The Luminous Infrared Composite Seyfert 2 Galaxy NGC 7679 Through

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22 2 with column density of NH > 10 cm− and these values range Table 1. Observiation details 22 2 24 2 from 10 cm− to higher than 10 cm− for about96 % of this class of objects (Risaliti et al. 1999, Bassani et al. 1999). The circumnuclear starburst should also play a major role in the ob- scuration processes – see for details Levenson et al. (2001) and image interference Fabry-Perot frames frame filtera) tuned references therein. However, there are Sy2 galaxies with col- × 22 2 wavelength exposure umn densities lower than 10 cm− . Panessa & Bassani (2002, hereafter PB02) present a sample of 17 type 2 SyGs showing λc/FWHM λFP time such low absorption in X-rays. The Compton thin nature of (Å)/(Å) (Å) (s) these sources is strongly suggested by some isotropic indica- tors such as FIR and ø3 emission. Hα 6662/55 6674.8 1 1800 / × The fraction of Composite Seyfert Starburst objects is esti- 2 900 mated to be in the range of 10% - 30% of the Sy2 population. × [N II]λ6548 6662/55 6659.9 1 900 The simple formulation of the Unified model for SyGs is not × applicable in such sources. The observed absorption is likely continuum 6719/33 6720.0 1 1800 × to originate at larger scales instead in the pc-scale molecular 1 900 torus. Probably the Broad Line Regions (BLRs) of these ob- × ø3 λ5007 5094/44 5092.4 2 900 jects are covered by some obscuring dusty material. × continuum 5002/41 4437.7 1 1200 × NGC 7679 is a nearby (z = 0.0177) nearly face-on SB0 Gunn rb) 6800/1110 1 60 Seyfert 2 type galaxy in which starburst and AGN activities co- × BG 39/2b) 4720/700 2 1500 exist. The IRAS fluxes show that the luminosity of NGC 7679 × 11 in the far infrared is about LFIR 10 L . This object is in- cluded in the large spectroscopic survey≈ of⊙ 200 luminous IRAS a) Used to separate Fabry-Perot working orders galaxies (Kim et al. 1995, Veilleux et al. 1995). NGC 7679 is b) Broad-band image taken without Fabry-Perot to reveal the mor- physically associated by a common stream of ionized gas with phology the Sy2 galaxy NGC 7682 at 4.5 arcmin eastward (PA ∼ ≈ 72◦) forming the pair Arp 216 (VV 329). The tidal interac- tions between both galaxies together with the existence of a bar in NGC 7679 could enhance the gas flow towards the nu- 2. Observations and data reduction clear regions and possibly trigger the starburst processes (Gu et NGC 7679 was observed by K. Jockers, T. Bonev, and T. al. 2001). Credner with the 2m RCC reflector of the Peak Terskol Observatory, Caucasus, Russia. The observations were carried The X-ray properties of the NGC 7679 based on the out in October 1996 with the Two-channel Focal Reducer of BeppoSAX observations and on the ASCA archive were dis- the former Max-Planck-Institut für Aeronomie, Germany (now cussed by Della Ceca et al. (2001, hereafter DC01). Their Max-Planck-Institut für Sonnensystemforschung, MPS). This conclusion is that NGC 7679 is a Seyfert-starburst composite instrument was primarily intended for cometary studies but it galaxy which implies the clear predominance of an AGN in the has repeatedly been used for observations of active galactic nu- X-ray regime connected with a starburst in the optical and IR clei (see for example Golev et al. 1995, 1996, and Yankulova regime. DC01 found that a simple power-law spectral model 1999). The technical data and the present capabilities of the with Γ 1.75 and small intrinsic absorption (N < 4 1020 H MPS Two-channel Focal Reducer are described in Jockers cm 2) provides∼ a good description of the spectral properties× of − (1997) and Jockers et al. (2000). NGC 7679 from 0.1 to 50 keV. The small X-ray absorption All observations were taken in Fabry-Perot (FP) mode us- and the absence of strong (EW 1 keV) Fe-lines suggest a ing tunable FP narrow-band imaging with spectral FWHM of Compton thin type 2 AGN in NGC∼ 7679 which clearly distin- the Airy profile δλ in order of 3 - 4 Å. The details of obser- guishes this galaxy from the other LIG Seyferts. vations are presented in Table 1 where the central wavelengths The main goal of this article is to investigate both gas distri- λc and the effective width ∆λ of the interference filters used bution and ionization structure in the circumnuclear regions of to separate the Fabry-Perot interference orders, the wavelength the luminous IR unabsorbed Seyfert galaxy NGC 7679 and to λFP at which the Fabry-Perot was tuned, and the exposures are look for tracers of the presence of a hidden Sy1-type nucleus. listed. The overall “finesse” of the system ∆λ/δλ is 15, ∆λ is Some information on the observations and data reduction the free spectral range of the FP. As one can see from≈ Table 1 procedures is presented in Section 2. The results are presented ∆λ is comparable to the filter’s band width and therefore all FP in Section 3 and discussed in Section 4. The combination of orders except the central one are efficiently suppressed. Two the data taken from recent literature and our Fabry-Perot ob- exposures of NGC 7679 were obtained through each filter to servations provides new insight in the circumnuclear region of eliminate cosmic ray events and to increase the signal-to-noise NGC 7679 and in the phenomena occurring there. ratio. Flatfield exposures were obtained using dusk and dawn Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ... 3

