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Astron. Astrophys. 344, 36-42 (1999)

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4. Discussion

4.1. Gas and dust in the narrow line region in Mkn 620

Previous studies of Mkn 620 have revealed a strong [O III ] [FORMULA] Å emission concentrated around the nucleus and circumnuclear extended emission of H[FORMULA] + [N II ] [FORMULA] ÅÅ (Pogge, 1989; MWT96).

The color map [FORMULA] (Fig. 2) infers the presence of a redder dusty ring-like structure around the AGN nucleus of Mkn 620. Otherwise, the innermost circumnuclear region is not influenced by extinction. We assume that the circumnuclear emission line region with diameter of about 700-800 pc , where the [O III ] [FORMULA] Å contours are situated is a high ionized Strömgren zone (Fig. 3). The Strömgren depth is defined as

[EQUATION]

where [FORMULA] cm3 sec-1 is the recombination coefficient to excited states of hydrogen, c is the speed of light and [FORMULA] is the ionization parameter. In order to estimate [FORMULA] we take the electron density [FORMULA] cm-3 following LLS92. As was mentioned in Sect. 3.1, the nonthermal component of the ionizing continuum [FORMULA] amounts 0.74 of the measured flux [FORMULA] Å). Taking a resonable value for [FORMULA] we estimate the ionizing parameter [FORMULA] which yields [FORMULA] [FORMULA].

The [N II ] [FORMULA] and [OI][FORMULA] + [FeX][FORMULA] emissions (Fig. 4a,b) arise both in the inner Strömgren zone and in the dusty ring. Thus the redder dusty ring-like structure appears to be a large partially ionized zone (hereafter PIZ) in which the ionized gas becomes neutral. The existence of this large PIZ results from harder photons of the ionizing continuum.

In our color map (Fig. 2 and see also Fig. 3) the Strömgren zone is homogeneous and we assume the measured flux ratio [FORMULA] is not reddened there. But in the PIZ, where the dust content is enhanced this ratio is influenced by dust extinction (absorption and scattering) and it is about 2.5. The dust opacity [FORMULA] is given by

[EQUATION]

where [FORMULA] (cm2 per hydrogen nucleus) is the total extinction cross section and [FORMULA] is the dust content of the medium expressed relative to the standart ISM dust-to-gas mass ratio (Binette et al. 1993). We assume [FORMULA] and take [FORMULA] from Draine & Lee (1984) (see their Fig. 7). Based on the reddened and unreddened flux ratios [FORMULA] we estimate the column density in the PIZ [FORMULA] cm -2. Then the total column density is

[EQUATION]

The column density [FORMULA] defines the depth of the complete "photoexcited" region, that is the depth at which the incoming ionizing flux is exhausted.

The extinction opacity at 5500 Å is [FORMULA] and the extinction is [FORMULA] 1[FORMULA] 04 if [FORMULA]. Following Spitzer (1978) we can estimate the mean dust density along the line of sight

[EQUATION]

where [FORMULA] is the density of the particular dust grains and we assume [FORMULA] g cm-3. The dielectric dust function [FORMULA] in the low frequency's limit is [FORMULA] (Spitzer 1978). The mean extent of the dusty ring-like structure measured on the color map is [FORMULA]. Then according to Eq. (4) the mean dust density along the line of sight is [FORMULA] g cm-3.

From Fig. 2 we can roughly estimate the volume occupied by the dusty ring [FORMULA] cm3 assuming a filling factor of [FORMULA]. Knowing [FORMULA] we determine the dust mass contained by the observed dusty ring [FORMULA] [FORMULA]. This value is an upper limit since a filling factor of [FORMULA] was assumed.

Balmer decrement. The Balmer emission lines could be affected by the presence of PIZ in the circumnuclear region of Mkn 620. The measured Balmer decrement by LLS92 in Mkn 620 is [FORMULA]. After a correction for the reddening we obtain a dereddened Balmer decrement [FORMULA].

Binette et al.,(1993) have shown the integrated Balmer lines of a power-law photoionized gas in a system of clouds with internal dust would be significantly affected because of dust and perspective. The same authors argue the intrinsic Balmer decrement could be as steep as 4-4.4 in the radiation bounded case where many clouds are seen from the back side.

4.2. Infrared emission of Mkn 620

The IRAS data show that Mkn 620 is a luminous IR galaxy [FORMULA](40 - 300 µm)[FORMULA][FORMULA] . The spectral indexes, defined as [FORMULA] are [FORMULA], [FORMULA] and [FORMULA] and place this galaxy in the typical range for Seyfert galaxies in the color-color diagrame [FORMULA] versus [FORMULA] (Miley et al. 1985).

