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

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3. Results

3.1. Broad and narrow-band continuum images

Contour map of the broad band r continuum is presented in Fig. 1. Several morphological features are clearly seen: the outermost ring-like structure at radius of 7.3 kpc ([FORMULA] arcsec); an ovally distorted disk with a radius of about 30 arcsec . The innermost visible structure is a bar with P.A.[FORMULA]. Weak spiral arms originate from the bar and form an inner ring-like with radius [FORMULA] kpc (20 arcsec). Similar features are noted and discussed by Gallagher & Wirth (1980) and van Driel & Buta (1991).

[FIGURE] Fig. 1. Contour plot of the Gunn r image. The outermost contour is at [FORMULA] of the background ([FORMULA] background noise level is [FORMULA] ergs cm-2 s- 1 arcsec-2Å-1). Next contours increase as a power series of 2

It should be noted, the Gunn r continuum contour map (Fig. 1) shows an isophotal twist in the innermost region of Mkn 620. Similar isophotal twists are observed in early type spirals and could be associated with inner Lindblad resonances (ILR) (Elmegreen & Elmegreen 1996).

The continua images obtained at [FORMULA] Å and [FORMULA] Å were used to create the narrow-band color map presented in Fig. 2, where "black" means excess of light while "white" means absorption. The color map reveals an inner redder structure reminding of dusty ring around the AGN nucleus of Mkn 620. This ring-like structure has a diameter of about 14 arcsec corresponding to [FORMULA] pc . Its mean projected thickness is about 500 pc . The dusty ring is disconnected in the east-west and it is much more prominent in the north-west of the nucleus.

[FIGURE] Fig. 2. Red/blue narrow-band color map of Mkn 620

A second dusty ring is clearly seen on the color map. It is approximately placed on the inward side of the ovally distorted disk and its radius is [FORMULA] arcsec (2.4 kpc). Gallagher & Wirth (1980) have also shown the presence of outer rather red ring.

We calibrated the continuum image in F 426 and estimated the observed flux [FORMULA]. On the other hand, according to Rowan-Robinson & Crawford (1989) the starburst and Seyfert components for Mkn 620 contain 0.74 of the whole infrared emission. We utilize this value to estimate the nonthermal AGN fraction of the observed flux. The later was used to calculate the energy density of the radiation field in the circumnuclear region occupied by the dusty ring.

3.2. Narrow-band emission line images

The emission line contours of [O III ] [FORMULA] Å, [N II ] [FORMULA] Å and [O I ] [FORMULA] + [Fe X ] [FORMULA] ÅÅ superimposed on the [FORMULA] color map are presented in Fig. 3 and Fig. 4. The corresponding [FORMULA] background noise levels in the emission line images are presented in Table 2. The lowest isophotal level is at [FORMULA] above the sky subtraction level and the following contours are multiplied with [FORMULA].

[FIGURE] Fig. 3. The [O III][FORMULA] Å-emission contours superimposed over the red/blue narrow-band color map

[FIGURE] Fig. 4. a  The [N II][FORMULA] Å emission contours superimposed over the red/blue narrow-band color map; b  The [OI][FORMULA] + [FeX][FORMULA] ÅÅ emission contours over the same color map


Table 2. M 620-observed emission line fluxes in units of [FORMULA].
The flux in [N II ] [FORMULA] Å has been obtained assuming [FORMULA]/[N II ] [FORMULA] Å/[NII] [FORMULA] Å = 3/1/3.

The [O III ] [FORMULA] Å emission arises entirely in the circumnuclear region enclosed by the dusty ring and it is strongly reduced there where the dust content is enhanced.

The [N II ] [FORMULA] Å emission (Fig. 4a) shows extended structure in diameter of about 18 arcsec . The morphology does not differ from that of the H[FORMULA] + [N II ] [FORMULA] ÅÅ image presented by MWT96. The outermost contour in Fig. 4a is at [FORMULA] above the noise level and includes entirely the dusty ring.

The [O I ] emission is a good diagnostic tool for the presence of hard nonthermal ionizing photons. Both emission lines [O I ] [FORMULA] Å and [Fe X ] [FORMULA] Å (Fig. 4b) are transmited through the F 642.

The Gunn r contours (Fig. 1), the color map (Fig. 2) and the presence of extended emission line regions suggest that the ionized region is a gaseous disk with an orientation of the major axis at P.A. [FORMULA]. The gaseous disk wound by the dusty ring appears to be viewed at an inclination of [FORMULA] to the line of sight derived from the ratio of major to minor axes (Fig. 1).

The width of the dusty ring projected onto the sky plane is comparable with its geometrical depth due to the small inclination angle to the line of sight. Note that the inclination angle for the galaxy stellar disk is [FORMULA] (Whittle 1992; de Vaucouleurs et al.1991 [RC3]).

From our calibrated images we estimated the total fluxes of the observed emission lines in apertures used by other authors. Optical spectral observations of Mkn 620 were made by LLS92, and Ho, Filippenko & Sargent, 1997 (HFS97), where the slit width and length are [FORMULA] arcsec and [FORMULA] arcsec , respectively. We performed an integration of the fluxes inside these apertures and in aperture [FORMULA] arcsec used by MWT96. The data are presented in Table 2. Our measured fluxes in [O III ] [FORMULA] Å and [N II ] [FORMULA] Å agree quite well with the fluxes presented by LLS92 and are larger than that measured by MWT96. The transmission of H[FORMULA] [FORMULA] Å in the interference filter F 657 Å centered near the wavelength of [N II ] [FORMULA] Å is about [FORMULA] which results in a larger flux.

We have to note that the measured fluxes by HFS97 in emission lines of [O III ] [FORMULA] Å and [N II ] [FORMULA] Å in aperture 2[FORMULA] [FORMULA] 4[FORMULA] exceed the fluxes of the same lines measured in larger apertures by LLS92 and MWT96. But on the other hand, the authors in HFS97 have stated the photometric accuracy of the line fluxes are only approximate.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999