Fig. 1. Contours of continuum-subtracted narrow-band Fig. 2. Contours of continuum-subtracted narrow-band Hα- ø3 λ5007-image superimposed on the gray-scale ø3 image superimposed on the gray-scale Hα-emission distribu- λ5007-emission distribution of the circumnuclear region tion of the circumnuclear region of NGC 7679. The back- 17 18 2 1 2 of NGC 7679. The background noise level is σ = 2.01 10− ground noise level is σ = 2.77 10− ergs cm− s− arcsec− . 2 1 2 × × ergs cm− s− arcsec− . The outermost contour is taken at 5σ The outermost contour is taken at 5σ above the sky level and above the sky level and the next contours increase by a factor the next contours increase by a factor of √2. North is up, East of √2. Note East-West elongation and two extrema decentered is to the left. of about 4 arcsec from the position of the nucleus marked by cross.∼ North is up, East is to the left. twilight for uniform illumination of the detector. No dark correction was required. The images were reduced following the usual reduction steps for narrow-band imaging. After flatfielding the frames were aligned by rebinning to a common origin. The final align- ment of all the images was estimated to be better than 0.1 px (the scale is 1 px = 0.8 arcsec). A convolution procedure was performed in order to match the Point-Spread Functions (PSFs) of each line-continuum pair which unavoidably degrades the final FWHM of the images to the mean value 3 3.3 arcsec (shown as ’seeing’ in Fig. 1). At the distance≈ of− NGC 7679 one arcsec corresponds to a distance of about 340 pc assuming 1 1 H0 = 75 km sec− Mpc− .

3. Results 3.1. Narrow-band emission-line images Fig. 3. Contours of continuum-subtracted narrow-band Gray-scale images of the narrow-band ﬂux distribution of the [N II]λ6548-image superimposed on the gray-scale extended circumnuclear region of NGC 7679 in the ø3 λ5007, [N II]λ6548-emission distribution of the circumnuclear region Hα, and [N II]λ6548 emission lines with superimposed con- of NGC 7679. The background noise level is σ = 4.75 10 18 tours are presented in Fig. 1, 2, and 3, respectively. − ergs cm 2 s 1 arcsec 2. The outermost contour is taken× at 5σ The ø3 λ5007 emission shown in Fig. 1 reveals a bright, − − − above the sky level and the next contours increase by a factor about 20 arcsec in size, extended emission-line region (EELR) of √2. North is up, East is to the left. which is elongated approximately in East direction (PA 80 ≈ ◦± 10◦). This region is similar to the analogous EELRs observed in many Sy2 type galaxies. Most probably it is powered by the 4 Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ...

