The luminous infrared composite Seyfert 2 galaxy NGC 7679 through the [O
  III] 5007 emission line
Astronomy & Astrophysics manuscript no. ngc7679-paper c© ESO 2018
November 1, 2018
The luminous infrared composite Seyfert 2 galaxy NGC 7679
through the [O III] λ 5007 emission line ⋆
I. M. Yankulova1, V. K. Golev1, and K. Jockers2
1 Department of Astronomy, St Kliment Okhridski University of Sofia, 5 James Bourchier Street, BG–1164 Sofia, Bulgaria
e-mail: yan@phys.uni-sofia.bg, valgol@phys.uni-sofia.bg
2 Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, D–37191 Katlenburg-Lindau
e-mail: jockers@linmpi.mpg.de
Accepted March 29, 2007
ABSTRACT
Context. NGC 7679 (Mrk 534) is a nearby (z = 0.0177) nearly face-on SB0 luminous infrared Sy2 galaxy in which starburst and AGN
activities co-exist. The ionization structure is maintained by both the AGN power-law continuum and starburst. The galaxy is a bright X-ray
source possessing a low X-ray column density NH < 4 × 1020 cm−2.
Aims. The Compton-thin nature of such unabsorbed objects infers that the simple formulation of the Unified model for SyGs is not applicable
in their case. The absorption is likely to originate at larger scales instead of the pc-scale molecular torus. The main goal of this article is to
investigate both gas distribution and ionization structure in the circumnuclear region of NGC 7679 in search for the presence of a hidden
Sy1-type nucleus, using the ø3 λ5007 luminosity as a tracer of AGN activity.
Methods. NGC 7679 was observed with the 2m RCC reflector of the Ukraine National Astronomical Observatory at peak Terskol, Caucasus,
Russia. The observations were carried out on October 1996 with the Focal Reducer of the Max-Planck-Institut für Sonnensystemforschung,
Germany. All observations were taken with tunable Fabry-Perot narrow-band imaging with spectral FWHM of the Airy profile δλ between 3
and 4 Å depending of the used wavelength.
Results. The ø3 λ5007 emission-line image of the circumnuclear region of NGC 7679 shows elliptical isophotes extended along the PA≈ 80◦
in the direction to the counterpart galaxy NGC 7682. There is a maximum of this emission which is shifted ∼ 4 arcsec from the center defined
by the continuum emission. The maximum of ionization by the AGN power-law continuum traced by ø3 λ5007/Hα ratio is displaced by
∼ 13 arcsec eastward from the nucleus. The direction where high ionization is observed at PA≈ 80◦ ± 10◦ coincides with the direction to the
companion galaxy NGC 7682 (PA≈ 72◦). On the contrary, at PA∼ 0◦ the ionization in the circumnuclear region is entirely due to hot stars.
Conclusions. Both the ratio (Nph/Nion)hν > 55 eV ≈ 0.2 − 20 of the number Nph of photons traced by ø3 to the number Nion of high-energy
ionizing photons and the presence of weak and elusive Hα broad wings indicate a hidden AGN. We conclude that the dust and gas in the high
ionization direction PA≈ 80◦ has a direct view to the central AGN engine. This possibly results in dust/star-formation decay. A large fraction
of the unabsorbed Compton-thin Sy2s with ø3 luminosity >∼ 1041 erg s−1 possesses a hidden AGN source.
Key words. galaxies: individual: NGC 7679 (Mrk 534) – galaxies: ISM – galaxies: starburst – galaxies: Seyfert
1. Introduction
Luminous infrared galaxies (LIGs) are characterized by ex-
treme IR luminosities LIR >∼ 1011L⊙ at mid- to far-infrared
(FIR) wavelengths. In their comprehensive spectroscopic sur-
vey of LIGs Kim et al. (1995) and Veilleux et al. (1995) have
shown a clear tendency for the more luminous objects to be
more Seyfert-like. The starburst and AGN are tightly connected
phenomena and the interaction between them is a matter of de-
bate.
Send offprint requests to: Ivanka Yankulova, e-mail:
yan@phys.uni-sofia.bg
⋆ Based on observations obtained at the Peak Terskol Observatory,
Caucasus, Russia.
Based on a large spectroscopic optical survey of bright
IRAS and X-ray sources from ROSAT All Sky Survey, Moran
et al. (1996) extracted low-redshift galaxies with optical spec-
tra characterized by the HII regions and X-ray luminosities
typical of AGNs and these objects were named Composite
Seyfert/Starburst galaxies. Other similar galaxies (i.e. with
bright X-ray emission together with the clear predominance of
a starburst in the optical and IR regime) have been found also in
the deep ROSAT fields (Boyle et al. 1995, Griffiths et al. 1996)
and in the Chandra and XMM-Newton deep fields (Rosati et al.
