A practicable estimation of opening angle of dust torus in Type-1.9 AGN with double-peaked broad H
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MNRAS 519, 4461–4466 (2023) https://doi.org/10.1093/mnras/stad024
Advance Access publication 2023 January 09
A practicable estimation of opening angle of dust torus in Type-1.9 AGN
with double-peaked broad Hα
Xue-Guang Zhang‹
Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, GuangXi University, No. 100, Daxue Road, 530004,
Nanning, China
Accepted 2023 January 5. Received 2023 January 4; in original form 2022 November 1
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ABSTRACT
In this manuscript, an independent method is proposed to estimate opening angle of dust torus in AGN, through unique properties
of Type-1.9 AGN with double-peaked broad Hα (Type-1.9 DPAGN) coming from central accretion disc. Type-1.9 AGN without
broad Hβ can be expected by the commonly accepted unified model of AGN, considering central BLRs seriously obscured
by dust torus with its upper boundary in the line of sight. For the unique Type-1.9 DPAGN, accretion disc originations of
double-peaked broad Hα can be applied to determine the inclination angle of the central accretion disc, which is well accepted
as substitute of the half opening angle of the central dust torus. Then, among low redshift Type-1.9 DPAGN in SDSS, SDSS
J1607+3319 at redshift 0.063 is collected, and the half opening angle of the central dust torus is determined to be around 46 ± 4◦ ,
after considering the disfavoured BBH system to explain the double-peaked broad Hα through long-term none variabilities and
disfavoured local physical conditions to explain the disappearance of broad Hβ through virial BH mass properties. The results
indicate that more detailed studying on dust torus of AGN can be appropriately done through Type-1.9 DPAGN in the near
future.
Key words: galaxies:active – galaxies:nuclei – quasars:emission lines – quasars: individual (SDSS J1607+3319).
kind of AGN, Type-1.9 AGN (first discussed in Osterbrock 1981),
1 I N T RO D U C T I O N
with broad Hα emission lines but no broad Hβ indicating central
An unified model of Active Galactic Nuclei (AGN) is well known BLRs seriously obscured by dust torus with its upper boundary in the
and widely accepted to explain different spectroscopic phenomena line of sight, besides the Type-1 and Type-2 AGN. Commonly, as a
between Type-1 AGN with optical both broad and narrow emission transition type, Type-1.9 AGN are considered as the best candidates
lines and Type-2 AGN with only optical narrow emission lines, after on studying properties, especially properties of spatial structures,
mainly considering obscurations on central Broad Line Regions of the unified model expected central dust torus. Actually, there
(BLRs) by central dust torus. The unified model has been first are some reports on the opening angles (covering factor) of the
discussed in Antonucci (1993), Urry & Padovani (1995), and more central dust torus in the literature. Arshakian (2005) have proposed
recently reviewed and discussed in Netzer (2015), Kuraszkiewicz a receding torus model, based on statistically significant correlation
et al. (2021), Zhang (2022a). The Unified model has been strongly between the half opening angle of the torus and [O III] emission-
supported by clearly detected polarized broad emission lines and/or line luminosity, and then followed and discussed in Simpson (2005),
clearly detected broad infrared emission lines in some Type-2 AGN Alonso-Herrero et al. (2011), Marin, Goosmann & Petrucci (2016),
(Tran 2003; Savic et al. 2018; Moran et al. 2020). Moreover, there is Matt et al. (2019). Zhuang, Ho & Shangguan (2018) have reported
observational/theoretical evidence to the support central dust torus as that the half opening angle of the torus declines with increasing
one fundamental structure in the unified model, such as the results in accretion rate until the Eddington ratio reaches 0.5, above which the
NGC1068 in Rouan et al. (1998), Marco & Alloin (2000), Gratadour trend reverses. Netzer et al. (2016), Stalevski et al. (2016) have found
et al. (2015) through direct Near-IR images and polarimetric images, no evidence for a luminosity dependence of the torus covering factor
the resolved dust torus in the Circinus galaxy in Tristram et al. in AGN not to support the receding torus model, similar conclusions
(2007), the reported diversity of dusty torus in AGN in Burtscher can also be found in Mateos et al. (2017). More recent interesting
(2013), the estimated covering factors of central dust torus in local discussions on central obscurations by dust torus can be found in
AGN in Ezhikode et al. (2017), the determined size of central dust Ricci et al. (2022) to support a radiation-regulated unification model
torus in H0507+164 in Mandal et al. (2018), the well discussed X- in AGN.
ray clumpy torus model in Ogawa et al. (2021) etc. A more recent Until now, there are rare reports on the opening angles of the
review on dust torus can be found in Almeida & Ricci (2017). central dust torus in AGN through direct spatial resolved images.