AGN-type activity of the nucleus. The emission-line peak of ø3 in Table 2. In this Table we have collected available measure- λ5007 is shifted at about 4 arcsec to the East with respect to ments of the emission lines observed by us up to now. Our the center defined by the∼ continuum emission and marked by measured fluxes are in good agreement with those of Kim et cross in Fig. 1. al. (1995) and differ from the measurements of Contini et al. At larger distances ( 37 arcsec) the ionized gas forms an (1998). Flux values given by Contini et al. (1998) are twice ∼ envelope which is extended along the direction PA 72◦ to larger than ours and those given by Kim et al. (1995). the NGC 7682, the counterpart of NGC 7679, as it was≈ already Recently Gu et al. (2006) measured the central flux in ø3 noted by Durret & Warin (1990). λ5007. We found a reasonable coincidence between their value 14 2 1 14 2 In Fig. 2 we present our very deep and high-contrast Hα (1.55 10− ergs cm− s− ) and ours (1.94 10− ergs cm− 1 × × continuum-subtracted image with numerous starburst regions s− ) in the much smaller aperture used by them. where because of both seeing and pixel size we are able to We estimated the flux of the continuum near ø3 λ5007 15 2 see only elliptical central isophotes instead of the “double nu- within the central 2 kpc to be F(λcont) = 6.74 10− ergs cm− 1 × cleus” observed recently by Buson et al (2006). Our analysis s− Å. Then the equivalent width of the emission line ø3 λ5007 of the unpublished Hα images taken from the archive of the is EW(λ 5007) = 7.6 Å. Baskin and Loar (2005) have used the Isaak Newton Group of telescopes at La Palma as well as the photoionization code CLOUDY to calculate the dependence of archive images of Buson et al. (2006) from the ESO La Silla EW(λ 5007) on the electron density ne, the ionization parame- NTT also revealed a “double nucleus” otherwise unseen in the ter U, and the covering factor CF. Following their Fig. 5 and known broad-band images. The separation between the nuclear our estimation of EW(λ 5007) we derive for the covering factor counterparts (in fact one is the active nucleus itself and the CF the range 0.016 CF 0.04 with the most probable value ≤ ≤ other one is a bright spiral-like extremely powerful starburst CF 0.024. ≈ region) is 3 arcsec. The existence of this “double nucleus” in There is a large quantity of absorbing matter in the central ≈ NGC 7679 could enhance the gas flows towards the nuclear region of NGC 7679 (Telesco et al. 1995) which modifies the regions and possibly trigger the starburst process itself. The Balmer emission lines. The Balmer decrement reported by Kim “double nucleus” can be also seen at very different wavelength et al. (1995)in the central2kpcis F(Hα)/F(Hβ) 17.4,but fol- rangeon 6 cm and 20 cm high-resolutionVLA radio continuum lowing Contini et al. (1998) this decrement is 8.5.≈ Kewley et al. map of NGC 7679 published by Stine (1992). The angular dis- (2000) give E(B V) = 0.47 which results to F(Hα)/F(Hβ) = tance and PA between two counterparts is quite the same. The 5.04. In Table 2 the− value of the parameter C is evaluated from radio spectral index is 0.37 and steepens away from the cen- the measured Balmer decrement and from the assumption that − ter which indicates that nonthermal emission leaks out of the in AGNs F(Hα)/F(Hβ) = 3.1 and the optical depth τλ = C f (λ) starburst region. where f (λ) is the reddening curve (Osterbrock 1989). The ex- The low-excitation gas traced by the emission in Hα re- tinction E(B V) derived from the Balmer decrement is also − veals differentmorphologyas compared to that of the ø3 λ5007 given in Table 2. emission. Inside of the region with radius of 6 – 8 arcsec from Contini et al. (1998) present measurements of emission- the center the contours of the Hα emission are nearly circular. lines fluxes made in the extranuclear region 9 arcsec off the nu- Outside this region to the West of the main body of NGC 7679 cleus at PA = 207◦ in an aperture of 3 arcsec. We estimated the a clearly outlined wide arc is observed at 16 arcsec ( 5 kpc) emission-line fluxes from our images in the same aperture at ∼ from the center. To the East this arc converts into a gaseous the same place in order to compare with those given by Contini envelope which forms a part of a circumnuclear starforming et al. (1998). The results are given in Table 2. The Contini’s ring mentioned by Pogge (1989). This arc is not detected on values are about 2 times larger than ours in the extranuclear the narrow-band continuum image next to the Hα. The same region as well as at the nucleus. morphology in Hα + [N II] with higher spatial resolution was Moustakas & Kennicutt (2006) report total emission-line observed by Buson et al. (2006). fluxes of Hα and ø3 λ5007 in a wide rectangular aperture The Fabry-Perot technique used by us makes possible to 30 80 arcsec oriented at PA = 90◦. Their Hα-flux F(Hα) = × 12 2 1 disentangle [N II]λ6548 from Hα. The pure [N II]λ6548 emis- (1.535 0.062) 10− ergs cm− s− coincides with our value sion (Fig. 3) shows extended structure 20 arcsec in diameter. (1.52 ±10 12 ergs× cm 2 s 1) in the same wide aperture after a ∼ − − − The starformingring revealed by the Hα image is not seen here. correction× for extinction with E(B V) = 0.065 used by them. − 13 As a rule the gas component in the starforming ring is ionized In ø3 λ5007 the coincidence is reasonably good (4.72 10− 13 2 1 × by stellar UV-emission and the [N II]λ6548 is weaker than that compared with ours 3.90 10− ergs cm− s− ). one where the gas is ionized by power-low AGN continuum. × On the other hand, this could be an effect due to the shorter exposure time of our [N II]λ6548 frame. 3.3. The ionization map F(ø3 λ5007) / F(Hα) Our flux-calibrated emission-line images are used to form the 3.2. Narrow-band emission-line total fluxes F(ø3 λ5007)/F(Hα) ionization map in order to analyse the mean level of ionization. This map is shown in the left panel of The total emission-line fluxes of Hα, ø3 λ5007and [N II]λ6583 Fig. 4. All pixels below 4σ of the background noise level were were estimated from our flux calibrated images in an aperture suppressed before the division of the corresponding images. of 2 kpc (r < 3 arcsec) like the one used by the authors cited The ionization map infers a presence of a maximum shifted ∼ Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ... 5

