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

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1. Introduction

In recent years, several arguments have been advanced to indicate that galaxy interactions and/or orbital resonances can lead to the enhanced fueling of various kinds of nuclear activity, especially the Seyfert and Starburst phenomena (Heckman 1990, 1991; Dultzin-Hacyan, 1997). Particularly interesting in this respect are Seyferts with rings and bars. The common ringed galaxies are believed to outline the locations of major orbital resonances with a bar or oval distortion in the galactic disk (Buta 1986a,b, Norman 1987).

In the theory of AGNs the luminous ([FORMULA][FORMULA]) IRAS galaxies play an important role. It appears that the luminous IR galaxies are extraordinarily rich in molecular hydrogen, highly concentrated towards their nuclei (Scoville & Soifer 1991; Scoville et al. 1994). This matter reservoir plays a central role in promoting star formation and possibly fueling accretion onto the central compact object (Scoville et al. 1991).

Studies of dust and gas distribution in the circumnuclear regions of luminous IRAS AGNs galaxies with rings and bars provide information both about the link between the AGN and star formation events and about the matter transport to the active nucleus.

NGC 2273 (Mkn 620) was discovered by Huchra et al.(1982) to be a type 2 Seyfert galaxy and classified by de Vaucouleurs et al., 1991 [RC3] as a morphological type SB(r)a:. The heliocentric recession velocity is [FORMULA] km sec-1 [RC3].

According to van Driel & Buta (1991) this galaxy shows faint doubled outer ring-like structure, which consists of two incompletely closed concentric structures formed by separated sets of spiral arms at radii of 7.3 and 10.2 kpc . There is foreshortened bar and an inner ring-like shaped by sets of spiral arms.

NGC 2273 has been detected by IRAS (Lonsdale et al., 1992 hereafter LLS92) with strong far infrared emission (FIR) [FORMULA][FORMULA].

Young & Devereux (1991) have presented new CO 2.6 mm observations of Mkn 620 and determined the mass of molecular gas confined to the central 45 arcsec to be [FORMULA] [FORMULA] . The CO line width is found to be less than 260 km s-1 and this may indicate that the molecular gas is confined to the central few arcseconds in this galaxy. The star formation rate per unit mass of molecular gas in the nucleus of Mkn 620 is [FORMULA].

The mid-infrared emission of Mkn 620 at 10 µm measured by Devereux (1987) in small aperture ([FORMULA]5 arcsec) is due to thermal dust reradiation of the UV/optical emission of the central source (Giuricin et al.,1995).

The continuum measurements by Krugel et al. (1988) at 1300 µm and the spectral index between 100 and 1300 µm clearly indicate that the emission must be thermal reradiation from dust.

CCD interference-band images isolating the emission lines of H[FORMULA] + [N II ] [FORMULA] ÅÅ and [O III ] [FORMULA] Å were obtained by Pogge (1989) and Mulchaey, Wilson and Tsvetanov (1996) (hereafter MWT96) to search for spatially extended circumnuclear emission regions.

In this paper we present the results of new narrow and broad-band imaging to study gas and dust distribution in the circumnuclear region of the ringed Seyfert 2 galaxy Mkn 620. Some information on the observations and data reduction procedures is presented in Sect. 2. The results are given and analyzed in Sects. 3 and 4, respectively. Their compatibility with the IR measurements is discussed.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999