2001).
A significant part of the observed FIR-emission of these
composites could be associated with circumnuclear starburst
events. The nuclear X-ray source there is generally absorbed
http://arxiv.org/abs/0704.0768v1
2 Yankulova I., Golev V., and Jockers, K.: The luminous infrared composite Seyfert 2 galaxy NGC 7679 through ...
with column density of NH > 10
22 cm−2 and these values range
from 1022 cm−2 to higher than 1024 cm−2 for about 96 % 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
references therein. However, there are Sy2 galaxies with col-
umn densities lower than 1022 cm−2. Panessa & Bassani (2002,
hereafter PB02) present a sample of 17 type 2 SyGs showing
such low absorption in X-rays. The Compton thin nature of
these sources is strongly suggested by some isotropic indica-
tors such as FIR and ø3 emission.
The fraction of Composite Seyfert/Starburst objects is esti-
mated to be in the range of 10% - 30% of the Sy2 population.
The simple formulation of the Unified model for SyGs is not
applicable in such sources. The observed absorption is likely
to originate at larger scales instead in the pc-scale molecular
torus. Probably the Broad Line Regions (BLRs) of these ob-
jects are covered by some obscuring dusty material.
NGC 7679 is a nearby (z = 0.0177) nearly face-on SB0
Seyfert 2 type galaxy in which starburst and AGN activities co-
exist. The IRAS fluxes show that the luminosity of NGC 7679
in the far infrared is about LFIR ≈ 1011L⊙. This object is in-
cluded in the large spectroscopic survey of 200 luminous IRAS
galaxies (Kim et al. 1995, Veilleux et al. 1995). NGC 7679 is
physically associated by a common stream of ionized gas with
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-
clear regions and possibly trigger the starburst processes (Gu et
al. 2001).
The X-ray properties of the NGC 7679 based on the
BeppoSAX observations and on the ASCA archive were dis-
cussed by Della Ceca et al. (2001, hereafter DC01). Their
conclusion is that NGC 7679 is a Seyfert-starburst composite
galaxy which implies the clear predominance of an AGN in the
X-ray regime connected with a starburst in the optical and IR
regime. DC01 found that a simple power-law spectral model
with Γ ∼ 1.75 and small intrinsic absorption (NH < 4 × 1020
cm−2) provides a good description of the spectral properties of
NGC 7679 from 0.1 to 50 keV. The small X-ray absorption
and the absence of strong (EW ∼ 1 keV) Fe-lines suggest a
Compton thin type 2 AGN in NGC 7679 which clearly distin-
guishes this galaxy from the other LIG Seyferts.
The main goal of this article is to investigate both gas distri-
bution and ionization structure in the circumnuclear regions of
the luminous IR unabsorbed Seyfert galaxy NGC 7679 and to
look for tracers of the presence of a hidden Sy1-type nucleus.
Some information on the observations and data reduction
procedures is presented in Section 2. The results are presented
in Section 3 and discussed in Section 4. The combination of
the data taken from recent literature and our Fabry-Perot ob-
servations provides new insight in the circumnuclear region of
NGC 7679 and in the phenomena occurring there.
Table 1. Observiation details
image interference Fabry-Perot frames
frame filtera) tuned ×
wavelength exposure
λc/FWHM λFP time
(Å)/(Å) (Å) (s)
Hα 6662/55 6674.8 1 × 1800
2 × 900
[N II]λ6548 6662/55 6659.9 1 × 900
continuum 6719/33 6720.0 1 × 1800
1 × 900
ø3 λ5007 5094/44 5092.4 2 × 900
continuum 5002/41 4437.7 1 × 1200
Gunn rb) 6800/1110 1 × 60
BG 39/2b) 4720/700 2 × 1500
a) Used to separate Fabry-Perot working orders
b) Broad-band image taken without Fabry-Perot to reveal the mor-
phology
2. Observations and data reduction
NGC 7679 was observed by K. Jockers, T. Bonev, and T.
Credner with the 2m RCC reflector of the Peak Terskol
Observatory, Caucasus, Russia. The observations were carried
out in October 1996 with the Two-channel Focal Reducer of
the former Max-Planck-Institut für Aeronomie, Germany (now
Max-Planck-Institut für Sonnensystemforschung, MPS). This
instrument was primarily intended for cometary studies but it
has repeatedly been used for observations of active galactic nu-
clei (see for example Golev et al. 1995, 1996, and Yankulova
1999). The technical data and the present capabilities of the
MPS Two-channel Focal Reducer are described in Jockers
(1997) and Jockers et al. (2000).