Table 2. Measured emission lines ﬂuxes in 2 kpc central aperture in NGC 7679

2 1 Emission Measured ﬂux F(λ), ergs cm− s− 2 kpc central aperture 9 arcsec oﬀ the nucleus 1 2 3 456 7

13 13 13 13 14 14 Hα 1.92 10− 1.9 10− 4.5 10− – 3.8 10− 3.73 10− 1.04 10− × × × × × × 14 13 13 15 15 [N II]λ6548 9.96 10− 1.08 10− 1.86 10− – – 9.8 10− 4.5 10− × × × × × 14 14 14 15 15 ø3 λ5007 5.2 10− 5.3 10− 8.8 10− – – 9 10− 4.6 10− × × × × × 14 14 14 15 Hβ – 1.1 10− 5.24 10− 1.0 10− – 5.9 10− – × × × × F(Hα)/F(Hβ) – 17.4 8.5 5.0 4.58 6.3 – F(Hγ)/F(Hβ) – 0.24 0.32 0.4 0.3 – –

C – 4.93 2.88 1.6 1.12 2.02 – E(B V) – 1.45 0.85 0.47 0.33 0.65 – −

Columns: 1 - this work; 2 - Kim et al. (1995); 3 - Contini et al. (1998); 4 - Kewley et al. (2000); 5 - Buson et al. (2006); 6 - Contini et al. (1998); 7 - this work, PA = 207◦.

Fig. 4. F(ø3 λ5007) / F(Hα) ionization map of NGC 7679. All pixels below 4σ of the background noise level were suppressed before image division (left). The ratio F(ø3 λ5007)/F(Hα) vs the axial distance from the nucleus along PA 80◦ (right). The positions labeled 1 to 5 are equidistant with step size of 3 arcsec. We refer to them later in the text (see Fig. 6).≈

to the East at PA 80◦ with respect to the photometric cen- 4. Discussion ter defined by the≈ integral light of the continuum images and marked by cross on the figure. 4.1. The ionizing flux from the central engine In order to estimate the number of ionizing photons emitted A slice of this map along the PA 80◦ versus the axial dis- from the central engine, we made use of the recentX-ray obser- tance from the nucleus is presented in≈ the right panel of Fig. 4. vations of NGC 7679. This object was observed by ASCA and Below we will discuss in more detail the behaviour of the ion- BeppoSAX in 1998, and by XMM-Newton in 2005. A detailed ization at positions 1 to 5. analysis of ASCA and BeppoSAX data sets is present in DC01. 6 Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ...

