A Compton-thick nucleus in the dual AGN of Mrk 266
K. Iwasawa, C. Ricci, G. C. Privon, N. Torres-Albà, H. Inami, V. Charmandaris, A. S. Evans, J. M. Mazzarella, T. Díaz-Santos
AAstronomy & Astrophysics manuscript no. ki_v2 c (cid:13)
ESO 2020July 20, 2020
A Compton-thick nucleus in the dual active galactic nuclei ofMrk 266
K. Iwasawa , , C. Ricci , , , G. C. Privon , , N. Torres-Albà , H. Inami , V. Charmandaris , , A. S. Evans , ,J. M. Mazzarella , and T. Díaz-Santos , , Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain Núcleo de Astronomía de la Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago,Chile Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China Department of Physics and Astronomy, George Mason University, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USADepartment of Astronomy, University of Florida, 211 Bryant Space Sciences Center, Gainesville, FL 32611, USA National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville, FL 32611, USA Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526,Japan Department of Physics, University of Crete, GR-71003 Heraklion, Greece Institute of Astrophysics, Foundation for Research and Technology—Hellas, Heraklion, GR-70013, Greece Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA IPAC, MC 100-22, California Institute of Technology, Pasadena, CA, 91125, USA Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101,ChinaJuly 20, 2020
ABSTRACT
We present the results from our analysis of
NuSTAR data of the luminous infrared galaxy Mrk 266, which contains two nuclei, south-western (SW) and north-eastern (NE), which were resolved in previous
Chandra imaging. Combining this with the
Chandra data, weintepret the hard X-ray spectrum obtained from a
NuSTAR observation to result from a steeply rising flux from a Compton-thick activegalactic nuclei (AGN) in the SW nucleus which is very faint in the
Chandra band, confirming the previous claim. This hard X-raycomponent is dominated by reflection, and its intrinsic 2-10 keV luminosity is likely to be ∼ × erg s − . Although it is brightin soft X-ray, only a moderately absorbed NE nucleus has a 2-10 keV luminosity of 4 × erg s − , placing it in the low-luminosityAGN class. These results have implications for understanding the detectability and duty cycles of emission from dual AGN in heavilyobscured mergers. Key words.
X-rays: galaxies - Galaxies: active - Galaxies:individual (Mrk 266)
1. Introduction
Mrk 266 ( = NGC 5256) is a major merger of two galaxieswith nearly equal masses with a nuclear separation of 10 arc-sec, corresponding to a projected physical distance of 6 kpcat z = . L − µ m = . × L (cid:12) and is a member of the Great Ob-servatories All-sky LIRG Survey (GOALS, Armus et al. 2009).While a significant portion of the large IR luminosity originatesin intense star formation, both galaxies host active galactic nu-clei (AGN). Dual AGN in galaxy mergers are relatively rare ininfrared-selected samples such as GOALS (four out of 54 multi-ple systems, Iwasawa et al. 2011; Torres-Albà et al. 2018; Iwa-sawa et al. 2018); this is likely due to heavy obscuration makingthe dual AGN di ffi cult to detect as opposed to the absence ofthem. Mrk 266 provides a good opportunity for a detailed studyof dual AGN in a luminous IR system. The two AGN, as de-scribed in this paper, are, however, found to have rather di ff erent characteristics despite their similar host galaxy masses; addition-ally, heavy obscuration does indeed play a significant role.Following the extensive multiwavelength analysis ofMrk 266 presented by Mazzarella et al. (2012, hearafter MIV12),here, we give a brief summary of known properties focusingon the two active nuclei residing in the south-western (SW)and north-eastern (NE) galaxies. The galaxies have compara-ble masses of around (5-6) × M (cid:12) , from which the black holemass of each nucleus is estimated to be ∼ × M (cid:12) , based ontheir bulge luminosities (Marconi & Hunt 2003). The individualgalaxies have L IR of 2 . × L (cid:12) (SW) and 0 . × L (cid:12) (NE).This seems to be compatible with the Herschel imaging at 70 µ mand 100 µ m, which marginally resolves them (Chu et al. 2017).While there is confusion surrounding the reversed classificationsgiven in the literature, the most likely optical classifications ofthe SW and NE nuclei are Seyfert 2 and LINER, respectively, assupported by works either dedicated to Mrk 266 or small sam-ples including Mrk 266 (Osterbrock & Dahari 1983; Kollatschny& Fricke 1984; Hutchings et al. 1988; Mazzarella & Boroson Article number, page 1 of 7 a r X i v : . [ a s t r o - ph . GA ] J u l & A proofs: manuscript no. ki_v2
001 0.077 0.23 0.54 1.1 2.4 4.8 9.6 19 39 7
NE SW
Fig. 1.