Under the framework of the unified model, considering different How to measure/determine the opening angle of the central dust
orientations of central dust torus in the line of sight, there is a special torus in an individual AGN is still an interesting challenge. Here,
based on the unique properties of Type-1.9 AGN with BLRs being
seriously obscured by the central dust torus, an independent method
E-mail: aexueguang@qq.com is proposed to estimate the opening angle of the central dust torus
© 2023 The Author(s)
Published by Oxford University Press on behalf of Royal Astronomical Society4462 X.-G. Zhang
It is exciting to check whether the method can be applied to
estimate the opening angle of the central dust torus in Type-
1.9 DPAGN, which is the main objective of the manuscript. And
the following three criteria are accepted to collect targets of the
manuscript. First, the targets are Type-1.9 DPAGN, with apparent
double-peaked broad Hα but no apparent broad Hβ. Second, there
are no signs for optical QPOs in the targets, indicating BBH systems
not preferred to explain the double-peaked broad Hα. Third, after
considering the BH mass properties which will be well discussed
in the Section 4, serious obscurations by the central dust torus are
well accepted to explain the seriously obscured broad Hβ in the
targets.
Among the low redshift (z < 0.35) broad line AGN listed in
Shen et al. (2011) with SPECIAL INTEREST FLAG = 1 and in Liu
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et al. (2019) with flag MULTI PEAK=2, there are 561 low redshift
DPAGN with reliable broad Hα emission lines (both reported line
width and line luminosity at least five times larger than their reported
uncertainties). Based on the main hypothesis and corresponding crite-
Figure 1. Top panel shows the SSP method determined descriptions (solid ria above, Type-1.9 DPAGN SDSS J160714.40+331909.12 (=SDSS
red line) to the SDSS spectrum (solid dark green line) with emission lines J1607+3319) at redshift 0.063 is collected as the unique target of the
being masked out. In top panel, solid blue line and dashed blue line show manuscript, based on two main unique features through properties
the determined host galaxy contributions and power-law AGN continuum of its spectroscopic and long-term variabilities, well discussed in the
emissions, respectively, solid cyan line shows the line spectrum calculated next section. On the one hand, among the Type-1.9 DPAGN, the
by the SDSS spectrum minus the sum of host galaxy contributions and AGN SDSS J1607+3319 has the most apparent double-peaked features
continuum emissions. Bottom panels show the best fitting results (solid red in broad Hα. On the other hand, there are no apparent variabilities
line) to absorption features (solid dark green line) of Ca II H + K (left-hand
in SDSS J1607+3319, which can be well applied to disfavour the
panel), Mg I (right-hand panel). In each panel, the determined χ 2 /dof and
BBH system in the SDSS J1607+3319, combining its double-peaked
stellar velocity dispersion are marked in red characters.
features in broad Hα.