They show that a single absorbed power-law function (with a not infer any preferred direction. In contrast, the ø3 λ5007 im- photon index 1.75) fits the observed spectrum very well and the age (Fig. 1) of the circumnuclear region of NGC 7679 shows 20 2 X-ray absorption is relatively small (NH 4 10 cm− ). elliptical isophotes extended along the PA 80◦ 10◦. Such The data for the X-ray observations≤ in× 2005 were taken difference in morphology of the emission-line≈ images± signals from the XMM-Newton public archive. The corresponding X- the presence of at least two distinct ionization components (see ray spectra for the PN and the two MOS detectors were ex- for example Pogge 1989). tracted following the standard procedures using the XMM- The extended morphology both of the ø3 λ5007 image Newton Science Analysis System software (SAS version (Fig. 1) and of the ø3 λ5007/Hα flux ratio image (Fig. 4) sug- 7.0.0). A single absorbed power-law function gave a good fit gests an anisotropy of the radiation field. In order to check (χ2/do f = 201/191) to all the three spectra which were fitted whether the ionizing field is collimated or not we have to com- simultaneously. The small X-ray absorption in the nucleus of pare the number of ionizing photons Nph, absorbed by the ex- NGC 7679 was confirmed, N = 5.6[4.0 7.5] 1020 cm 2, tended emission line gas with the number of ionizing photons H ÷ × − and no change in the shape of the spectrum was found, a pho- Nion, emitted by the central AGN engine. Usually, the hydro- ton indexof 1.81[1.70 1.92](the 90%-confidence intervals are gen line flux F(Hα ) or F(Hβ ) is used to find Nph. But the given in brackets). The÷ absorbing X-ray column density along NGC 7679 high resolution Hα image reveals a central circum- the line of sight is about an order of magnitude smaller than nuclear star-forming spiral ring capable of producing about that one estimated from the observed Balmer decrement which 75% of the optical line emission within a radius of 1kpc∼ is N 8 1021 cm 2 and 5 1021 cm 2 following Kim et (Buson et al. 2006). For this reason it is not quite correct∼ to use H ∼ × − ∼ × − al. (1995) and Contini et al. (1998), respectively. the F(Hα) in order to make the Nph estimate. Interestingly, the observed X-ray flux has decreased by a Kauffmann et al. (2003) focus on the luminosity of the ø3 factor 10 over a time period of 7 years: FX = 3.8 λ5007 as a tracer of AGN activity. We can estimate the number 13 ∼ 13 2 ∼1 × 10− and 5.8 10− ergs cm− s− correspondingly in the Nph of ionizing photons with energy above hν = 55eV from 0.1-2.0 keV and× 2.0-10.0 keV energy intervals. Since, on the the observed ø3 λ5007 luminosity after correction for extinc- one hand, there is only about 5% scatter of the fluxes for all tion. A dust correction to ø3 based on the ratio F(Hα) / F(Hβ) the three detectors (one PN and two MOS) around the average should be regarded as best approximation (Kauffmann et al. values given above, and, on the other hand, NGC 7679 shows 2003). According to Draine & Lee (1984) (Fig.7 therein) the an appreciable X-ray variability (DC01), it is then likely that optical depth is τ5007 = 0.96 C = 2.76. Here we adopt the the detected decrease of the X-ray flux is real and not an instru- value of C= 2.88 followingContini et al. (1998) as a more com- mental effect. promising reddening value among the different Balmer decre- The extrapolation of the DC01’s power law to the UV spec- ment assessments. Then the luminosity, corrected for extinc- tral domain (that is to hν = 13.6 eV) yields Fnt = F (ν /ν)α tion, Lcorr([O+2]λ5007) = 4.4 1041 ergs s 1. We note that 0 ν ν0 0 × − where α = 0.75 and F = 2.0 10 28 erg cm 2 s 1 Hz 1. The PB02 give 5.7 1041ergs s 1 for the ø3 λ5007 luminosity. ν0 − − − − × − same extrapolation for the XMM-Newton× spectrum results in The total number of ionizing photons that must be available nt α 29 Fν = Fν0 (ν0/ν) where α = 0.81 and Fν0 = 2.8 10− erg to produce the observed ø3 λ5007 emission is given by the 2 1 1 × cm− s− Hz− . expression The number of ionizing photons with hν> 55eV provided α (O+2, T )Lcorr([O+2]λ5007) CF 1 by the central AGN source is defined as = G e − Nph eff α5007(ne, Te) hν5007 ∞ F nt Fnt ν 2 hν=55eV 52 1 Nion = dν = 4πRG (1) 2 10 ph s− (2) Z hν hα ≈ × 55 eV +2 12 3 1 where αG(O , Te) = 5.1 10− cm s− (Aldrovandi & × 4 where RG is the distance to the NGC 7679. For the BeppoSAX Pequignot 1973) is the recombination coefficient at Te 10 K 52 1 eff 9 3 1 ≈ data this estimation is Nion 10 ph s− and for the XMM- and α (ne, Te) = 1.1 10− cm s− is the effective recombi- 51 ∼ 1 5007 × Newton data Nion 10 ph s− . These values are averaged be- nation coefficient at n = 105 cm 3 and T = 104 K. This coeffi- ∼ e − e tween all BeppoSAX and XMM-Newton bands, respectively. cient strongly depends on the electron density and temperature. The number of ionizing photons decrease from the BeppoSAX 4 eff 3 If we accept Te = 10 K then α5007(ne) = 5.14 10− A21/ne 51 < < 3 1 1 × time to the XMM-Newton time in the range of 10 Nion cm s− where A21 = 0.021 s− . As the critical electron density 1052 ph s 1. ∼ ∼ cr 5 3 − is ne (5007) = 5 10 cm− we assume that the electron density × 4 3 is not lower than ne 10 cm− in order to emit the ø3 λ5007. ≈ 51 1 4.2. Physical conditions in the circumnuclear region of Then the lower limit for Nph is 2 10 ph s− . For NGC 7679 the covering factor CF = 0.024.≈ × NGC 7679 The photon ratio Nph/Nion is a probe of the collimation hy- The extended emission-line region in NGC 7679 has a rather pothesis. In the anisotropic case this ratio is considerably larger different morphology when observed in Hα (low ionization than 1. Under the above assumptions about ne and Te we esti- emission line) as compared to ø3 λ5007 (high ionization emis- mateforNGC7679 0.2 < (Nph/Nion)hν>55eV < 20 but the lower sion line). The Hα image (Fig. 2) contains a compact circum- limit could increase if the∼ luminosity L([O+∼2]λ5007) is inte- nuclear region ( 20 arcsec in diameter) whose isophotes do grated over the whole image. The increase of the upper limit of ∼ Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ... 7