All observations were taken in Fabry-Perot (FP) mode us-
ing tunable FP narrow-band imaging with spectral FWHM of
the Airy profile δλ in order of 3 - 4 Å. The details of obser-
vations are presented in Table 1 where the central wavelengths
λc and the effective width ∆λ of the interference filters used
to separate the Fabry-Perot interference orders, the wavelength
λFP at which the Fabry-Perot was tuned, and the exposures are
listed.
The overall “finesse” of the system ∆λ/δλ is ≈ 15, ∆λ is
the free spectral range of the FP. As one can see from Table 1
∆λ is comparable to the filter’s band width and therefore all FP
orders except the central one are efficiently suppressed. Two
exposures of NGC 7679 were obtained through each filter to
eliminate cosmic ray events and to increase the signal-to-noise
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
ø3 λ5007-image superimposed on the gray-scale ø3
λ5007-emission distribution of the circumnuclear region
of NGC 7679. The background noise level is σ = 2.01 × 10−17
ergs cm−2 s−1 arcsec−2. The outermost contour is taken at 5σ
above the sky level and the next contours increase by a factor
2. Note East-West elongation and two extrema decentered
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 cor-
rection 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 fi-
nal 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
H0 = 75 km sec
−1 Mpc−1.
3. Results
3.1. Narrow-band emission-line images
Gray-scale images of the narrow-band flux distribution of the
extended circumnuclear region of NGC 7679 in the ø3 λ5007,
Hα, and [N II]λ6548 emission lines with superimposed con-
tours are presented in Fig. 1, 2, and 3, respectively.
The ø3 λ5007 emission shown in Fig. 1 reveals a bright,
about 20 arcsec in size, extended emission-line region (EELR)
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
Fig. 2. Contours of continuum-subtracted narrow-band Hα-
image superimposed on the gray-scale Hα-emission distribu-
tion of the circumnuclear region of NGC 7679. The back-
ground noise level is σ = 2.77 × 10−18 ergs cm−2 s−1 arcsec−2.
The outermost contour is taken at 5σ above the sky level and
the next contours increase by a factor of
2. North is up, East
is to the left.
Fig. 3. Contours of continuum-subtracted narrow-band
[N II]λ6548-image superimposed on the gray-scale
[N II]λ6548-emission distribution of the circumnuclear region
of NGC 7679. The background noise level is σ = 4.75 × 10−18
ergs cm−2 s−1 arcsec−2. The outermost contour is taken at 5σ
above the sky level and the next contours increase by a factor
2. North is up, East is to the left.
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
λ5007 is shifted at about ∼ 4 arcsec to the East with respect to
the center defined by the continuum emission and marked by
cross in Fig. 1.
At larger distances (∼ 37 arcsec) the ionized gas forms an
envelope which is extended along the direction PA ≈ 72◦ to
the NGC 7682, the counterpart of NGC 7679, as it was already
noted by Durret & Warin (1990).
In Fig. 2 we present our very deep and high-contrast Hα
continuum-subtracted image with numerous starburst regions
where because of both seeing and pixel size we are able to
see only elliptical central isophotes instead of the “double nu-
cleus” observed recently by Buson et al (2006). Our analysis
of the unpublished Hα images taken from the archive of the
Isaak Newton Group of telescopes at La Palma as well as the
archive images of Buson et al. (2006) from the ESO La Silla
NTT also revealed a “double nucleus” otherwise unseen in the
known broad-band images. The separation between the nuclear
counterparts (in fact one is the active nucleus itself and the
other one is a bright spiral-like extremely powerful starburst
region) is ≈ 3 arcsec. The existence of this “double nucleus” in
NGC 7679 could enhance the gas flows towards the nuclear
regions and possibly trigger the starburst process itself. The
“double nucleus” can be also seen at very different wavelength
range on 6 cm and 20 cm high-resolution VLA radio continuum
map of NGC 7679 published by Stine (1992). The angular dis-
tance and PA between two counterparts is quite the same. The
radio spectral index is −0.37 and steepens away from the cen-
ter which indicates that nonthermal emission leaks out of the
starburst region.
The low-excitation gas traced by the emission in Hα re-
veals different morphology as compared to that of the ø3 λ5007
emission. Inside of the region with radius of 6 – 8 arcsec from
the center the contours of the Hα emission are nearly circular.