The main components of FIR emission are nonthermal AGN continuum continued to IR domain and thermal dust reemission. We favore the nonthermal UV continuum as the energy source which heats the dust.

The temperature of a dust particle is determined by the equilibrium between the absorbed and emitted energy

[EQUATION]

where

[EQUATION]

is the energy density of the radiation field and [FORMULA] Mpc is the distance to the galaxy. The mean intensity [FORMULA] in ergs cm-2 s-1 Hz-1 of the radiation field at distance [FORMULA] pc from the central nonthermal point source is obtained from the measured flux [FORMULA] Å) extrapolating it to the UV with spectral index [FORMULA]. In Eq. (5) [FORMULA] Hz = 1 µm and [FORMULA] Hz = 100 Å are the assumed upper and lower limits to the frequencies which can heat the dust. [FORMULA] is the Planck function at grain temperature [FORMULA]. The absorption efficiency [FORMULA] in first approximation varies as [FORMULA] for a lot of dust particles in the visual and IR ranges (Spitzer 1978). Eq. (5) yields an equilibrium dust temperature [FORMULA] K.

Most of the 12 and 25 µm emission in Seyfert galaxies is from dust in ionized gas, heated both by nonthermal AGN continuum and by stellar UV photons (Mouri & Taniguchi, 1992; Granato & Danese, 1994).

It is natural to speculate that the IR [FORMULA] µm emission arises in the dusty ring-like structure inferred by the color map. Assuming dust thermal reradiation we make an estimation of the dust mass [FORMULA] responsible for the observed IRAS flux [FORMULA]. Taking [FORMULA] K and the measured IRAS flux [FORMULA] Jy we derive [FORMULA] [FORMULA] if the dust particles consist of astronomical silicates, and [FORMULA] [FORMULA] if the dust particles are graphites with small radii [FORMULA] µm. The estimated dust mass depends on the dust temperature and is an order of magnitude greater than the value [FORMULA] [FORMULA] , obtained in Sect. 4.1 assuming that the observed extinction is entirely due to the dusty ring. If the [FORMULA] is larger than 110 K then the required dust mass, needed to explain the 25 µm flux, would be smaller. Both larger nonthermal ionizing flux and star formation events could result in higher dust temperature [FORMULA]. We have to note, that according to Young & Devereux (1991) Mkn 620 exhibits levels of nuclear star formation activity for radii less than 700 pc. Also there is a high 10 µm luminosity in the circumnuclear region ([FORMULA] arcsec) of Mkn 620 which is an indication of nuclear star formation activity (Giuricin et al. 1995).

Dust ring-like morphologies. Dust ring-like morphologies in Seyfert galaxies could be traced both by red/blue color maps and by mid-infrared radiation. The 10.8 µm maps of the central region of the infrared-luminous barred galaxies NGC 1068 and NGC 1097 show the morphologies like kiloparsec-size rings which are intimately associated with the dense neutral interstellar gas inferred from CO maps (Telesco et al. 1993).

Moreover, in these two galaxies the observed dust ring-like morphologies coincide with the ILRs and demonstrate the role played by bars and oval distortions in the genesis of starbursts (Telesco et al. 1993).

If there are extended dusty clouds around Seyfert nuclei, they would show excess emission at FIR because the equilibrium temperature is about 100 K (Granato et al. 1996; Taniguchi et al.1997). In many objects, these dusty clouds are also connected with the occurrence of the circumnuclear starburst regions. Their typical radii range from several 100 pc to 1 kpc (Telesco et al. 1984; Boer & Schulz 1993; Genzel et al. 1995; Storchi-Bergmann et al. 1996). Arguments favoring the enhanced star formation around the nuclei of Seyfert galaxies as an alternative of the Unified model are presented by Dultzin-Hacyan (1995).

On the other hand, the accumulation of large amount of gas and dust near the ILR is connected with the supply of gas to the nucleus of Seyfert galaxy. Wada and Habe (1992;1995) have found a new fueling mechanism from kpc to several tens of pc, which is induced by a weak bar and the self-gravity of the gas in a massive gaseous disc. In their model the first ILR is essential for gas accumulation near the centre. The second ILR occupies, roughly, a diameter of about [FORMULA] kpc.

The dust ring-like structure ([FORMULA] pc in diameter (Fig. 2)) observed in Mkn 620 could be associated with a shocked dust connected with the ILRs.

As a rule, the dust ring-like morphologies favore type 2 Seyfert galaxies which possess SB morphology.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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