HST ACS / WFC image of Mrk 266 obtained with the F814Wfilter, overlaid by Chandra 4-7 keV image contours. The five contourlevels are linearly spaced between 0.15 to 3.8 counts pixel − . North isup, and east is to the left. The SW and NE nuclei are labelled. The scalebar of 10 arcsec is shown. v ] λ . , . µ m, and [O iv ] λ . µ m for the SW nucleus and warm dust continuum for theNE nucleus (see also Imanishi et al. 2009). A Chandra observa-tion, which resolves the two nuclei, shows that a hard-spectrumsource is clearly seen at the NE nucleus (Fig. 1), supporting thepresence of a moderately obscured AGN with N H = (7 ± × cm − (Brassington et al. 2007; MIV12; Torres-Albà et al. 2018).On the other hand, the SW nucleus is ∼
13 times fainter than theNE nucleus and barely detected in the Chandra hard-band (4-7keV) image (Fig. 1). The SW nucleus is also fainter than the NEnucleus in the optical and ultraviolet (UV), but it becomes pro-gressively brighter at longer wavelengths ≥ µ m, leading to theSW nucleus to be ∼ ) = . × M (cid:12) , which is ∼ F l u x den s i t y Energy (keV) . F l u x den s i t y Energy (keV) 10.5 2 5Energy (keV)
XMM-Newton + NuSTARNE SWFe K
Fig. 2.
Upper panel: Flux density spectrum of integrated emission ofMrk 266, containing both NE and SW nuclei. Data from XMM-Newton(blue) and NuSTAR (red squares) are plotted. The flux density is in unitsof 10 − erg s − cm − keV − . For display purposes, the NuSTAR dataused for the spectral analysis were rebinned further. The Fe K emissionline feature at 6.5 keV is marked. Bottom panel: Spectra of the NE andSW nuclei obtained from the Chandra ACIS. Since Fe K line emissionis absent in the NE spectrum (the line energy is denoted by the verti-cal dotted line), the Fe K line seen in the XMM-Newton and NuSTARspectra seems to originate predominantly from the SW. major IR source of the merger system (e.g. U et al. 2013; Iwa-sawa et al. 2011, 2018; Liu et al. 2019). In this paper, we exam-ine this hypothesis for Mrk 266 using new NuSTAR data whichcover the 3-50 keV band, expecting a sharp rise in the X-rayflux of the SW nucleus above 10 keV, which would exceed theflux from the NE nucleus in the NuSTAR band. The cosmologyadopted here is H =
70 km s − Mpc − , Ω Λ = . Ω M = .
2. Observations
Mrk 266 was observed with NuSTAR, XMM-Newton, and theChandra X-ray Observatory and the observation log is shown inTable 1. The NuSTAR data were taken as part of the NuSTARObscured Seyferts Survey (PI: J. Miller) and we retrieved thedata from the public archive. We used the event file processedby the standard pipeline. Spectral data extracted from a circu-lar aperture with a radius of 30 (cid:48)(cid:48) of the two focal plane modules,FPMA and FPMB, were combined for the analysis presented be-low. The spectral data of the two modules are in agreement witheach other within a statistical error, and the low-energy e ff ectivearea problem reported for FPMA (Madsen et al. 2020) seems tohave little impact. Since the NE and SW nuclei are unresolvedwith the NuSTAR beam, the spectrum contains emission fromboth nuclei, as in the XMM-Newton spectrum. The spectral dataobtained from XMM-Newton and Chandra are the same as thosepresented in MIV12. Article number, page 2 of 7. Iwasawa et al.: Dual AGN in Mrk 266
Table 1.
X-ray observations of Mrk 266.
Observatory Date ObsID Exposure Counts BandChandra 2001-11-02 2044 20 124 3-8 keVXMM-Newton 2002-05-15 0055990501 13 /
18 562 3-10 keVNuSTAR 2019-02-08 60465005002 64 441 3-50 keV
Notes.
