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Astron. Astrophys. 339, 19-33 (1998)

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2. NGC 946

NGC 6946 is a nearby Scd galaxy with a low inclination, [FORMULA] (Considère & Athanassoula 1988). At the adopted distance of 5 Mpc, 1 arcmin on the sky corresponds to a linear size of 1.5 kpc. The main characteristics of NGC 6946 are summarized in Table 1. It shows a weak bar (Martin 1995) and a prominent spiral structure with both an [FORMULA] and an [FORMULA] pattern (Considère & Athanassoula 1988). With this open spiral structure, the arm and interarm regions are resolved at moderate spatial resolution (30"). The gas distribution is well known since NGC 6946 has been mapped at high spatial resolution in HI (Boulanger & Viallefond 1992), and CO (Casoli et al. 1990, Clausset et al. 1991). Fig. 1 presents a CO(2-1) map obtained at the IRAM 30m telescope (Clausset et al. 1991), and Fig. 2 a V-I color map obtained at the Observatoire de Haute-Provence (OHP) (P. Boissé, private communication). NGC 946 is marginally resolved in the far infrared maps obtained by IRAS and the KAO (Engargiola 1991). It also belongs to the normal galaxy sample studied by ISO and as such has been extensively mapped in the mid and far infrared (Malhotra et al. 1996, Helou et al. 1996, Lu et al. 1996, Tuffs et al. 1996). NGC 6946 is forming stars actively, as revealed by its bright H[FORMULA] emission (Bonnarel et al. 1988a, 1988b, Kennicutt 1989) and the numerous supernovae (Li & Li 1995). Madden et al. (1993) have mapped the C+ 158 [FORMULA] emission at 45" resolution with the KAO. Bright C+ emission is found in the nucleus and in the disk. At this scale, the C+ emission is correlated with the CO emission in the spiral arms and with the HI emission in the outer disk.

[FIGURE] Fig. 1. CO(J=2-1) map at 13" resolution obtained with the IRAM 30m radiotelescope.

[FIGURE] Fig. 2. V-I image, obtained with the 1.20m telescope at the Observatoire de Haute Provence (P. Boissé, private communication). The field of view is 10´ by 10´, the pixel size is 2.3". The gray scale runs from white for blue colors to black is for red colors.


Table 1. Observed parameters for NGC 946.
We use the value 3.8 [FORMULA] 1033 erg s-1 for the solar luminosity at any wavelength.
1) We use [FORMULA] = 2.3 1020 mol cm- 2/(K km s-1) and a total intensity [FORMULA]= 569 K km s-1 in a 45" beam, we do not account for projection effects.
2) We use MHI = 1.9 [FORMULA] 1010 [FORMULA] at a distance of 10 Mpc, corresponding to [FORMULA] [FORMULA] for a distance of 5 Mpc. By performing an integration over the HI map, we estimate that the part with R [FORMULA] 6´ contributes to 55% of the global emission.
3) The 2000 Å flux, 6.92 [FORMULA] 10-12 erg cm- 2 s-1 Å-1, is corrected for the Galactic extinction and integrated over a 100 Å band. Note that Donas & Deharveng (1987) have reported a flux at 2000 Å twenty times lower (1.66 [FORMULA] 10-13 erg cm-2 s- 1 Å-1). Measurements at other UV wavelengths are available in Rifatto et al. (1995).
4) We use the uncorrected flux [FORMULA]= 1.31 [FORMULA] 10- 10 erg cm-2 s-1 and the relationship [FORMULA] = 3.13 [FORMULA] 1016 [FORMULA] [FORMULA] from Young et al. (1996). We have subtracted the contribution from the nucleus, estimated to be 20% of the total luminosity. Note that Young et al. (1996) give a lower total flux, [FORMULA] erg cm-2 s- 1, while Kennicutt (1989) reports a mean surface brightness of [FORMULA] erg s-1 pc- 2 corresponding to a total H[FORMULA] luminosity of [FORMULA] [FORMULA] at the adopted distance.
5) We estimate the luminosity in the band centered on frequency [FORMULA], by [FORMULA]. We use the IRAS and KAO fluxes reported by Engargiola (1991) for a radius R [FORMULA] 5.6´ and correct from the contribution of the nucleus (central 45") to obtain 12 Jy at 12µm, 15 Jy at 25µm, 114 Jy at 60µm, 283 Jy at 100µm and 288 Jy at 200µm. The global color for the disk is S60/S100 = 0.40. The far infrared luminosity is [FORMULA] + [FORMULA] = [FORMULA] [FORMULA], [FORMULA] = [FORMULA] [FORMULA]
6) We have excluded the contribution of the nucleus and rescaled the data by Madden et al. (1993) for the adopted distance.
7) We use an estimation of 625 mJy for the total flux at 6 cm (Klein et al. 1982), subtract the nuclear contribution estimated to be 33% of the total flux, and we keep 20% of the resultant disk flux as the thermal component.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998