2 Observations

All observations described in this paper were carried out with the 15 m James Clerk Maxwell Telescope (JCMT) on Mauna Kea (Hawaii). Details are given in Table 2. Up to 1993, we used a 2048 channel AOS backend covering a band of 500 MHz ( $650\,{\rm {km\,s^{-1}}}$ at 230 GHz). After that year, the DAS digital autocorrelator system was used in bands of 500 and 750 MHz. Integration times given in Table 2 are typical values used in mapping; central positions were usually observed more than once and thus generally have significantly longer integration times. Values listed are on+off. When sufficient free baseline was available, we subtracted second order baselines from the profiles. In all other cases, linear baseline corrections were applied. All spectra were scaled to a main-beam brightness temperature, $T_{\rm mb}$ = $T_{\rm A}^{*}$/ $\eta _{\rm mb}$; relevant values for $\eta _{\rm mb}$ are given in Table 2. Spectra of the central positions in both galaxies are shown in Fig. 1 and summarized in Table 3. In Table 2, we have also listed the parameters describing the various maps obtained. All maps are close to fully sampled with the exception of the J=3-2 CO map of NGC 6946 where we sampled the outer parts every other grid point only. In all maps except the J=2-1 CO map of NGC 6946, the mapping grid was rotated by the angle given in Table 2 so that the Y axis coincided with the galaxy major axis. The velocity-integrated maps shown in Figs. 2 and 3 have been rotated back, so that north is (again) at top and the coordinates are right ascension and declination. As a consequence of the interpolation involved in the rotation, the maps are shown at a resolution degraded by 5-10$\%$. For NGC 6946, the map grid origin is identical to the optical centre listed in Table 1. The radio centre occurs in the maps at offsets $\Delta \alpha$, $\Delta \delta$ = +2'', -1''; this is to all practical purposes within the pointing error. For M 83, the grid origin is at 13$^{\rm h}$34$^{\rm m}$11.3$^{\rm s}$, $-29^{\circ}$36'39'', roughly halfway between the optical and radio centres, which occur in the maps at $\Delta \alpha$, $\Delta \delta = +4''$, -3'' and -3'', +4'' respectively.

Figure 2: Contour maps of emission from NGC 6946 integrated over the velocity range $V_{\rm LSR} = -100$ to 220 $\,{\rm {km\,s^{-1}}}$. North is at top. Left to right: CO J=2-1, CO J=3-2, CO J=4-3 (top) and [CI] (bottom). Contour values are linear in $\int T_{\rm mb}$dV. Contour steps are 20 $\,{\rm {K\,km\,s^{-1}}}$ (2-1), 15 $\,{\rm {K\,km\,s^{-1}}}$ (3-2 and 4-3) and 6 $\,{\rm {K\,km\,s^{-1}}}$ (CI) and start at step 1

Figure 3: Contour maps of emission from M 83 integrated over the velocity range $V_{\rm LSR} = 400$ to 620 $\,{\rm {km\,s^{-1}}}$. North is at top. Left to right: CO J=2-1, CO J=3-2, CO J=4-3 (top) and [CI] (bottom). Contour values are linear in $\int T_{\rm mb}$dV. Contour steps are 15 $\,{\rm {K\,km\,s^{-1}}}$ (2-1), 20 $\,{\rm {K\,km\,s^{-1}}}$ (3-2 and 4-3) and 5 $\,{\rm {K\,km\,s^{-1}}}$ (CI) and start at step 1

 

 