The ’Exposure’ column shows a useful exposure time in each observation in units of 10 seconds. The ’Counts’ column gives background-corrected source counts in the energy band, which is shown in the following column. The two exposure times shown for the XMM-Newtonobservation are of the EPIC pn and the two EPIC MOS cameras. The source counts for XMM-Newton and NuSTAR are the sums of the threeEPIC cameras and two FPMs, respectively.
3. Results
The NuSTAR spectrum is in agreement with the XMM-NewtonEPIC spectrum in the overlapping 3-10 keV band, both in con-tinuum and the Fe K line (Fig. 2). The 3-50 keV NuSTAR spec-trum can be well described by a power-law of energy index α = . ± . plus a narrow Gaussian line at 6 . ± . χ = . . ± . × − ph cm − s − , corresponding to EW = . ± .
27 keV.A hard spectrum with a slope, as observed at lower ener-gies, would be interpreted as evidence of moderate absorptionof N H ≤ cm − since an intrinsic AGN spectrum is known tohave a well-defined range of slope around α = . .
15 (e.g. Ueda et al. 2014; Nandra & Pounds1994). The measured spectral slope, α ≈ .
0, which stretches upto 50 keV, is, however, unusual because a spectral slope wouldstart to approach the intrinsic value towards higher energies asthe e ff ect of such moderate absorption diminishes. Therefore, itis unlikely that a single source, that is the NE nucleus, domi-nates the NuSTAR bandpass as it does in the 4-7 keV Chandraimage. Instead, the NuSTAR spectrum strongly suggests, besidesthe NE source, the presence of an extra component rising abovethe Chandra bandpass, which elevates the hard band continuumemission. This is the same line of reasoning used to convey thepresence of a Compton-thick AGN in the northern nucleus ofMrk 273 using NuSTAR data (Iwasawa et al. 2018). As arguedin MIV12, the SW nucleus is a likely source of this extra hardcomponent, which can be attributed to a Compton-thick AGN.According to the above hypothesis, we modelled the NuS-TAR spectrum with two components from the NE and SW nu-clei. To avoid over-fitting, we tested the two-component model,leaving a minimum set of parameters free. Many model parame-ters were set to reasonable or known values. A first assumption isthat both X-ray sources in the NE and SW nuclei have the iden-tical, standard AGN slope of α = .
9. This is the intrinsic slopeof the power-law source, not the apparent slope of α = . ff ected by a contribution of reflectionfrom a surrounding medium, as is found in Nandra & Pounds(1994).The NE spectrum is mildly absorbed. The absorbing columndensity is found to be N H = (6 . ± . × cm − by fittingthe Chandra NE spectrum and we adopted the best-fit N H . Thisleaves the normalisation of the power-law as the only free pa-rameter for the NE component.We are primarily interested in the SW component. How-ever, as shown below, various configurations of a Compton-thick torus can describe the NuSTAR spectrum, as long as N H ≥ × cm − is met. Therefore we tested the following two We used energy-index α for a power-law spectral slope since it isappropriate for the flux density spectra shown in this article. It is relatedto the conventional photon index Γ by Γ = + α . θ op θ i SW-ASW-R
R1R2DR2D
Observer
Fig. 3.
Cross-section view of X-ray absorbing torus configurations as-sumed for the absroption-dominated (
SW-A ) and reflection-dominated(
SW-R ) models. The torus parameters of etorus given in Table 2 areillustrated. The inclination angle and half-opening angle are denoted as θ i and θ op ; the labels of D , R , and R ff from the part of the inner sur-face of the torus, which is hidden from direct view (‘Reflection 1’ asdefined in Ikeda et al. 2009), and reflected light coming from the vis-ible inner surface (‘Reflection 2’), respectively. In SW-A , an observedspectrum is dominated by D and weaker R
1, which has practically thesame spectral shape as D . In SW-R , R
2, which has a Compton-scatteredlow-energy tail, is the dominant component since D (and a minor R modelling options for a Compton-thick AGN: SW-A , which isa strongly absorbed spectrum, and
SW-R , which is a reflection-dominated spectrum. In both cases, we used etorus (Ikedaet al. 2009; Awaki et al. 2009), which is one of the Monte-Carlo codes, to compute X-ray spectra emerging from an X-ray source surrounded by an absorber with a torus geometry,including Compton-scattered light. The etorus o ff ers flexibleparameterisations of torus geometry which are similar to thosein another recent code borus02 (Balokovi´c et al. 2018); ad-ditionally, Balokovi´c et al. (2018) report that the two codesproduce well-matched reflection spectra, apart from fluorescentemission-lines, which are not included in the current version of etorus . We modelled the most prominent Fe K line with a nar- Article number, page 3 of 7 & A proofs: manuscript no. ki_v2
105 20 F l u x den s i t y Energy (keV)
NE SW
105 20 F l u x den s i t y Energy (keV)
NE SW
NE + SW: absorption NE + SW: reflection
Fig. 4.