in a special kind of Type-1.9 AGN, the Type-1.9 AGN with double-
peaked broad Hα (Type-1.9 DPAGN). The manuscript is organized 3 S P E C T RO S C O P I C R E S U LT S O F T H E
as follows. Section 2 presents our main hypothesis to estimate the T Y P E - 1 . 9 D PAG N S D S S J 1 6 0 7 +3 3 1 9
half opening angle of the central dust torus in special Type-1.9 SDSS J1607+3319 has its SDSS spectrum (plate-mjd-
DPAGN. Section 3 shows the spectroscopic results of the Type- fiberid = 1419-53144-0453) with signal-to-noise about 34
1.9 DPAGN SDSS J160714.40+331909.12 (=SDSS J1607+3319) shown in Fig. 1. In order to measure emission lines, the commonly
at redshift 0.063. Section 4 gives the main discussions. Section 5 accepted SSP (Simple Stellar Population) method is applied to
gives our final conclusions. And the cosmological parameters have determine host galaxy contributions. More detailed descriptions
been adopted as H0 = 70km s−1 Mpc−1 , = 0.7, and m = 0.3. on the SSP method can be found in Bruzual & Charlot (2003),
Kauffmann et al. (2003), Cid Fernandes et al. (2005), Cappellari
(2017). And the SSP method has been applied in our previous papers
2 M A I N H Y P OT H E S I S
Zhang (2021a, 2021b, 2021d, 2022a, 2022b). Here, we show simple
Accretion disc originations have been well accepted to double- descriptions on SSP method as follows. The 39 SSP templates
peaked broad emission lines, as well discussed in Chen & Halpern from Bruzual & Charlot (2003), Kauffmann et al. (2003) have
(1989), Eracleous et al. (1995), and Storchi-Bergmann et al. (2003, been exploited, combining with a power-law component applied
2017). The inclination angle of the central accretion disc can be to describe intrinsic AGN continuum emissions. When the SSP
well estimated through double-peaked broad line emission features. method is applied, optical narrow emission lines are masked out
Meanwhile, considering the serious obscurations from the central by full width at zero intensity about 450 km s−1 , and the spectrum
dust torus in Type-1.9 DPAGN, the accretion disc origination with wavelength range from 6250–6750 Å are also masked out due
determined inclination angle should be well accepted to trace the to the strongly broad Hα. Then, through the Levenberg–Marquardt
half opening angle of the central dust torus. Certainly, besides the least-squares minimization technique, SDSS spectra with emission
accretion disc origination, a commonly known binary black hole lines being masked out can be well described by combinations
(BBH) system can also be applied to explain double-peaked broad of broadened stellar population templates and the power-law
emission lines, such as the results shown in Shen & Loeb (2010). component. The best descriptions are shown in Fig. 1 with χ 2 /dof ∼
However, the assumed BBH system should lead to optical quasi- 0.91 (the summed squared residuals divided by degree of freedom)
periodic oscillations (QPOs) with periodicities about hundreds to and with determined stellar velocity dispersion (the broadening
thousands of days, such as the results shown in Graham et al. velocity) about 224 ± 5 km s−1 .
(2015a, 2015b), Zhang (2022), and will be discussed to disfavour Moreover, in order to determine reliable stellar velocity dispersion,
the BBH system in the target in the manuscript. Moreover, it should absorption features of around Ca II H + K from 3750–4200 Å and
be confirmed that the seriously obscured broad Hβ are not due to around Mg I from 5050–5250 Å are applied to re-measure stellar
local intrinsic physical conditions (such as the case in H1320+551 velocity dispersions, through the same SSP method above. The best
discussed in Barcons, Carrera & Ceballos 2003) but due to serious fitting results are shown in bottom panels of Fig. 1 with determined
obscurations by the central dust torus. stellar velocity dispersions in units of km s−1 about 222 ± 11 and
MNRAS 519, 4461–4466 (2023)Opening angle of dust torus 4463
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Figure 2. Top panels show the best fitting results (solid red line) to the emission lines (solid dark green line), and bottom panels show the corresponding
residuals. In top left panel, solid blue line shows the determined narrow Hβ, solid green lines show the determined [O III] doublet. In top right panel, solid blue
line shows the determined narrow Hα, solid cyan line shows the determined double-peaked broad Hα described by the elliptical accretion disc model, solid
green lines show the determine [O I], [N II], and [S II] doublets, dashed purple lines show the determined broad Hα described by two broad Gaussian functions.
In each bottom panel, horizontal dashed lines show residuals = 0, ± 1, respectively.