Fig. 5. Spectral energy distribution (SED) from the radio to the X-ray band of the composite Starburst/Sy2 galaxy NGC 7679 (open diamonds). The radio values at 6 cm and 20 cm are from VLA (Stine, 1992). The X-ray band data are from ASCA and BeppoSAX (DC02 and Risaliti, 2002). Filled diamonds repre- Fig. 6. The ø3 λ5007/Hβ vs. [N II]λ6583/Hα diagnostic dia- sent recent X-ray observations taken from the XMM-Newton gram of Veilleux & Osterbrock (1987). The dashed and dotted archive. All other data are taken from NED. The SED has been theoretical lines demarcate between Starbursts and AGNs ac- compared with a normal spiral galaxy template (dotted line) cording to Kauﬀmann et al. (2003) and Kewley et al. (2001), taken from Elvis et al.(1994), with Starburst and Sy2 galaxy respectively. The line dividing between LINERs and SyGs is templates (dashed line and thin solid line) taken from Schmitt taken according to PA02. The label “Comp” indicates the re- et al. (1997), and with Sy1 galaxy template (thick solid line) gion of the diagram in which composite objects are expected to taken from Mas-Hesse et al. (1995). be found. The diagnostic value measured by us is denoted by thick triangle. See text for other designations.

4.3. Ionization structure in the circumnuclear region of the NGC 7679 this ratio is due to the XMM-Newton data which are 8 times ∼ lower than ASCA/BappoSAX ones. The ionization map (the right panel of Fig. 4) displays the clear signature of highly-excited gas. The ø3 λ5007/Hα-ratio Both the ratio (Nph/Nion)hν>55eV and the presence of weak and elusive broad Hα-wings (Kewley et al. 2000) indicate a increases in the direction of the counterpart galaxy NGC 7682 reaching a maximum of 2.5 at about 12 arcsec off the nu- hidden AGN in the NGC 7679. Contrary, the NGC 7679 X- ≈ 20 2 cleus. More than 15 years ago Durret & Warin (1990) also re- ray spectrum is not highly absorbed and NH < 4 10 cm− (see discussion in section 4.1). As a matter of fact× Bian & Gu ported about the presence of high-ionization gas in this direc- (2006) recently found a very high detectability of hidden BLRs tion (see their Fig.3a) but their result seemingly did not attract ( 85%) for Compton-thin Sy2s with higher ø3 luminosity of attention. ∼ +2 41 1 L([O ]λ5007) > 10 erg s− . On the other hand at PA 0◦ our map shows values around ø3 λ5007/Hα 0.3 and the≈ ionization in this direction is en- We have to note that NGC 7679 resembles in many respects tirely due to the≈ young hot stars. the galaxy IRAS 12393+3520. In this galaxy direct X-ray evi- The ø3 λ5007/Hβ vs. [N II]λ6583/Hα diagnostic diagram dence suggests the presence of a hidden AGN (Guainazzi et al., (Veilleux & Osterbrock, 1987) helps to delineate the different 2000). This homology can be seen in Fig. 5 where the spectral ionization mechanisms maintaining the ionization of gaseous energy distribution (SED) from the radio to the X-ray band of component in AGNs and in Starbursts. In Fig. 6 such a dia- NGC 7679 is shown. gram is shown for NGC 7679. Kewley et al. (2001) distinguish The composite nature of NGC 7679 is clearly seen. between Starbursts and AGN using a theoretical upper limit de- Whereas the starburst component dominates in the FIR-IR rived from star forming models. This limit is shown as a dotted range, the X-ray band emission is well below that of a typi- line in Fig. 6. Objects with emission-line ratios above this limit cal Sy1. The extrapolation of the power-low X-ray spectrum to cannot be explained by any possible combination of parame- 13.6 eV shows a much lower value than the typical Sy2 emis- ters in a star forming model. Kauffmann et al. (2003) published sion at this wavelength. This again favors the idea about a hid- an updated estimate for the starburst boundary derived from den central engine. Guainazzi et al. (2000) suppose that a dusty the SDSS observations. In Fig. 6 this boundary is shown as a ionized absorber is able to obscure selectively the optical emis- dashed line. The location of the Composites is expected to lie sion, leaving the X-rays almost unabsorbed. between these two lines (see e.g. Panessa et al., 2005). 8 Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ...