NuSTAR spectrum of Mrk 266 and the best-fitting two-component model (grey line). Left:
SW-A ; Right:
SW-R . The contributions of theNE (dotted line) and SW (dash-dotted line) nuclei are overplotted. Same as in Fig. 2, the NuSTAR data used for the analysis were rebinned furtherfor display purposes. Relative contributions of the NE and SW nuclei change dramatically across the energy range.
Table 2.
Two-component fits to the NuSTAR spectrum. NE N H . × SW SW-A SW-RN H . + . − . × × θ op ◦ ◦ θ i ◦ ◦ Fe K line E . ± .
09 6 . ± . I . ± . . ± . χ / dof 46.8 /
50 45.9 / Notes.
The 3-50 keV NuSTAR spectrum was fitted by NE and SW com-posite models with two modelling options for the SW component. 1)
SW-A : absorption-dominated; and 2)
SW-R : reflection-dominated spec-trum. The spectral slope for both components were assumed to be α = .
9. The SW spectrum was computed by the torus spectrum model etorus (Ikeda et al. 2009). The torus configuration was determined byan opening angle, θ op , and inclination, θ i . Apart from the absorbing col-umn for SW-A , free parameters are normalisations of the respective NEand SW continuum and the centroid energy and intensity of a Gaussianfor the Fe K line. The values of N H , line energy ( E ), and intensity ( I )are in units of cm − , keV, and 10 − ph cm − s − , respectively. row Gaussian. The torus parameters were chosen so that a result-ing model gives a representative spectral shape for each option.They are illustrated in Fig. 3 (see Figures 3 and 4 from Ikedaet al. 2009 for a visualisation of the simulated spectral compo-nents). Since the line-of-sight is obscured, the absorbing torusis expected to be highly inclined and we assume its inclinationangle to be θ i = ◦ , which is approximately in the middle of the45 ◦ -90 ◦ range. In SW-A , a relatively narrow torus-opening angleof θ op = ◦ is assumed so that the inner surface of the torus ishardly visible and thus the observed spectrum is dominated byan absorbed component of the central source. The absorbing col-umn and the power-law normalisation were fitted. The absorbingcolumn density is found to be N H = . + . − . × cm − . In SW-R ,a wide opening angle of θ op = ◦ and a large N H = × cm − were assumed. This results in a spectrum being reflection dom-inated since the highly Compton-thick opacity assures that thedirect light from the central source is strongly suppressed andthat it has a negligible contribution to the observed spectrum. Table 3.
Decomposed fluxes of the NE and SW nuclei.
Band NE SW
SW-A
SW-R
Notes.
Fluxes for the NE and SW components in the 3-10 keV and10-30 keV bands were obtained from the spectral decompositions ofthe NuSTAR data for the two spectral modelling options for SW,
SW-A, and
SW-R (see text and Table 2). The fluxes are in units of 10 − erg s − cm − . The 3-10 keV flux for the SW nucleus is for the continuumonly and it does not include the Fe K line flux, which is 0 . × − erg s − cm − . Whereas the wide θ op makes the inner wall of the torus well ex-posed to our view; however, the central source is still hiddenfrom direct view by a small margin of ∆ θ = ◦ . The only freeparameter is the power-law normalisation.These spectral modellings are good matches for the NuSTARdata with a comparable quality, as is shown in Fig. 4 and Table2. The Fe K line EW is found to be (cid:39) . ± .
