aa r X i v : . [ a s t r o - ph . GA ] M a y Research in Astron. Astrophys. R esearchin A stronomyand A strophysics Low-mass and High-mass Supermassive Blackholes In Radio-LoudAGNs Are Spun-up in Different Evolution Paths
J. Wang , , , M. Z. Kong , S. F. Liu , D. W. Xu , , Q. Zhang and J. Y. Wei , Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology,Guangxi University, Nanning 530004, China; [email protected] Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories,Chinese Academy of Sciences, Beijing 100012, China School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing,China Department of Physics, Hebei Normal University, No. 20 East of South 2nd Ring Road,Shijiazhuang, 050024, China; confucious [email protected]
Abstract
How Supermassive Blackholes (SMBHs) are spun-up is a key issue of modernastrophysics. As an extension of the study in Wang et al. (2016), we here address the issueby comparing the host galaxy properties of nearby ( z < . ) radio-selected Seyfert2 galaxies. With the two-dimensional bulge+disk decompositions for the SDSS r -bandimages, we identify a dichotomy on various host galaxy properties for the radio-powerfulSMBHs. By assuming the radio emission from the jet reflects a high SMBH spin, whichstems from the well-known BZ mechanism of jet production, high-mass SMBHs (i.e., M BH > . M ⊙ ) have a preference for being spun-up in classical bulges, and low-mass SMBHs (i.e., M BH = 10 − M ⊙ ) in pseudo-bulges. This dichotomy suggests andconfirms that high-mass and low-mass SMBHs are spun-up in different ways, i.e., a major“dry” merger and a secular evolution. Key words: galaxies: bulges — galaxies: nuclei — galaxies: Seyfert
Both merger and secular evolutionary scenarios have been proposed to understand the growth of super-massive blackholes (SMBHs) located at the centers of host galaxies, which stem from the widelyaccepted conception of co-evolution of active galactic nuclei (AGNs) and their host galaxies (e.g.,Heckman & Best 2014; Alexander & Hickox 2012; Sanders et al. 1988). Although a high fractionof merger is found in luminous quasars and ultra-luminous infrared galaxies (ULIRGs) (e.g., Liu etal. 2008; Mainieri et al. 2011; Treister et al. 2012; Veilleux et al. 2009; Hao et al. 2005), the studiesfrom Sloan Digital Sky Survey (SDSS) clearly suggest that the growth of local less-massive SMBHs aremainly resulted from gas accretion occurring in small host galaxies (see a review in Heckman & Best2014), which implies a prevalence of a disk-like bugle (i.e., a pseudo-bugle, e.g, Kormendy & Kennicutt2004) for these host galaxies. In fact, observations with high spatial resolution reveal a pseudo-buglein the galaxies of some narrow-line Seyfert 1 galaxies (NLS1s) that are believed to have less-massiveSMBHs and high Eddington ratios (e.g., Zhou et al. 2006; Ryan et al. 2007; Mathur 2000; Orban deXivry et al. 2011; Mathur et al. 2012). Wang et al. (2016) recently claimed that for less-massive SMBHspowerful radio emission is favored for occurring in pseudo-bugles, which implies that the less-massive
Wang et al.
SMBHs are spun up by a gas accretion due to the significant disk-like rotational dynamics of the hostgalaxy in the secular evolution scenario.Are high-mass SMBHs spun up in the same way, or not? Although there was accumulating ob-servational evidence supporting that radio-loud quasars could be spun up by a BH-BH merger (e.g.,Laor 2000; Best et al. 2005; Chiaberge & Marconi 2011; Chiaberge et al. 2015), a comparison studybetween low-mass and high-mass SMBHs is still rare. The morphology of the host galaxies of quasarsis in fact hard to be studied because the host galaxies are overwhelmed by the luminous emission fromthe central SMBH accretion, even though previous studies indicate that the bulge morphology keep theevolutionary information well (e.g., Kormendy & Kennicutt 2004; Kormendy & Ho 2013). Barisic etal. (2019) recently reveals a higher radio-loud fraction in elliptical galaxies with larger mass and higherstellar velocity dispersion than in disk galaxies with smaller mass and lower velocity dispersion, whichimplies that the star formation in the elliptical galaxies is suppressed by the feedback energy depositedby AGN’s jet.In this paper, by following our previous study in Wang et al. (2016), we attempt to explore therole of both merger and secular evolution scenarios on the spinning-up of a SMBH in a sample ofnearby radio-selected Seyfert 2 galaxies with powerful radio emission, in which the bulge morphologyof high-mass SMBHs are compared to that of low-mass SMBHs. Our study is based on the well-knownBlandford-Znajek model (Blandford & Znajek 1977) in which the observed powerful jet is resultedfrom an extraction of the rotational energy of the central SMBH. In fact, Martinez-Sansigre & Rawlings(2011) indicates