208 ± 26 through the Ca II H + K and Mg I, respectively. Therefore, the Levenberg–Marquardt least-squares Minimization technique, the
in the manuscript, the inverse variance weighted mean stellar velocity Maximum Likelihood method combined with the MCMC (Markov
dispersion σ = 222 ± 26 km s−1 in SDSS J1607+3319 is accepted, Chain Monte Carlo) technique (Foreman-Mackey et al. 2013) is
which is consistent with the SDSS pipeline reported 230 km s−1 . applied. The evenly prior distributions of the seven model parameters
After subtractions of host galaxy contributions and AGN con- in the elliptical accretion disc model are accepted with the following
tinuum emissions, emission lines in the line spectrum can be well limitations, log (r0 ) ∈ [2, 4], log (r1 ) ∈ [2, 6] (r1 > r0 ), log (sin (i))
measured. Similar as what we have previously done in Zhang (2021a, ∈ [−3, 0], log (q) ∈ [−1, 1], log (σ L ) ∈ [2, 4], log (e) ∈ [−5,
2021b, 2022a, 2022b, 2022c), for the emission lines within rest 0], log (φ 0 ) ∈ [−5, log (2 × π )]. The determined best fitting results
wavelength range from 4600–5150 Å, there are one broad and one and corresponding residuals to the emission line around Hα are
narrow Gaussian functions applied to describe probable broad and shown in the right-hand panels of Fig. 2 with χ 2 /dof ∼ 0.48. The
apparent narrow Hβ, two Gaussian functions applied to describe MCMC technique determined posterior distributions of the model
[O III]λ4959, 5007 Å doublet. When the functions above are applied, parameters in the elliptical accretion disc model are shown in Fig. 3.
each component has line intensity not smaller than zero, and the And the half width at half maximum of each parameter distribution
[O III] components have the same redshift and the same line width is accepted as the uncertainty of the parameter. The determined
and have flux ratio to be fixed to the theoretical value 3. Then, through parameters and corresponding uncertainties of each model parameter
the Levenberg–Marquardt least-squares minimization technique, the are listed in Table 1. Moreover, as discussed in Zhang (2022a),
best fitting results to the emission lines and the corresponding clean double-peaked broad line emission features can lead to solely
residuals (line spectrum minus the best fitting results and then divided determined model parameters in the elliptical accretion disc model.
by uncertainties of SDSS spectrum) are shown in left-hand panels Therefore, there are no further discussions on whether is there a
of Fig. 2 with χ 2 /dof ∼ 0.70. Based on the fitting results, it is not solely determined model parameter of sin (i).
necessary to consider broad Gaussian component in Hβ, because
the determined line width and line flux (around to zero) of the
broad Gaussian component are smaller than their corresponding 4 MAIN DISCUSSIONS
uncertainties, indicating there are no apparent broad Hβ in SDSS
In the section, two points are mainly considered. First, it is necessary
J1607+3319.
to determine that the accretion disc origination is favoured to explain
Meanwhile, Gaussian functions can be applied to describe the
the double-peaked broad Hα in SDSS J1607+3319 rather than a
narrow emission lines within rest wavelength range from 6200–
BBH system. Second, it is necessary to determine that the large
6850 Å, the [O I], [N II], [S II], and narrow Hα. But the commonly
broad Balmer decrement (flux ratio of broad Hα to broad Hβ)
accepted elliptical accretion disc model with seven model parameters
is due to serious obscurations rather than due to local physical
well discussed in Eracleous et al. (1995) is applied to describe
conditions because that BLRs modelled with relatively low optical
the double-peaked broad Hα, because the model can be applied
depths and low ionization parameters can reproduce large broad
to explain almost all observational double-peaked broad Hα of the
Balmer decrements, as well discussed in Kwan & Krolik (1981),
SDSS J1607+3319. The seven model parameters are inner and
Canfield & Puetter (1981), Goodrich (1990) without considering
out boundaries [r0 , r1 ] in the units of RG (Schwarzschild radius),
serious obscurations and see the unobscured central regions in a
inclination angle i of disc-like BLRs, eccentricity e, orientation angle
Type-1.9 AGN in Barcons et al. (2003).