23 keV with re-spect to the total continuum. We note that depending on the SWmodelling options, decomposed fluxes of the NE and SW nucleivary (Table 3).The NuSTAR data cannot distinguish between
SW-A and
SW-R . The primary reason is that, although the spectral mod-els for the SW nucleus that were expected from
SW-A and
SW-R di ff er at some degree at energies below the Fe K band (see Fig.4), this di ff erence is compensated for by varying normalisationof the NE model. Here we investigate if the Chandra data ofthe SW nucleus can tell the di ff erence between SW-A and
SW-R , albeit with low source counts. The Chandra data for the SWnucleus have only six counts in 3-7 keV and there are no countsat higher energies; the six counts are composed of four countsfor the Fe K line and two counts in the 4-5 keV continuum. The
SW-A model has a sharp decline below 6 keV and its expectedcount rate in 4-5 keV is 5 × − ct s − while the SW-R modelhas a Compton-scattered tail towards lower energies giving anexpected count rate of 5 × − ct s − . The observed count rate1 . × − ct s − lies closer to the latter. Among 1000 simulations Article number, page 4 of 7. Iwasawa et al.: Dual AGN in Mrk 266 (deg) θ i L ′ SW-A % C I Fig. 5.
Intrinsic 2-10 keV luminosity of the SW nucleus estimated forthe reflection-dominated model
SW-R , as a function of assumed incli-nation angle ( θ i ) of the obscuring torus. The intrinsic luminosity, L (cid:48) ,is in units of 10 erg s − . For θ i = ◦ -72 ◦ , θ i = θ op + ◦ is assumed.For the point for θ i = ◦ , θ op = ◦ is used as it is the upper bound ofthe etorus model. The thick horizontal line indicates the best-estimateof L (cid:48) X = . × erg s − , which was obtained by combining the X-ray and mid-IR feature based estimates (see Sect. 4.1) and the 95% CIis indicated by two dotted-lines. The L (cid:48) X value derived from the nom-inal SW-A model (see Table 2) is 0 . × erg s − , i.e. the bottom ofthe panel, but it could go up within the shaded area depending on anassumed geometry (see text) in the absorption model. of Chandra data based on the absorption model, there were noincidences of two or more counts in 4-5 keV, which, in contrast,occur for 19% of simulations under the reflection model. Thisseems to favour the refelction model unless the 4-5 keV countscome from any other external sources, for example, X-ray binaryemission from a starburst, which cannot be ruled out.Estimating an intrinsic 2-10 keV luminosity, L (cid:48) X , of an ab-sorbed X-ray source is straightforward when its absorption opac-ity is significantly smaller than a Thomson depth or N H (cid:28) cm − . For example, in the case of the NE X-ray source, L (cid:48) X (NE)is (3.8-4.4) × erg s − , depending on the choice of the spectralmodel for the SW nucleus. In the Compton-thick regime, the ab-sorption correction has a dependency on an assumed torus geom-etry (Matt et al. 1999). This applies to the absorption-dominatedmodel ( SW-A ) for the SW nucleus. In our assumed torus ge-ometry, which has a large covering fraction (Table 2), L (cid:48) X (SW) = . × erg s − . However, a similar absorption-dominatedspectrum can be obtained with a smaller covering torus with, forinstance, θ op = ◦ if the torus is precisely edge-on ( θ i (cid:39) ◦ ). Inthis configuration, since less light is Compton-scattered into theline of sight from a small-covering torus than from that of large-covering, the (absorbed) illuminating source has to be more lu-minous, resulting in a factor of ∼ L (cid:48) X (SW) of 3 . × erg s − . This is a special configuration and it sets the upper boundof L (cid:48) X for SW-A .Uncertainty in the intrinsic luminosity estimate is even largerwhen observed light is reflection-dominated as no direct lightfrom the obscured source is visible. In the
SW-R model, θ op = ◦ and θ i = ◦ are assumed, but as long as θ i = θ op + ◦ ismaintained, θ op = ◦ -70 ◦ gives a similar quality of fit. However,since the visible surface of the torus inner wall becomes small asthe torus inclination θ i increases (while the spectral shape of re-flected light remains similar), the illuminating source needs tobe more luminous to produce the same reflection luminosity andvice versa, as is shown in Fig. 5. For θ i = ◦ , the instrinsic Table 4.
AGN luminosities of the NE and SW nuclei. log L (cid:48) X log L AGN log λ Edd log ( L AGN / L IR )NE 41.60 42.80 − . − . − . − . Notes.