Outside this region to the West of the main body of NGC 7679
a clearly outlined wide arc is observed at 16 arcsec (∼ 5 kpc)
from the center. To the East this arc converts into a gaseous
envelope which forms a part of a circumnuclear starforming
ring mentioned by Pogge (1989). This arc is not detected on
the narrow-band continuum image next to the Hα. The same
morphology in Hα + [N II] with higher spatial resolution was
observed by Buson et al. (2006).
The Fabry-Perot technique used by us makes possible to
disentangle [N II]λ6548 from Hα. The pure [N II]λ6548 emis-
sion (Fig. 3) shows extended structure ∼ 20 arcsec in diameter.
The starforming ring revealed by the Hα image is not seen here.
As a rule the gas component in the starforming ring is ionized
by stellar UV-emission and the [N II]λ6548 is weaker than that
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.
3.2. Narrow-band emission-line total fluxes
The total emission-line fluxes of Hα, ø3 λ5007and [N II]λ6583
were estimated from our flux calibrated images in an aperture
of 2 kpc (r <∼ 3 arcsec) like the one used by the authors cited
in Table 2. In this Table we have collected available measure-
ments of the emission lines observed by us up to now. Our
measured fluxes are in good agreement with those of Kim et
al. (1995) and differ from the measurements of Contini et al.
(1998). Flux values given by Contini et al. (1998) are twice
larger than ours and those given by Kim et al. (1995).
Recently Gu et al. (2006) measured the central flux in ø3
λ5007. We found a reasonable coincidence between their value
(1.55 × 10−14 ergs cm−2 s−1) and ours (1.94 × 10−14 ergs cm−2
s−1) in the much smaller aperture used by them.
We estimated the flux of the continuum near ø3 λ5007
within the central 2 kpc to be F(λcont) = 6.74×10−15 ergs cm−2
s−1Å. Then the equivalent width of the emission line ø3 λ5007
is EW(λ 5007) = 7.6 Å. Baskin and Loar (2005) have used the
photoionization code CLOUDY to calculate the dependence of
EW(λ 5007) on the electron density ne, the ionization parame-
ter U, and the covering factor CF. Following their Fig. 5 and
our estimation of EW(λ 5007) we derive for the covering factor
CF the range 0.016 ≤ CF ≤ 0.04 with the most probable value
CF ≈ 0.024.
There is a large quantity of absorbing matter in the central
region of NGC 7679 (Telesco et al. 1995) which modifies the
Balmer emission lines. The Balmer decrement reported by Kim
et al. (1995) in the central 2 kpc is F(Hα)/F(Hβ)≈ 17.4, but fol-
lowing Contini et al. (1998) this decrement is 8.5. Kewley et al.
(2000) give E(B − V)= 0.47 which results to F(Hα)/F(Hβ)=
5.04. In Table 2 the value of the parameter C is evaluated from
the measured Balmer decrement and from the assumption that
in AGNs F(Hα)/F(Hβ)= 3.1 and the optical depth τλ = C f (λ)
where f (λ) is the reddening curve (Osterbrock 1989). The ex-
tinction E(B − V) derived from the Balmer decrement is also
given in Table 2.
Contini et al. (1998) present measurements of emission-
lines fluxes made in the extranuclear region 9 arcsec off the nu-
cleus at PA = 207◦ in an aperture of 3 arcsec. We estimated the
emission-line fluxes from our images in the same aperture at
the same place in order to compare with those given by Contini
et al. (1998). The results are given in Table 2. The Contini’s
values are about 2 times larger than ours in the extranuclear
region as well as at the nucleus.
Moustakas & Kennicutt (2006) report total emission-line
fluxes of Hα and ø3 λ5007 in a wide rectangular aperture
30 × 80 arcsec oriented at PA = 90◦. Their Hα-flux F(Hα) =
(1.535± 0.062)× 10−12 ergs cm−2 s−1 coincides with our value
(1.52 × 10−12 ergs cm−2 s−1) in the same wide aperture after a
correction for extinction with E(B − V) = 0.065 used by them.
In ø3 λ5007 the coincidence is reasonably good (4.72 × 10−13
compared with ours 3.90 × 10−13 ergs cm−2 s−1).
3.3. The ionization map F(ø3 λ5007) / F(Hα)
Our flux-calibrated emission-line images are used to form the
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
Fig. 4. All pixels below 4σ of the background noise level were
suppressed before the division of the corresponding images.
