objects with shifted broad emission lines are coincident with what we expect from the partly obscured accretion disk model. Thus, we provide an optional better model to explain the appearance of shifted broad emission lines, especially for those objects with underestimated virial BH masses.

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arXiv2008

Iron line profiles and self-shadowing from relativistic thick accretion discs

We present Fe Kalpha line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and light bending. We find that the separation and the relative height between the blue and red peaks of the line profile diminish as the thickness of the disc increases. This code is also well-suited to produce accretion disc images. We calculate the redshift and flux images of the accretion disc and find that the observed image of the disc strongly depends on the inclination angle. The self-shadowing effect appears remarkable for a high inclination angle, and leads to the black hole shadow being completely hidden by the disc itself.

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arXiv2008

Broad reprocessed Balmer emission from warped accretion discs

Double peaked broad emission lines in active galactic nuclei are generally considered to be formed in an accretion disc. In this paper, we compute the profiles of reprocessing emission lines from a relativistic, warped accretion disc around a black hole in order to explore the possibility that certain asymmetries in the double-peaked emission line profile which can not be explained by a circular Keplerian disc may be induced by disc warping. The disc warping also provides a solution for the energy budget in the emission line region because it increases the solid angle of the outer disc portion subtended to the inner portion of the disc. We adopted a parametrized disc geometry and a central point-like source of ionizing radiation to capture the main characteristics of the emission line profile from such discs. We find that the ratio between the blue and red peaks of the line profiles becoming less than unity can be naturally predicted by a twisted warped disc, and a third peak can be produced in some cases. We show that disc warping can reproduce the main features of multi-peaked line profiles of four active galactic nuclei from the Sloan Digital Sky Survey.

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arXiv2008

The Correlation Between Spectral Index And Accretion Rate For AGN

In this paper, we present a correlation between the spectral index distribution (SED) and the dimensionless accretion rate defined as $\dot{m}={L_{bol}/L_{Edd}}$ for AGN. This quantity is used as a substitute of the physical accretion rate. We select 193 AGN with both broad H$α$ and broad H$β$, and with absorption lines near MgI$\lambda5175Å$ from SDSS DR4. We determine the spectral index and dimensionless accretion rate after correcting for both host galaxy contribution and internal reddening effects. A correlation is found between the optical spectral index and the dimensionless accretion rate for AGN, including low luminosity AGN ($L_{Hα}<10^{41}{\rm erg\cdot s^{-1}}$ sometimes called "dwarf AGN" (Ho et al. 1997)). The existence of this correlation provides an independent method to estimate the central BH masses for all types of AGN. We also find that there is a different correlation between the spectral index and the BH masses for normal AGN and low luminosity AGN, which is perhaps due to the different accretion modes in these two types of nuclei. This in turn may lead to the different correlations between BH masses and optical continuum luminosity reported previously (Zhang et al. 2007a), which invalidates the application of the empirical relationship found by Kaspi et al. (2000, 2005) to low luminosity AGN in order to determine their BLR sizes.

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arXiv2007

Iron Line Profiles from Relativistic Thick Accretion Disk

We present a new code for calculating the Fe Kalpha line profiles from relativistic accretion disks with finite thickness around a rotating black hole. The thin Keplerian accretion disk must become thicker and sub-Keplerian with increasing accretion rates. We here embark on, for the first time, a fully relativistic computation which is aimed at gaining an insight into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. This code is also well-suited to produce accretion disk images.

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arXiv2007

SDSS J1130+0058 an X-shaped Radio Source With Double-Peaked Low-Ionization Emission Lines: A binary Black Hole System?

In this paper we study the object SDSS J1130+0058 which is the only AGN known to have both double-peaked low-ionization broad emission lines, and also X-shaped radio structures. Emission from an accretion disk can reproduce the double-peaked line profile of broad H$α$, but not the radio structure. Under the accretion disk model, the period of the inner emission line region is about 230 years. Using a new method to subtract the stellar component from the data of the SDSS DR4, we obtain an internal reddening factor which is less than previously found. The implied smaller amount of dust disfavors the backflow model for the X-shaped radio structure. The presence of a Binary Black Hole (BBH) system is the most natural way to explain ${\it both}$ the optical and radio properties of this AGN. Under the assumption of the BBH model, we can estimate the BBH system has a separation of less than 0.04 pc with a period less than 59 years, this may pose some problem to the BLRs sizes, still we conclude that the BBH model is favored on the basis of the present limited information.