The best-estimates for the intrinsic 2-10 keV luminosity L (cid:48) X andX-ray based AGN bolometric luminosity L AGN in units of erg s − , Ed-dington ratio λ Edd , and the AGN luminosity and IR luminosity ratio forthe NE and SW nuclei. See text and Fig. 5 for details on the estimatesof L (cid:48) X , their uncertainties, and the bolometric corrections. luminosity was estimated to be L (cid:48) X = . × erg s − , but itcould be smaller if θ i were smaller. At a given θ i , decreasing θ op also makes the visible reflection surface smaller and thus pushesthe illuminating luminosity up. However, if θ op is too small, itwould loose the quality of fit for the NuSTAR data due to a mis-match in the Fe K absorption edge depth; a small θ op results ina relative increase in reflection light coming through the torusover directly visible reflection leading to a deeper Fe K edge,which overestimates the observed edge depth. For θ i = ◦ , weobtained a constraint of θ op > ◦ (the 90% lower limit). By de-creasing θ op , the data start to favour the absorption-dominatedspectrum with a smaller N H , as is similar to what was obtainedin SW-A . Thus, although there is a small forbidden area in theparameter space, various combinations of the torus parametersare possible to describe the NuSTAR spectrum. Accordingly, the2-10 keV intrinsic luminosity of the obscured nucleus in SW, L (cid:48) X (SW), could range from 2 × erg s − to 4 × erg s − .
4. Discussion
Our analysis of the NuSTAR spectrum suggests that the SW nu-cleus contains a Compton-thick nucleus, which is intrinsicallymore luminous than the NE nucleus, as speculated by MIV12.Before examining their AGN properties in turn, some basicparameters of the two nuclei are summarised. The black holemasses for the two nuclei, which were estimated from their hostspheroid luminosities, are similarly M BH = × M (cid:12) , for whichthe Eddington luminosity is L Edd ∼ × erg s − . The IR lu-minosity estimated for the NE and SW nuclei are L IR (NE) = . × erg s − and L IR (SW) = . × erg s − . The AGNbolometric luminosity estimated from X-ray L AGN = κ L (cid:48) X where κ is the X-ray bolometric correction, along with the Eddingtonratio λ Edd , and the AGN and IR luminosity ratio L AGN / L IR foreach nucleus, as discussed below, are given in Table 4. As shown in Sect. 3, the SW nucleus is likely a Compton-thickAGN with N H ≥ × cm − , but the intrinsic X-ray luminosity L (cid:48) X of the heavily obscured source remains uncertain between 2 × erg s − and 4 × erg s − . AGN-related mid-IR diagnosticsseem to have a close relation to AGN bolometric power (e.g.Gruppioni et al. 2016). Since some of them have been reportedto have a good correlation directly with intrinsic X-ray emission,we examine their predictions for L (cid:48) X below.On account of being dust re-radiation from an obscuringtorus, the mid-infrared continuum emission of AGN has beenfound to tightly correlate with the intrinsic 2-10 keV luminosity(Lutz et al. 2004; Horst et al. 2008; Gandhi et al. 2009; Mateoset al. 2015). However, one di ffi culty is isolating an AGN compo-nent from other continuum sources at the same wavelength. In Article number, page 5 of 7 & A proofs: manuscript no. ki_v2
Mrk 266, the available mid-IR photometric data were obtainedfrom apertures with greater than a few arcsecs of the Spitzer-IRAC or IRAS, and we faced the di ffi culty mentioned above.First, we directly adopted the available photometric data, whichgives an upper limit on L (cid:48) X . From the SED given in Fig. 9 ofMIV12, luminosities of the SW nucleus at 6 µ m and 12 µ m wereestimated to be L = . × erg s − and L = . × erg s − , respectively. Using the correlation for 6 µ m by Mateoset al. (2015), L (cid:48) X (cid:39) × f AGN erg s − was obtained, where f AGN denotes the AGN fraction at the mid-IR wavelength. Sim-ilarly, the correlation for 12 µ m of Gandhi et al. (2009) gave L (cid:48) X (cid:39) × f AGN erg s − . We note that Mateos et al. (2015)and Gandhi et al. (2009) used distinct samples of AGN and theirscaling-relations were calibrated independently. The AGN frac-tion, f AGN , in the SW nucleus was estimated using various mid-IR diagnostics (MIV12; Díaz-Santos et al. 2017) but the ob-tained values of f AGN spread widely, likely due to aperture ef-fects. Here, we tentatively adopt the median of the six diagnos-tics, f AGN = .