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
Emission Measured flux F(λ), ergs cm−2 s−1
2 kpc central aperture 9 arcsec off the nucleus
1 2 3 4 5 6 7
Hα 1.92 × 10−13 1.9 × 10−13 4.5 × 10−13 – 3.8 × 10−13 3.73 × 10−14 1.04 × 10−14
[N II]λ6548 9.96 × 10−14 1.08 × 10−13 1.86 × 10−13 – – 9.8 × 10−15 4.5 × 10−15
ø3 λ5007 5.2 × 10−14 5.3 × 10−14 8.8 × 10−14 – – 9 × 10−15 4.6 × 10−15
Hβ – 1.1 × 10−14 5.24 × 10−14 1.0 × 10−14 – 5.9 × 10−15 –
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-
ter defined by the integral light of the continuum images and
marked by cross on the figure.
A slice of this map along the PA ≈ 80◦ versus the axial dis-
tance from the nucleus is presented in the right panel of Fig. 4.
Below we will discuss in more detail the behaviour of the ion-
ization at positions 1 to 5.
4. Discussion
4.1. The ionizing flux from the central engine
In order to estimate the number of ionizing photons emitted
from the central engine, we made use of the recent X-ray obser-
vations of NGC 7679. This object was observed by ASCA and
BeppoSAX in 1998, and by XMM-Newton in 2005. A detailed
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
photon index 1.75) fits the observed spectrum very well and the
X-ray absorption is relatively small (NH ≤ 4 × 1020 cm−2).
The data for the X-ray observations in 2005 were taken
from the XMM-Newton public archive. The corresponding X-
ray spectra for the PN and the two MOS detectors were ex-
tracted following the standard procedures using the XMM-
Newton Science Analysis System software (SAS version
7.0.0). A single absorbed power-law function gave a good fit
(χ2/do f = 201/191) to all the three spectra which were fitted
simultaneously. The small X-ray absorption in the nucleus of
NGC 7679 was confirmed, NH = 5.6[4.0 ÷ 7.5] × 1020 cm−2,
and no change in the shape of the spectrum was found, a pho-
ton index of 1.81[1.70÷1.92] (the 90%-confidence intervals are
given in brackets). The absorbing X-ray column density along
the line of sight is about an order of magnitude smaller than
that one estimated from the observed Balmer decrement which
is NH ∼ 8 × 1021 cm−2 and ∼ 5 × 1021 cm−2 following Kim et
al. (1995) and Contini et al. (1998), respectively.
Interestingly, the observed X-ray flux has decreased by a
factor ∼ 10 over a time period of ∼ 7 years: FX = 3.8 ×
10−13 and 5.8 × 10−13 ergs cm−2 s−1 correspondingly in the
0.1-2.0 keV and 2.0-10.0 keV energy intervals. Since, on the
one hand, there is only about 5% scatter of the fluxes for all
the three detectors (one PN and two MOS) around the average
values given above, and, on the other hand, NGC 7679 shows
an appreciable X-ray variability (DC01), it is then likely that
the detected decrease of the X-ray flux is real and not an instru-
mental effect.
The extrapolation of the DC01’s power law to the UV spec-
tral domain (that is to hν0 = 13.6 eV) yields F
ν = Fν0(ν0/ν)
where α = 0.75 and Fν0 = 2.0 × 10
−28 erg cm−2 s−1 Hz−1. The
same extrapolation for the XMM-Newton spectrum results in
Fntν = Fν0(ν0/ν)
α where α = 0.81 and Fν0 = 2.8 × 10
−29 erg
cm−2 s−1 Hz−1.
The number of ionizing photons with hν > 55 eV provided
by the central AGN source is defined as
Nion =
55 eV
F ntν
dν = 4πR2G
hν=55 eV
where RG is the distance to the NGC 7679. For the BeppoSAX
data this estimation is Nion ∼ 1052 ph s−1 and for the XMM-
Newton data Nion ∼ 1051 ph s−1. These values are averaged be-
tween all BeppoSAX and XMM-Newton bands, respectively.
The number of ionizing photons decrease from the BeppoSAX
time to the XMM-Newton time in the range of 1051 <∼ Nion <∼
1052 ph s−1.
4.2. Physical conditions in the circumnuclear region of
NGC 7679
The extended emission-line region in NGC 7679 has a rather
different morphology when observed in Hα (low ionization
emission line) as compared to ø3 λ5007 (high ionization emis-
sion line). The Hα image (Fig. 2) contains a compact circum-
nuclear region (∼ 20 arcsec in diameter) whose isophotes do
not infer any preferred direction. In contrast, the ø3 λ5007 im-
age (Fig. 1) of the circumnuclear region of NGC 7679 shows
elliptical isophotes extended along the PA ≈ 80◦ ± 10◦. Such
difference in morphology of the emission-line images signals
the presence of at least two distinct ionization components (see
for example Pogge 1989).