Table 3: Central CO and CI line intensities in NGC 6946 and M 83

			NGC 6946			M 83
Transition		Resolution	$T_{\rm mb}$a	$\int T_{\rm mb}$dVa	$L_{\rm tot}$b	$T_{\rm mb}$a	$\int T_{\rm mb}$dVa	$L_{\rm tot}$b
		( $\hbox{$^{\prime\prime}$ }$)	(mK)	( $\,{\rm {K\,km\,s^{-1}}}$)	$\,{\rm {K\,km\,s^{-1}}}$ kpc2	(mK)	( $\,{\rm {K\,km\,s^{-1}}}$)	$\,{\rm {K\,km\,s^{-1}}}$ kpc2
J=2-1	12CO	21	1384	$222\pm20$	124	2540	$261\pm15$	55
J=3-2	12CO	14	1428	$209\pm25$	56	2176	$262\pm15$	38
		21		$145\pm15$			$167\pm15$
J=4-3	12CO	11	1798	$216\pm20$	33	2669	$270\pm20$	21
		14		$170\pm17$			$189\pm20$
		21		$112\pm11$			$122\pm15$
J=2-1	13CO	21	141	$22.2\pm3$		247	$28.5\pm3$
J=3-2	13CO	14	105	$11.4\pm2$		194	$22.3\pm1$
3P1-3P0	CI	10	465	$85\pm9$	7.2	685	$83\pm14$	4.4
		14		$60\pm10$			$75\pm10$
		21		$44\pm8$			$55\pm8$

Notes to Table 3: ^a Beam centered on nucleus; ^b Total central concentration.

 

 

Table 4: Integrated line ratios in the centres of NGC 6946 and M 83

Transitions	NGC 6946			M 83
	Nucleus	Total Center	+10'', +10''	Nucleus	Total Center	+7'', -7''	-14'', -14''
$\,{\rm ^{12}CO}$ (1-0)/(2-1)a	$1.1\pm0.2$	0.95	1.0	$0.9\pm0.2$	1.1	--	--
$\,{\rm ^{12}CO}$ (3-2)/(2-1)b	$0.65\pm0.10$	0.5	0.5	$0.65\pm0.13$	0.7	0.8	0.5:
$\,{\rm ^{12}CO}$ (4-3)/(2-1)b	$0.45\pm0.15$	0.3	0.4:	$0.48\pm0.10$	0.4	0.6	--
$\,{\rm ^{12}CO}$/$\13co$ (1-0)c	$11.8\pm1.3$	--	--	$10.4\pm1.6$	--	--	--
$\,{\rm ^{12}CO}$/$\13co$ (2-1)b	$9.8\pm1.5$	--	15	$9.2\pm1.0$	--	11	9
$\,{\rm ^{12}CO}$/$\13co$ (3-2)d	$13.0\pm1.4$	--	--	$11.8\pm1.1$	--	--	--
CI/CO(2-1)b	$0.20\pm0.04$	0.06	--	$0.18\pm0.04$	0.07	0.2:	--
CII/CO(2-1)e	0.08			0.55

Notes: ^a From J=1-0 data by Weliachew et al. (1988), Sofue et al. (1988), Wild (1990); Handa et al. (1990) and Israel et al. (unpublished); ^b This Paper, JCMT at 21'' resolution; ^c Sage $\&$ Isbell (1991); NRAO 12 m at 57'' resolution; Young $\&$ Sanders (1986); FCRAO at 45'' resolution; Israel et al. (unpublished); SEST at 43'' resolution; Rickard $\&$ Blitz (1985); NRAO at 65'' resolution; ^d This Paper; JCMT at 14'' resolution. ^e From Crawford et al. (1985) and Stacey et al. (1991), KAO at 55'' resolution.

Figure 4: Position-velocity maps of CO emission from NGC 6946 in position angle 55$^{\circ }$. Left to right: CO J=2-1, CO J=3-2 and CO J=4-3. Contour values are linear in $T_{\rm mb}$. Contour steps 4 K (2-1), 3 K (3-2) and 2.5 K (4-3) and start at step 1

Figure 5: Position-velocity maps of CO emission from M 83 in position angle 45$^{\circ }$. Left to right: CO J=2-1, CO J=3-2 and CO J=4-3. Contour values are linear in $T_{\rm mb}$. Contour steps are 4 K (2-1 and 3-2) and 2.5 K (4-3) and start at step 1