35, which yields L (cid:48) X estimates of 0 . × erg s − and 1 . × erg s − , based on L and L , respectively.Another X-ray luminosity estimator is the [O iv ] λ . µ memission line, which also shows a tight correlation with L X (Meléndez et al. 2008; Rigby et al. 2009; Diamond-Stanic et al.2009; Liu et al. 2014). As [O iv ] is a high-excitation line, whichlikely arises from AGN irradiation, it should be largely freefrom external contamination (Pereira-Santaella et al. 2010), un-like the continuum emission; therefore, it directly scales with theAGN luminosity. Being a mid-IR line, it is also relatively robustagainst dust extinction. Therefore it is deemed to give a reliableestimate of the AGN intrinsic luminosity. The [O iv ] flux mea-sured with the IRS aperture only for the SW nucleus is available(MIV12; Bernard-Salas et al. 2009), which gives [O iv ] lumi-nosity of 8 × erg s − . The [O iv ]- L (cid:48) X correlation of Liu et al.(2014) for Seyfert 2 galaxies gives L (cid:48) X (cid:39) . × erg s − .The above three predictions for L (cid:48) X from the mid-IR fea-tures all lie around ∼ × erg s − and within the rangeinferred by the modelling of the NuSTAR data. This supportsthe reflection-dominated spectrum of SW-R for modelling thedata (see Fig. 5). By combining these estimates, we lookedinto the most likely value of L (cid:48) X (SW) and its uncertainty andthen assessed how probable the reflection-dominated hypothe-sis might be. We label the X-ray and the mid-IR estimates as E i ( i = , , , µ m based, E2 is 12 µ m based, and E3 is [O iv ] based. In E0,we assume that log L (cid:48) X is equally likely between 42.3 and 43.6[erg s − ], or E0 ∼ Uni f (42 . , . L (cid:48) X ( ≡ µ i )follows a Gaussian distribution of E i ∼ N ( µ i , σ i ) ( i = , , ∼ N (42 . , . ∼ N (43 . , . ∼ N (43 . , . f AGN to σ and σ in addition to the scatter of the correlations. Theposterior probability (cid:81) E i gives the most likely estimate of log L (cid:48) X = .
13 (the 95% compatible interval (CI): 42.7-43.6). Thisis illustrated in Fig. 5. The L (cid:48) X range that can be obtained in SW-A lies below the 95% CI, indicating that
SW-R is preferred and areflection-dominated spectrum is a more probable description ofthe NuSTAR data for the SW nucleus.Assuming a bolometric correction of 20 (Marconi et al.2004), the L AGN (SW) is found to be 2 . × erg s − , based onthe best-estimate of L (cid:48) X . The Eddington ratio is then 0.9%, which is typical of Seyferts. The luminosity ratio L AGN / L IR (cid:39) . L AGN is all absorbed by circumnuclear dust and reradiatedin the IR.
The X-ray luminosity of the NE nucleus is relatively low, about4 × erg s − . Although the hard Chandra spectrum indicates asignificant AGN contribution in the Chandra band (Torres-Albàet al. 2018) since intense star formation is also taking place inthe NE nucleus, we first examine how much the star-formingactivity might contribute to the observed hard X-ray emission.At energies above 3 keV, starburst emission mainly comes fromhigh-mass X-ray binaries. Their collective X-ray luminosity canbe estimated using the empirical relation with the star forma-tion rate (Grimm et al. 2003; Ranalli et al. 2003; Lehmer et al.2010). The star formation rate of the entire Mrk 266 system is65 M (cid:12) yr − (Howell et al. 2010). By scaling with L IR , we esti-mate the star formation rate of the NE nucleus to be 15 M (cid:12) yr − .Using the formula of Lehmer et al. (2010) and given the star for-mation rate, L X , is dominated by high-mass X-ray binaries andgiven the contribution of low-mass X-ray binaries is minor, theexpected 2-10 keV luminosity from X-ray binaries is found to be2 . × erg s − or ∼ ≤ ∼
25% higherthan that of XMM-Newton and NuSTAR. Although the increaseis marginal, X-ray variability would support the dominance ofAGN emission.The Eddington ratio of NE is rather low ( λ Edd ∼ × − ).Among nearby AGN, the λ Edd range around this value is pop-ulated by LINERs and low-luminosity Seyferts (Ho 2008).Although the radio to mid-IR flux-density ratio, q obs = log( S . / S µ m ) = .