The extended morphology both of the ø3 λ5007 image
(Fig. 1) and of the ø3 λ5007/Hα flux ratio image (Fig. 4) sug-
gests an anisotropy of the radiation field. In order to check
whether the ionizing field is collimated or not we have to com-
pare the number of ionizing photons Nph, absorbed by the ex-
tended emission line gas with the number of ionizing photons
Nion, emitted by the central AGN engine. Usually, the hydro-
gen line flux F(Hα ) or F(Hβ ) is used to find Nph. But the
NGC 7679 high resolution Hα image reveals a central circum-
nuclear star-forming spiral ring capable of producing about ∼
75% of the optical line emission within a radius of ∼ 1 kpc
(Buson et al. 2006). For this reason it is not quite correct to use
the F(Hα) in order to make the Nph estimate.
Kauffmann et al. (2003) focus on the luminosity of the ø3
λ5007 as a tracer of AGN activity. We can estimate the number
Nph of ionizing photons with energy above hν = 55 eV from
the observed ø3 λ5007 luminosity after correction for extinc-
tion. A dust correction to ø3 based on the ratio F(Hα) / F(Hβ)
should be regarded as best approximation (Kauffmann et al.
2003). According to Draine & Lee (1984) (Fig.7 therein) the
optical depth is τ5007 = 0.96 C = 2.76. Here we adopt the
value of C= 2.88 following Contini et al. (1998) as a more com-
promising reddening value among the different Balmer decre-
ment assessments. Then the luminosity, corrected for extinc-
tion, Lcorr([O+2]λ5007) = 4.4 × 1041 ergs s−1. We note that
PB02 give 5.7×1041ergs s−1 for the ø3 λ5007 luminosity.
The total number of ionizing photons that must be available
to produce the observed ø3 λ5007 emission is given by the
expression
Nph =
+2, Te)L
corr([O+2]λ5007) CF−1
αeff5007(ne, Te) hν5007
≈ 2 × 1052 ph s−1 (2)
where αG(O
+2, Te) = 5.1 × 10−12 cm3 s−1 (Aldrovandi &
Pequignot 1973) is the recombination coefficient at Te ≈ 104 K
and αeff5007(ne, Te) = 1.1× 10
−9 cm3 s−1 is the effective recombi-
nation coefficient at ne = 10
5 cm−3 and Te = 10
4 K. This coeffi-
cient strongly depends on the electron density and temperature.
If we accept Te = 10
4 K then αeff5007(ne) = 5.14 × 10
−3A21/ne
cm3 s−1 where A21 = 0.021 s
−1. As the critical electron density
is ncre (5007) = 5×10
5 cm−3 we assume that the electron density
is not lower than ne ≈ 104 cm−3 in order to emit the ø3 λ5007.
Then the lower limit for Nph is ≈ 2×1051 ph s−1. For NGC 7679
the covering factor CF = 0.024.
The photon ratio Nph/Nion is a probe of the collimation hy-
pothesis. In the anisotropic case this ratio is considerably larger
than 1. Under the above assumptions about ne and Te we esti-
mate for NGC 7679 0.2 <∼ (Nph/Nion)hν>55 eV <∼ 20 but the lower
limit could increase if the luminosity L([O+2]λ5007) is inte-
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-
sent recent X-ray observations taken from the XMM-Newton
archive. All other data are taken from NED. The SED has been
compared with a normal spiral galaxy template (dotted line)
taken from Elvis et al.(1994), with Starburst and Sy2 galaxy
templates (dashed line and thin solid line) taken from Schmitt
et al. (1997), and with Sy1 galaxy template (thick solid line)
taken from Mas-Hesse et al. (1995).
this ratio is due to the XMM-Newton data which are ∼8 times
lower than ASCA/BappoSAX ones.
Both the ratio (Nph/Nion)hν>55 eV and the presence of weak
and elusive broad Hα-wings (Kewley et al. 2000) indicate a
hidden AGN in the NGC 7679. Contrary, the NGC 7679 X-
ray spectrum is not highly absorbed and NH < 4 × 1020 cm−2
(see discussion in section 4.1). As a matter of fact Bian & Gu
(2006) recently found a very high detectability of hidden BLRs
(∼ 85%) for Compton-thin Sy2s with higher ø3 luminosity of
L([O+2]λ5007) > 1041 erg s−1.