that the efficiency with which the jet is produced is required to increase with SMBH’sspin to reproduced the observe quasar’s “radio-loudness” range, although a direct correlation betweenradio power and the measured spin has not been found in AGNs (see Reynolds 2019 for a recent review).The lack of the correlation implies an importance of the strength and geometry of the magnetic field inthe production of a jet.The paper is organized as follows. Section 2 presents the sample selection and analysis. The resultsand discussions are given in Section 3. A Λ CDM cosmology with parameters H = 70 km s − Mpc − , Ω m = 0 . , and Ω Λ = 0 . is adopted throughout the paper. A sample of nearby radio-selected Seyfert 2 galaxies is used in the current study, which is consist oftwo sub-samples with small and large M BH . The sub-sample with small M BH comes from our previ-ous study, in which Wang et al. (2016) selected a sample of radio-selected nearby ( z < . ) “pure”Seyfert 2 galaxies with small M BH ( − M ⊙ ) by cross-matching the value-added SDSS Data Release7 Max-Planck Institute for Astrophysics/Johns Hopkins University (MPA/JHU) catalog (see Heckman &Kauffmann 2006 for a review) with the FIRST survey catalog (Becker et al. 2003). Briefly speaking, notonly the three widely used Baldwin-Phillips-Terlevich diagnostic diagrams (e.g., Veilleux & Osterbrock1987), but also the [OIII] λ /[OII] λ line ratio corrected by the local extinction (Heckman etal. 1981) is used to remove starforming galaxies, composite galaxies and LINERs (e.g, Kewley et al.2001, 2006). The M BH of each galaxy is obtained from the measured velocity dispersion σ ⋆ of the bulgethrough the well-calibrated M BH − σ ⋆ relationship (Magorrian et al. 1998; McConnell & Ma 2013 andreferences therein) log( M BH /M ⊙ ) = (8 . ± .
05) + (5 . ± .
32) log( σ ⋆ /
200 km s − ) that is validfor M BH in a range of − M ⊙ . Although there is evidence that pseudo-bulges deviate from the M BH − σ ⋆ relationship established in classical bulges (e.g., Kormendy & Ho 2013), we argue that thedeviation is not a serious issue for the current study because the M BH − σ ⋆ relationship is only used byus to select SMBHs at both high-mass and low-mass ends.We selected a sub-sample with large M BH by following the scheme adopted in Wang et al. (2016).The M BH that are obtained again from the M BH − σ ⋆ relationship are required to be larger than . M ⊙ .This lower limit of M BH is adopted by taking into account of a balance between threshold and samplesize. pinning-up of Supermassive Blackholes 3 fpC-53794-2418-116fpC-53499-2011-492 Fig. 1 The two SDSS r -band images for SDSS J130125.26+291849 (the left panel) andSDSS J080446.40+104635.8 (the right panel) both with a strongly disturbed profile.After the selection on M BH , the sub-sample with large M BH is further filtered out according totheir nuclear accretion properties. By using the [OIII] λ line luminosity as a proxy of the bolomet-ric luminosity (e.g., Kauffmann et al. 2003), we finally focus on the objects located within a bin of log L [OIII] = 40 . − . , where the intrinsic extinction due to the host galaxy has been corrected bythe standard method based on both the Balm er decrement in the standard case B recombination andthe Galactic extinction curve with R V = 3 . . The used bin size is determined by a balance between thedistribution of log L [OIII] and the size of our finally used sample.Finally, there are 31 objects in the sub-sample with a small M BH , and 26 in the sub-sample with alarge M BH . As the same as in Wang et al. (2016), we model the surface brightness profile of each galaxy by a linearcombination of an exponential radial profile for the disk component and a Sec profile with an indexof n B for the bulge component. The two-dimensional bulge+disk decomposition is performed for the r -band images of each of the objects listed in the large M BH sub-sample by using the publicly avail-able SEX TRACTOR and KIM2D packages (Beaten & Aunts 1996; Smart et al. 2002), except for fourcases. The decomposition is ignored for SDSS J081937.87+210651.4 and SDSS J111349.74+093510.7because of their heavy obstruction. The other two objects (i.e., SDSS J080446.40+104635.8 andSDSS J130125.26+291849.4) are ignored in our decomposition since their host galaxies are stronglydisturbed due to an on-going merger of two galaxies. The SDSS r -band images are displayed in Figure1 for the two objects with an on-going merger. The seeing effect has been taken into account of byconvolving the model with a simple point-spread function described by a Gaussian profile that is deter-mined from the field stars. The resulted reduced χ is very close to unit for all the remaining 22 hostgalaxies. With the bulge+disk decomposition of the surface brightness, Figure 2 reproduces the Figure 2 in Wanget al. (2016) by complementing the objects listed in the large M BH sub-sample, in which the modeledSersic index n B of the surface brightness profile of the bulge of the host galaxies is plotted againstthe radio loudness R ′ (the left panel), the rest-frame [OIII] line luminosity L [OIII] (the middle panel),and the rest-frame radio power P . at 1.4GHz (the right panel). A k -correction is performed in thecalculation of P . by adopting a universal spectral slope α = − . ( f ν ∝ ν α , Ker et al. 2012): P . = 4 πd L f ν (1 + z ) − − α , where d L is the luminosity distance, z the redshift and f ν the observedintegrated flux density. Wang et al. -1 -0.5 0 0.5 1 40.5 41 41.5 21 21.5 22 22.5