1, is consistent with those of star form-ing galaxies (Sargent et al. 2010), the radio source is compactand unresolved at HPBW = . (cid:48)(cid:48) × . (cid:48)(cid:48) ffi cient accretion flow, suggested by the low λ Edd ,producing the radio emission. The observed radio flux of the NEnucleus is comparable with that of the SW nucleus with similarspectral slopes: − . − . L X is ∼ . ffi cient accretionflow (Ho 2008).When the bolometric correction κ =
16, suggested forLLAGN by Ho (2008) is used, L AGN = × erg s − is ob-tained. The L AGN / L IR is therefore ∼ f AGN ∼
60% obtained from the mid-IR diag-nostics (MIV12). However, the Spitzer IRS spectrum of the NEnucleus alone was taken only from the SL module and it lacksthe longer wavelength coverage where the key AGN diagnosticlines [Ne v ] λ . , . µ m and [O iv ] λ . µ m are present. Thisleaves the PAH strengths as the only mid-IR AGN diagnosticsthat may be biased. Given the low accretion rate inferred fromthe X-ray observations, strong outflows are expected from the Article number, page 6 of 7. Iwasawa et al.: Dual AGN in Mrk 266
AGN in the NE nucleus, which fit observed optical and radiosignatures pointed out by MIV12. The AGN energy output ofthe NE nucleus may be dominated by the mechanical power ofthose outflowing winds.
Mrk 266 exhibits properties that have close relevance to themerger-driven formation of dual AGN and their detectability.Two important characteristics of this merger system are 1) aclose nuclear separation of 6 kpc in projection; and 2) the massratio of the host spheroids are close to unity and thus, presum-ably, as is the black hole mass ratio. These two are required con-ditions for forming dual AGN deduced from cosmological sim-ulations (Steinborn et al. 2016; Volonteri et al. 2016, see alsoe.g. Hopkins et al. 2006; Solanes et al. 2019). As a substantialamount of cold molecular gas ( ∼ a few 10 M (cid:12) ) is available be-tween the galaxies (Imanishi et al. 2009), this reservoir providesindividual galaxies with gas for further accretion, helping thesystem evolve into an ULIRG, as argued by MIV12. However,it is interesting to note that the two AGN appear to have distinctcharacteristics: The SW nucleus is (cid:39)
30 times more luminousthan the NE nucleus and they are possibly accreting in di ff er-ent modes as discussed above, suggesting that the accretion con-ditions might actually be unbalanced between the galaxies. Interms of detectability of dual AGN, heavy obscuration certainlyplays a role. Ricci et al. (2017) found an elevated proportion ofCompton-thick AGN in advanced mergers with nuclear separa-tions of <
10 kpc. Mrk 266 fits in the merger stage and the SWnucleus is indeed a Compton-thick AGN, which had not beenverified until the NuSTAR observation. The spatial resolutionof Chandra and the hard X-ray sensitivity of NuSTAR comple-ment each other, helping to identify a Compton-thick AGN inadvanced merger systems such as Mrk 266 and Mrk 273. A mul-tiwavelength approach, as employed by GOALS, is obviouslye ff ective at overcoming the obscuration issue (e.g. Hickox &Alexander 2018), which would naturally be expected in lumi-nous merger systems. We refer readers to an extensive review ofboth observational and theoretical aspects of dual AGN by DeRosa et al. (2019) who further discuss related X-ray observa-tional works on samples that were selected using various tech-niques (e.g. Koss et al. 2010, 2012; Comerford et al. 2015; Satya-pal et al. 2017). Acknowledgements.
This research made use of data obtained from NuSTAR,XMM-Newton and Chandra X-ray Observatory, software packages of HEASoftand R (R Core Team 2017), and the NASA / IPAC Extragalactic Databases (NED),which is funded by the National Aeronatutics and Space Administration and op-erated by the California Institute of Technology. KI acknowledges support by theSpanish MICINN under grant PID2019-105510GB-C33. T.D-S. acknowledgessupport from the CASSACA and CONICYT fund CAS-CONICYT Call 2018.C.R. acknowledges support from the Fondecyt Iniciacion grant 11190831. ASE,GCP and KI acknowledge NASA Astrophysics Data Analysis Program (ADAP)Grant 80NSSC20K0450 (PI: U).
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