We have to note that NGC 7679 resembles in many respects
the galaxy IRAS 12393+3520. In this galaxy direct X-ray evi-
dence suggests the presence of a hidden AGN (Guainazzi et al.,
2000). This homology can be seen in Fig. 5 where the spectral
energy distribution (SED) from the radio to the X-ray band of
NGC 7679 is shown.
The composite nature of NGC 7679 is clearly seen.
Whereas the starburst component dominates in the FIR-IR
range, the X-ray band emission is well below that of a typi-
cal Sy1. The extrapolation of the power-low X-ray spectrum to
13.6 eV shows a much lower value than the typical Sy2 emis-
sion at this wavelength. This again favors the idea about a hid-
den central engine. Guainazzi et al. (2000) suppose that a dusty
ionized absorber is able to obscure selectively the optical emis-
sion, leaving the X-rays almost unabsorbed.
Fig. 6. The ø3 λ5007/Hβ vs. [N II]λ6583/Hα diagnostic dia-
gram of Veilleux & Osterbrock (1987). The dashed and dotted
theoretical lines demarcate between Starbursts and AGNs ac-
cording to Kauffmann et al. (2003) and Kewley et al. (2001),
respectively. The line dividing between LINERs and SyGs is
taken according to PA02. The label “Comp” indicates the re-
gion of the diagram in which composite objects are expected to
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
The ionization map (the right panel of Fig. 4) displays the
clear signature of highly-excited gas. The ø3 λ5007/Hα-ratio
increases in the direction of the counterpart galaxy NGC 7682
reaching a maximum of ≈ 2.5 at about 12 arcsec off the nu-
cleus. More than 15 years ago Durret & Warin (1990) also re-
ported about the presence of high-ionization gas in this direc-
tion (see their Fig.3a) but their result seemingly did not attract
attention.
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-
tirely due to the young hot stars.
The ø3 λ5007/Hβ vs. [N II]λ6583/Hα diagnostic diagram
(Veilleux & Osterbrock, 1987) helps to delineate the different
ionization mechanisms maintaining the ionization of gaseous
component in AGNs and in Starbursts. In Fig. 6 such a dia-
gram is shown for NGC 7679. Kewley et al. (2001) distinguish
between Starbursts and AGN using a theoretical upper limit de-
rived from star forming models. This limit is shown as a dotted
line in Fig. 6. Objects with emission-line ratios above this limit
cannot be explained by any possible combination of parame-
ters in a star forming model. Kauffmann et al. (2003) published
an updated estimate for the starburst boundary derived from
the SDSS observations. In Fig. 6 this boundary is shown as a
dashed line. The location of the Composites is expected to lie
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
measured in an aperture of 3 arcsec in steps of 3 arcsec both
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
obtained with 3 arcsec slit width, we estimate the observed
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
from the nucleus but in opposite direction, is located nearly on
the dividing line.
All points which refer to the PA = 0◦ are situated between
Kauffmann’s and Kewley’s demarcation lines in the region of
Composites.
In Fig. 6 we also plot with asterisks the nuclear diagnostic
ratios according to the data of authors presented in Table 2. The
thick triangle refers to the nucleus according to our measure-
ments under the assumption of F(Hα)/F(Hβ) = 8.5 (Contini et
al., 1998). The large scattering of nuclear values is probably
due to the variations of the strength of Hβ absorption line of
the star-forming stellar population.
4.4. Unabsorbed SyGs with and without hidden BLRs
The unabsorbed Sy2 galaxies with low absorption in X-rays
(NH < 10
22 cm−2) possess a hidden or nonhidden central en-
gine 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
way as it was done for NGC 7679 (Subsections 4.1 and 4.2).
We derive the ratio (Nph/Nion)hν>55 eV following equations (1)
and (2) under the assumtions of ne ≈ 5 × 104 cm−3 (which is
an order of magnitude smaller than the critical electron density
for the ø3 λ5007 emission), Te ≈ 104 K, and CF ≈ 10−2. These
assumptions refer to the inner circumnuclear clouds of AGNs.
The ratios are presented in Table 3. For the objects dis-
cussed in Panessa et al. (2005) the most popular (i.e. as in
NED) galaxy names are used. The Lcorr([O+2]λ5007) values
are taken from PB02 and Panessa et al. (2005). In the case of
NGC 7679 we have used both their and our determinations of
Lcorr([O+2]λ5007).