In Fig. 6 we plot the emission-line flux ratios of NGC 7679 Table 3. The photon deficiency for unabsorbed Sy2s discussed measured in an aperture of 3 arcsec in steps of 3 arcsec both by Panessa and Bassani (2002) and Panessa et al. (2005) along the PA 80◦ (with crosses) and PA= 0◦ (with diamonds). ≈ The labels 1 - 5 for PA 80◦ correspond to the labels in the right panel of Fig 4. Using≈ spectra taken from the Smithsonian Astrophysical Observatory data Center Z-Machine Archive galaxy (Nph/Nion)hν>55eV Nrec/Nion obtained with 3 arcsec slit width, we estimate the observed (lower limit) F(Hα)/F(Hβ) 5 in NLR. On Fig. 6 positions 1 and 2 at PA ∼ = 80◦ off the nucleus lie well within the region occupied by the Sy 2 galaxies. The position 5, which is at the same distance ESO 540-G001 4.2 13.0 from the nucleus but in opposite direction, is located nearly on CGCG 551-008 1.0 the dividing line. MCG -03-05-007 2.2 UGC 03134 19.5 = All points which refer to the PA 0◦ are situated between IRAS 20051-1117 1.6 1.2 Kauffmann’s and Kewley’s demarcation lines in the region of CGCG 303-017 1.3 2.0 Composites. IC 1631 0.3 In Fig. 6 we also plot with asterisks the nuclear diagnostic NGC 2992 2.0 ratios according to the data of authors presented in Table 2. The NGC 3147 0.4 thick triangle refers to the nucleus according to our measure- NGC 4565 6.7 ments under the assumption of F(Hα)/F(Hβ) = 8.5 (Contini et NGC 4579 0.2 NGC 4594 1.7 al., 1998). The large scattering of nuclear values is probably NGC 4698 0.3 due to the variations of the strength of Hβ absorption line of NGC 5033 1.3 the star-forming stellar population. MRK 273x 0.4 NGC 5995 0.4 NGC 6221 0.02 4.4. Unabsorbed SyGs with and without hidden BLRs NGC 6251 6.0 NGC 7590 0.4 The unabsorbed Sy2 galaxies with low absorption in X-rays 22 2 NGC 7679 3.4 (2.0 from our data) (NH < 10 cm− ) possess a hidden or nonhidden central engine and BLRs. We have used the ø3 λ5007 emission to test the presence of hidden or nonhidden AGN sources in unabsorbed Sy2 galaxies in the sample of PB02 (14 objects) and Panessa et al., 2005 (6 objects selected by Moran et al., 1996) in the same It is still not clear what kind of physicalprocess is related to way as it was done for NGC 7679 (Subsections 4.1 and 4.2). the presence of hidden central engines in Sy2s. PB02 suggest / We derive the ratio (Nph Nion)hν>55eV following equations (1) two scenarios for the unabsorbed Sy2s (i) the central engine and (2) under the assumtions of n 5 104 cm 3 (which is e ≈ × − and their BLR must be hidden by an absorbing medium with an order of magnitude smaller than the critical electron density high value of the A /N ratio, and (ii) the BLR is very weak or λ 4 2 V H for the ø3 5007 emission), Te 10 K, and CF 10− . These absent. assumptions refer to the inner circumnuclear≈ clouds≈ of AGNs. The ratios are presented in Table 3. For the objects discussed in Panessa et al. (2005) the most popular (i.e. as in 5. Conclusions NED) galaxy names are used. The Lcorr([O+2]λ5007) values We present a new ø3 λ5007 emission-line image of the are taken from PB02 and Panessa et al. (2005). In the case of circumnuclear region of NGC 7679 which shows elliptical NGC 7679 we have used both their and our determinations of isophotes extended along the PA 80 10 in the direction to Lcorr([O+2]λ5007). ◦ ◦ the counterpart galaxy NGC 7682.