Fig. 2 The modeled Sersic index n B plotted against radio loudness R ′ defined in Equation (1) (the leftpanel), rest-frame [OIII] λ n B are marked by triangles for n B = 1 and by squares for n B = 4 .By combining the two traditionally used bolometric corrections: L bol ≈ L [OIII] and L bol =9 λL λ (5100 ˚ A) (Kaspi et al. 2000; Heckman & Best 2014), the radio loudness R ′ based on the [OIII]line luminosity is defined as log R ′ = log (cid:18) P . / W Hz − L [OIII] / erg s − (cid:19) + 19 . (1)A comparison between small- M BH and large- M BH sub-samples in the occupation in the diagramsindicates that 1) almost all the objects with large M BH are associated with a classical bulge with n B > . (e.g., Kormendy & Kennicutt 2004; Fisher & Drory 2008); 2) the radio-powerful (i.e., log R ′ > or log P . > . ) low-mass SMBHs tend to be associated with a pseudo-bulge with n B < . , even though the approximation to identify pseudo-bulges by the threshold of n B was proposedby Gadotti (2009). To reveal the dichotomy on pseudo-bulges and classical bulges, we perform a two-sample Gehan’s generalized Wilcoxon test on the distributions of n B for the radio-powerful objectswith either log R ′ > or log P . > . . The statistical results are tabulated in Table 1. Columns(2) and (3) list the probability that the two samples are drawn from the same parent population andthe corresponding Z -value, respectively. The average value and the corresponding standard deviationare listed in the first row in Column (4) for the small- M BH sub-sample, and in the second row for thelarge- M BH sub-sample.In addition to the revealed dichotomy on the Sersic index n B , the discrepancy between the small- M BH and large- M BH subsamples can be further verified in Figure 3 for the radio-powerful SMBHs. Thefigure plots radio loudness R ′ as a function of the stellar population age (the upper panel) as assessed bythe lick 4000 ˚A break index defined as D n (4000) = R f λ dλ/ R f λ dλ (e.g., Bruzual & Charlot2003; Coelho det al. 2007 and references therein) and the scalelength ratio h d /R e between discs andbulges (the lower panel). As revealed in Wang et al. (2016), the radio-powerful low-mass SMBHs (i.e., pinning-up of Supermassive Blackholes 5 Table 1 Statistical results of two-sample Gehan’s generalized Wilcoxon tests for radio-powerful SMBHs( log R ′ > or log P . > . ). Parameter
P Z -value Mean(1) (2) (3) (4) n B × − . ± . . ± . h d /R e ) × − . ± . . ± . D n (4000) 5 × − . ± . . ± . -1 -0.5 0 0.5 1 Fig. 3
Upper panel: the measured stellar population ages as assessed by the D n (4000) index is plottedas a function of radio loudness R ′ . The symbols are the same as in Figure 2. Lower panel: the same asthe upper panel but for the scalelength ratio between discs and bulges h d /R e . log R ′ > and n B < ) are associated with young stellar populations with D n (4000) < . . While, thehigh-mass counterparts (i.e., log R ′ > and n B > ) found to be associated with both young and oldstellar populations in the current study. Compared to the radio-powerful high-mass SMBHs, the low-mass counterparts tend to have smaller h d /R e ratio. In fact, by performing 2-dimensional bulge+diskdecomposition for a large sample of 1000 galaxies from SDSS, Gadotti (2009) indicates that comparedto the classical bulges, the pseudo-bulges tend to have younger stellar population and higher R e /h d ratio at the same B/T ratio, even though the author instead separates pseudo-bulges and classical bulgesin terms of the h µ e i − R e relation firmly established in elliptical galaxies (i.e., the Kormendy relation,Kormendy 1977). The statistical results based on the same two-sample test are again listed in Table 1. It is widely accepted that the SMBH’s powerful radio emission is likely generated by an energy extrac-tion from BH’s spin through the Blandford-Znajek (BZ) mechanism (e.g., Blandford & Znajek 1977, An alternative is the Blandford-Payne mechanism in which the observed powerful radio emission is resulted from an energyextraction from a disk wind (e.g., Blandford & Payne 1982; Wang et al. 2003; Cao 2016)
Wang et al.