For three objects with estimated broad Hα component
LbroadHα (Panessa et al., 2005, Table 1 therein) we derive also the
number of recombinations Nrec resulting in the Hα emission.
We assume Te = 10
4 K and CF = 1 which leads to the estima-
tion of the lower limit of the value of Nrec. The Nrec/Nion lower
limits are also presented in Table 3.
One can see that 17 out of 20 objects of the unabsorbed
Sy2s discussed here reveal Nph/Nion)hν>55 eV > 0.3. This indi-
cates that the central AGN sources in a considerable part of
the unabsorbed Sy2s are obscured. The NGC 7679 does not
make an exception and also possesses a hidden AGN engine
suggested both by the ø3 λ5007 morphology and by the photon
deficiency.
Table 3. The photon deficiency for unabsorbed Sy2s discussed
by Panessa and Bassani (2002) and Panessa et al. (2005)
galaxy (Nph/Nion)hν>55 eV Nrec/Nion
(lower limit)
ESO 540-G001 4.2 13.0
CGCG 551-008 1.0
MCG -03-05-007 2.2
UGC 03134 19.5
IRAS 20051-1117 1.6 1.2
CGCG 303-017 1.3 2.0
IC 1631 0.3
NGC 2992 2.0
NGC 3147 0.4
NGC 4565 6.7
NGC 4579 0.2
NGC 4594 1.7
NGC 4698 0.3
NGC 5033 1.3
MRK 273x 0.4
NGC 5995 0.4
NGC 6221 0.02
NGC 6251 6.0
NGC 7590 0.4
NGC 7679 3.4 (2.0 from our data)
It is still not clear what kind of physical process is related to
the presence of hidden central engines in Sy2s. PB02 suggest
two scenarios for the unabsorbed Sy2s (i) the central engine
and their BLR must be hidden by an absorbing medium with
high value of the AV/NH ratio, and (ii) the BLR is very weak or
absent.
5. Conclusions
We present a new ø3 λ5007 emission - line image of the
circumnuclear region of NGC 7679 which shows elliptical
isophotes extended along the PA≈ 80◦ ± 10◦ in the direction to
the counterpart galaxy NGC 7682. The maximum of this emis-
sion is displaced by about 4 arcsec from the photometric center
defined by the continuum emission.
The ratio of the quantity of ionizing photons inferred from
the observed extinction corrected ø3 λ5007 luminosity to the
number of ionizing photons with hν > 55 eV provided by the
central AGN source (Nph/Nion)hν> 55 eV ≈ 0.2 − 20 as well as
the presence of weak and elusive Hα broad wings probably
indicate a hidden AGN.
The high ionization inferred by the flux ratio ø3 λ5007/Hα
in the direction of about PA≈ 80◦ ± 10◦ coincides with the di-
rection to the counterpart galaxy NGC 7682. It is possible that
the dust and gas in this direction has a direct view to the central
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
two galaxies.
In the direction PA≈ 0◦ the ionization is entirely caused by
hot stars.
A large part of the unabsorbed Compton-thin Sy2s with
higher ø3 luminosity (>∼ 1041 erg s−1) possesses a hidden AGN
source.
Acknowledgements. We are grateful to the referee, Lucio Buson, for
his valuable comments which improved both the content and the clar-
ity of this manuscript.
We would like to thank T. Bonev, Institute of Astronomy of
Bulgarian Academy of Sciences, for kindly providing the Fabry-Perot
observations and for useful discussions. We are grateful to S. Zhekov,
Space Research Institute of Bulgarian Academy of Sciences, for the
numerous fruitful discussions and especially for the analysis of the
X-ray properties of NGC 7679.
Our work was partially based on data from the La Palma ING,
ESO NTT, and XMM-Newton Archives.
This research has made use of the SIMBAD database, operated
at CDS, Strasbourg, France, and of the NASA/IPAC Extragalactic
Database (NED) which is operated by the Jet Propulsion Laboratory,
California Institute of Technology, under contract with the National
Aeronautics and Space Administration.
We acknowledge the support of the National Science Research
Fund by the grant No.F-201/2006.
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http://arxiv.org/abs/astro-ph/0611199
	Introduction
	Observations and data reduction
	Results
	Narrow-band emission-line images
	Narrow-band emission-line total fluxes
	The ionization map F(ø3 5007)/F(H)
	Discussion
	The ionizing flux from the central engine
	Physical conditions in the circumnuclear region of NGC 7679
	Ionization structure in the circumnuclear region of the NGC 7679
	Unabsorbed SyGs with and without hidden BLRs
	Conclusions