≈ The± maximum of this emis- For three objects with estimated broad Hα component sion is displaced by about 4 arcsec from the photometric center broad LHα (Panessa et al., 2005, Table 1 therein) we derive also the defined by the continuum emission. number of recombinations Nrec resulting in the Hα emission. The ratio of the quantity of ionizing photons inferred from 4 We assume Te = 10 K and CF = 1 which leads to the estima- the observed extinction corrected ø3 λ5007 luminosity to the tion of the lower limit of the value of Nrec. The Nrec/Nion lower number of ionizing photons with hν > 55eV provided by the limits are also presented in Table 3. central AGN source (N /N ) ν> 0.2 20 as well as ph ion h 55eV ≈ − One can see that 17 out of 20 objects of the unabsorbed the presence of weak and elusive Hα broad wings probably Sy2s discussed here reveal Nph/Nion)hν>55eV > 0.3. This indi- indicate a hidden AGN. cates that the central AGN sources in a considerable part of The high ionization inferred by the flux ratio ø3 λ5007/Hα the unabsorbed Sy2s are obscured. The NGC 7679 does not in the direction of about PA 80◦ 10◦ coincides with the di- make an exception and also possesses a hidden AGN engine rection to the counterpart galaxy≈ NGC± 7682. It is possible that suggested both by the ø3 λ5007 morphologyand by the photon the dust and gas in this direction has a direct view to the central deficiency. AGN engine. It suggests that starburst and dust decay in this di- Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ... 9 rection have occurred because of tidal interaction between the Jockers, K., Credner, T., Bonev, T., Kiselev, N., Korsun, P., Kulik, I., two galaxies. Rosenbush, V., Andrienko, A., Karpov, N., Sergeev, A., & Tarady, In the direction PA 0◦ the ionization is entirely caused by V. 2000, Kinematika i Fizika Nebesnykh Tel, Suppl, No. 3, 13 hot stars. ≈ Kauffmann, G., Heckman, T. M., Tremonti, C., et al. 2003, MNRAS, A large part of the unabsorbed Compton-thin Sy2s with 346, 1055 41 1 Kewley, L. J., Heisler, C. A., Dopita, M. A., Sutherland, R. Norris, R., higherø3 luminosity (> 10 erg s− ) possesses a hidden AGN ∼ Reynolds, J., & Lumsden, S. 2000, ApJ, 530, 704 source. Kewley, L. J., Heisler, C. A., Dopita, M. A., & Lumsden, S. 2001, ApJS, 132, 37 Acknowledgements. We are grateful to the referee, Lucio Buson, for Kim, D.-C., Sanders, D. B., Veilleux, S., Mazzarella, J. M., & Soifer, his valuable comments which improved both the content and the clar- B. T. 1995, ApJS, 98, 129 ity of this manuscript. Kotilainen, J. K., & Prieto, M. A. 1995, A&A, 295, 646 We would like to thank T. Bonev, Institute of Astronomy of Levenson, N., Weaver, K., & Heckman, T. 2001, ApJ, 550, 230 Bulgarian Academy of Sciences, for kindly providing the Fabry-Perot Lipari, S., Bonatto, Ch., & Pastoriza, M. 1991, MNRAS, 253, 19 observations and for useful discussions. We are grateful to S. Zhekov, Mas-Hesse, J. M., Rodriguez-Pascual, P. M., Sanz Fernandez de Space Research Institute of Bulgarian Academy of Sciences, for the Cordoba, L., Mirabel, I. F., Wamsteker, W., Makino, F., & Otani, numerous fruitful discussions and especially for the analysis of the C. 1995, A&A 298, 22 X-ray properties of NGC 7679. Moran, E. C., Halpern, J. P.,& Helfand, D. 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