φ 0 of elliptical rings, local broadening velocity σ L in units of km s,
For the first point on BBH system, the following discussions are
line emissivity slope q (fr ∝ r−q ). Meanwhile, we have also applied
given. The double-peaked broad Hα can also be well described by
the very familiar elliptical accretion disc model in our more recent
two broad Gaussian functions shown as dashed purple lines in top
studies on double-peaked lines in Zhang (2021c, 2022a), and there
right panel of Fig. 2 with model parameters listed in Table 1. Under
are no further discussions on the elliptical accretion disc model in
the assumption of BBH system in SDSS J1607+3319, considering
the manuscript. Then, in order to obtain more reliable uncertainties
the strong linear correlation between broad Hα luminosity and
of model parameters in the complicated model functions, rather than
continuum luminosity as discussed in Greene & Ho (2005), there
MNRAS 519, 4461–4466 (2023)4464 X.-G. Zhang
Figure 3. MCMC technique determined 2D posterior distributions in contour of the model parameters in the elliptical accretion disc model applied to describe
the double-peaked broad Hα. In each panel, sold circle plus error bars in red mark the positions of the accepted values and corresponding uncertainties of the
model parameters. The number densities related to different colours are shown in colour bar in top region of each panel.
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Table 1. Parameters of the emission line components. Therefore, rather than the BBH system, the elliptical accretion disc
model is preferred to explain the double-peaked broad Hα in SDSS
Model parameters of elliptical accretion disc model for broad Hα J1607+3319.
r0 = 2035 ± 240, r1 = 3766 ± 500, sin (i) = 0.71 ± 0.04 For the second point, properties of virial BH mass are mainly
q = 3.35 ± 0.19, e = 0.81 ± 0.08, σL = 796 ± 70 km s−1 , φ 0 = 190 ± 6◦ discussed. Based on accepted virialization assumptions to properties
Model parameters of Gaussian emission components of observed broad Hα as discussed in Vestergaard (2002), Peterson
Line λ0 σ flux et al. (2004), Greene & Ho (2005), Shen et al. (2011), Mejia-Restrepo
Broad Hα 6505.6 ± 1.1 41.4 ± 1.2 897 ± 25 et al. (2022), virial BH mass can be estimated by
6643.9 ± 1.1 34.9 ± 1.2 699 ± 24 0.55
LH α σH α −1 2.06
Narrow Hα 6564.2 ± 0.5 5.6 ± 0.6 311 ± 54 MBH = 15.6 × 106 M
1042 erg s−1 1000 km s
Narrow Hβ 4862.4 ± 0.3 4.2 ± 0.4 45 ± 8
= (5.5 ± 0.6) × 107 M , (1)
[O III]λ5007Å 5008.8 ± 0.3 3.9 ± 0.3 172 ± 10
with LHα = (1.39 ± 0.05) × 1041 erg s−1 as line luminosity of ob-
[O I]λ6300Å 6301.9 ± 1.1 7.6 ± 1.2 113 ± 14
served broad Hα and σ Hα = (3100 ± 110) km s−1 as second moment
[N II]λ6583Å 6585.5 ± 0.2 6.5 ± 0.3 642 ± 55 of observed broad Hα, after considering more recent empirical RL
[S II]λ6716Å 6719.2 ± 0.9 6.6 ± 0.9 260 ± 33 relation to estimate BLRs sizes in Bentz et al. (2013). Uncertainty of
virial BH mass is determined by uncertainties of the LHα and σ Hα . If
[S II]λ6731Å 6734.5 ± 0.8 4.6 ± 0.7 157 ± 29 large broad Balmer decrement was due to local physical conditions,
Note. For the Gaussian emission components, the first column shows which the estimated virial BH mass should be simply consistent with the
line is measured, the second, third, fourth columns show the measured line MBH −σ relation (Ferrarese & Merritt 2000; Gebhardt et al. 2000;
parameters: The centre wavelength λ0 in unit of Å, the line width (second Kormendy & Ho 2013; Batiste et al. 2017; Bennert et al. 2021)
moment) σ in unit of Å and the line flux in unit of 10−17 erg s−1 cm−2 . expected value otherwise there should be smaller virial BH mass.