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arXiv2007

The sizes of BLRs and BH masses of double-peaked broad low-ionization emission line objects

In this paper, the sizes of the BLRs and BH masses of DouBle-Peaked broad low-ionization emission line emitters (dbp emitters) are compared using different methods: virial BH masses vs BH masses from stellar velocity dispersions, the size of BLRs from the continuum luminosity vs the size of BLRs from the accretion disk model. First, the virial BH masses of dbp emitters estimated by the continumm luminosity and line width of broad H$β$ are about six times (a much larger value, if including another dbp emitters, of which the stellar velocity dispersions are traced by the line widths of narrow emission lines) larger than the BH masses estimated from the relation $M_{BH} - σ$ which is a more accurate relation to estimate BH masses. Second, the sizes of the BLRs of dbp emitters estimated by the empirical relation of $R_{BLR} - L_{5100Å}$ are about three times (a much larger value, if including another dbp emitters, of which the stellar velocity dispersions are traced by the line widths of narrow emission lines) larger than the mean flux-weighted sizes of BLRs of dbp emitters estimated by the accretion disk model. The higher electron density of BLRs of dbp emitters would be the main reason which leads to smaller size of BLRs than the predicted value from the continuum luminosity.

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arXiv2007

Metal Enriched Outflows and Dusty starburst in the type II quasar Q 1321+058

We present an analysis of the emission line property and the broad band spectral energy distribution of the ultra-luminous infrared Type II quasar Q132+058. The optical and ultraviolet emission lines show four distinct components: a LINER-like component at the systematic velocity, a heavily reddened HII-like component blueshifted by about 400 km/s, and two broad components blueshifted by about 400 km/s and 1900 km/s, respectively. The emission line ratios suggest that broad components are produced in dense and alpha-enriched outflows with a metalicity of 3-5 Zsun and a density of n_H ~ 10^7 cm^-3. The optical--UV continuum is dominated by stellar light and can be modeled with a young (<1 Myr) plus an intermediate age (0.5-0.8Gyr) stellar populations. The near to mid-infrared light is dominated by hot and warm dust heated by the hidden quasar. We derive a star formation rate (SFR) of 300-500 M\sun yr$^{-1}$ from the UV spectrum and far-infrared luminosity, which is two orders of magnitude larger than that indicated by reddening uncorrected [OII] luminosity.

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arXiv2006

Polarization of Quasars: Rotated and Funnel-shaped Outflow

Polarization is a useful probe to investigate the geometries and dynamics of outflow in BAL QSOs. We perform a Monte-Carlo simulation to calculate the polarization produced by resonant and electron scattering in BALR. We find: 1)A rotated and funnel-shaped outflow is preferred to explain many observed polarization features. 2)The resonant scattering can contribute a significant part of NV emission line in some QSOs.

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arXiv2006

The properties of optical FeII emission lines of AGN with double-peaked broad emission lines

We study the FeII properties of double-peaked broad low-ionization emission line AGN (dbp emitters) using a sample of 27 dbp emitters from SDSS (DR4), with mean value $σ_{Hα_{B}}\sim3002\pm139{\rm km\cdot s^{-1}}$. Our first result is that the line spectra in the wavelength range from 4100$Å$ to 5800$Å$ can be best fitted by an elliptical accretion disk model, assuming the same double-peaked line profiles for H$β$, FeII, H$γ$ and HeII$\lambda4686Å$ as that of double-peaked broad H$α$ for all the 27 dbp emitters, except the object SDSS J2125-0813 which we have discussed in a previous paper. The best fitted results indicate that the optical FeII emission lines of dbp emitters originate from the same region in the accretion disk where the double-peaked Balmer emission lines originate. Some correlations between FeII emission lines and the other broad emission lines for normal AGN can be confirmed for dbp emitters. However, these results should be taken with caution due to the small number of objects and the bias in selecting strong FeII emitters. We show that for dbp emitters, BH masses seems to have more influence on FeII properties than dimensionless accretion rate. We also find that the dbp emitters in the sample are all radio quiet quasars except one dbp emitter with $R_r> 1$ according to the definition by Ivezi$\rm{\acute{c}}$ et al. (2002) and 6 objects undiscovered by FIRST.

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