Chiaberge & Marconi 2011; Chisellini et al. 2014). In the model, the power of the jet L jet is predictedto be L jet ∝ j B p , where j is the dimensionless black hole spin and B p is the poloidal magnetic fieldstrength at the horizon of the SMBH (e.g., Thorne et al. 1986; Meier 2001; Koide et al. 2002; Daly2009, 2016). The validation of the BZ mechanism for the jet production is supported by some numericalsimulations and observations (e.g., Hawley & Krolik 2006; Sadowski et al. 2015; Martinez-Sansigre &Rawlings 2011). By assuming the BZ mechanism, the clearly revealed dichotomy on the bugle mor-phology therefore predicates a profound dichotomy on the spinning-up mechanisms in low-mass andhigh-mass SMBHs. We argue that the dichotomy on spinning-up mechanisms is related with the twotypes of evolutionary scenario, which is described as follows.On the one hand, a low-mass SMBHs is more likely spun-up within a pseudo-bugle with a sig-nificant disk-like rotational dynamics. The pseudo-bugle can be produced in the secular evolution of adisk galaxy possibly through either a second hump instability or a vertical dynamical resonance (e.g.,Kormendy & Kennicutt 2004; Fisher & Drory 2011; Silverman et al. 2011; Kormendy & Ho 2013;Sellwood 2014). From the theoretical ground, a less-massive SMBH can be spun-up efficiently by theaccreted gas through the frame-dragging effect that realigns the BH-disk system through the interac-tion between the Lense-Thirring torque and the strong disk viscous stress (e.g., King et al. 2005, 2008;Volonteri et al. 2007; Perego et al. 2009; Li et al. 2015), once the mass of the gas accreted onto theSMBH exceeds the alignment mass limit m align ∝ a / ( L/L
Edd ) / M / (King et al. 2005),where a = cJ/GM BH is the dimensionless angular momentum, and L Edd the Eddington luminosity.On the other hand, a classical bulge that is widely believed to be resulted from a major “dry” mergerof two galaxies (Toomre 1977) is responsible for the spinning-up of a high-mass SMBH. A “dry” mergerof two galaxies is argued to be the origination of a ‘core” galaxy since the deficit of star light can beresulted from an ejection of stars away from the central region during the merger (e.g., Faber et al. 1997;Kormendy et al. 2009). A spinning SMBH can be produced by the subsequent BH-BH merger if themasses of the two involved SMBH are comparable (e.g., Hughes & Blandford 2003; Baker et al. 2006;Li et al. 2010). After the coalescences of the two SMBHs, a formation and a maintenance of a powerfuljet results in a spinning-down due to an extraction of its rotational energy. By using the HST imageswith high spatial resolution, Capetti & Balmaverde (2006, 2007) pointed out that radio-loud AGNs tendto be associated with a “core” galaxy that has a small logarithmic slope of the nuclear surface brightnessprofile (see also in de Ruiter et al. 2005). In addition to the implication discussed above, the mergerscenario is further supported by the current two cases with an on-going merger (see Figure 1). In fact,Chiaberge et al. (2015) pointed out that ∼ radio-loud AGNs at z > are associated with an eitherrecent or on-going merger system. Finally, the merger scenario is further validated by that fact that thereis accumulating evidence supporting that radio-loud AGNs have richer environment than radio-quietAGNs (e.g., Shen et al. 2009; Donson et al. 2010). By comparing the host galaxies of a sample of nearby ( z < . ) radio-selected Seyfert 2 galaxies.we identify a dichotomy on the host galaxy properties for radio-powerful SMBHs, in which high-massSMBHs ( > . M ⊙ ) favor a spinning-up in classical bulges, and low-mass SMBHs ( − M ⊙ ) inpseudo-bulges, based on the assumption that a high spin of SMBH can be reflected by its powerful jet.We argue that high-mass and low-mass SMBHs are likely spun-up and grown up in different ways, i.e.,a major “dry” merger and a secular evolution, respectively. Acknowledgements
JW & DWX are supported by the National Natural Science Foundation ofChina under grants 11773036 and 11473036. MZK is supported by NSFC Youth Foundation (No.11303008) and by Astronomical Union Foundation under grant No. U1831126. This study is sup-ported by the National Basic Research Program of China (grant 2014CB845800), the NSFC undergrants 11533003, and the Strategic Pioneer Program on Space Science, Chinese Academy of Sciences,Grant No.XDA15052600. The study is supported by the National Basic Research Program of China(grant 2009CB824800). This study uses the SDSS archive data that was created and distributed by the pinning-up of Supermassive Blackholes 7
Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S.Department of Energy Office of Science.
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