Then, Fig. 5 shows virial BH mass properties of SDSS J1607+3319
in the MBH −σ space. In order to show clearer results, the 89 quiescent
galaxies from Savorgnan & Graham (2015) and the 29 reverberation
mapped (RM) AGN from Woo et al. (2015) and the 12 tidal disruption
events (TDEs) from Zhou et al. (2021) are considered to draw the
linear correlation between stellar velocity dispersion and BH mass
σ
MBH
log = (−2.89 ± 0.49) + (4.83 ± 0.22) log ,
M km s−1
(2)
through the Least Trimmed Squares robust technique (Cappellari
et al. 2013). And then the 3σ , 4σ , and 5σ confidence bands to the
linear correlation are determined and shown in Fig. 5. Therefore,
Figure 4. CSS V-band light curve of SDSS J1607+3319. Horizontal solid the estimated viral BH mass of SDSS J1607+3319 is lower than
and dashed red lines show the mean value and corresponding 2RMS scatters MBH −σ expected value with confidence level higher than 4σ . There-
of the light curve. fore, locate physical conditions are disfavoured to explain the large
broad Balmer decrement in SDSS J1607+3319.
are totally equal (ratio about 897:699 from emission fluxes of the Based on the double-peaked broad Hα in the Type-1.9 DPAGN
two broad Gaussian components) continuum luminosities related to SDSS J1607+3319, half opening angle of central dust torus is well
central two BH accreting systems, indicating there should be strong estimated as (46 ± 4)◦ (sin (i) ∼ 0.71 ± 0.04), roughly consistent
variabilities with QPOs due to orbital rotating effects. However, there with statistical mean value in Zhuang et al. (2018). Therefore, it
are none variabilities in the collected 8.4 yr-long CSS (Catalina Sky is interesting to study properties of opening angles of dust torus
Survey, Drake et al. 2009) V-band light curve shown in Fig. 4 through Type-1.9 DPAGN in the near future, after many efforts to
with almost all data points lying within 2RMS scatter ranges. disfavour BBH systems to explain their double-peaked broad Hα
MNRAS 519, 4461–4466 (2023)Opening angle of dust torus 4465
disc model applied to describe the double-peaked broad Hα in SDSS
J1607+3319, the half opening angle of the central dust torus can be
well estimated as (46 ± 4)◦ in SDSS J1607+3319. The results in the
manuscript strongly indicate that the proposed independent method
is practicable and can be applied to study detailed properties of the
opening angles of the central dust torus through a sample of Type-1.9
DPAGN, which will be studied in the near future.
AC K N OW L E D G E M E N T S
X-GZ gratefully acknowledges the anonymous referee for giving us
constructive comments and suggestions to greatly improve our paper.
X-GZ gratefully acknowledges the kind funding support NSFC-
Figure 5. On the correlation between stellar velocity dispersion measured
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12173020. This research has made use of the data from the SDSS
through absorption features and virial BH mass of SDSS J1607+3319. Solid
(https://www.sdss.org/) funded by the Alfred P. Sloan Foundation,
five-point-star in dark green shows the virial BH mass of SDSS J1607+3319
determined by properties of observed broad Hα. Dot–dashed lines in magenta
the Participating Institutions, the National Science Foundation and
and in black represent the MBH −σ relations through the quiescent galaxies the U.S. Department of Energy Office of Science, and use of
in Kormendy & Ho (2013) and through the RM AGNs in Woo et al. (2015), the data from CSS http://nesssi.cacr.caltech.edu/DataRelease/. The
respectively. Solid circles in red, in blue, and in pink show the values for research has made use of the MPFIT package https://pages.ph
the 89 quiescent galaxies in Savorgnan & Graham (2015), the 29 RM AGNs ysics.wisc.edu/∼craigm/idl/cmpfit.html, and of the LTS LINEFIT
in Woo et al. (2015), and the 12 TDEs in Zhou et al. (2021), respectively. package https://www-astro.physics.ox.ac.uk/∼cappellari/software/,
Thick solid red line shows the best fitting results to all the objects, and thick and of the emcee package https://pypi.org/project/emcee/.
dashed, dotted, and dot–dashed red lines show corresponding 3σ , 4σ , and 5σ
confidence bands to the best fitting results.
DATA AVA I L A B I L I T Y
and to disfavour local physical conditions to explain disappearance The data underlying this article will be shared on request to the
of broad Hβ. corresponding author (aexueguang@qq.com).
Before ending of the manuscript, an additional point is noted.
Before giving clear physical information of materials in the central
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