1 Introduction

Molecular gas is a major constituent of the interstellar medium in galaxies and the dominating component in regions of star formation and the inner disks of spiral galaxies. Within the inner kiloparsec, many spiral galaxies also exhibit a strong concentration of molecular gas towards their nucleus. It is generally thought that such concentrations are the result of angular momentum losses caused by e.g. encounters or mergers with other galaxies, or by bar-like potentials in the central part of the galaxy. However, in some cases, such as the Sb spiral galaxies M 31 and NGC 7331, most or all of the central gas may have originated from mass loss by evolved stars in the bulge (cf. Israel & Baas 1999). In order to determine the physical condition of molecular gas in the centers of galaxies, and its amount, we have conducted a programme to observe a number of nearby galaxies in various CO transitions, as well as the 492 GHz ³P₁-³P₀ CI transition. Results for the Sc galaxy NGC 253 (Israel et al. 1995) and the Sb galaxy NGC 7331 (Israel & Baas 1999) have already been published, as well as preliminary results on the Sc galaxy NGC 3628 (Israel et al. 1990). In this paper, we present the results for the Sc galaxies NGC 6946 and M 83. Basic properties of these galaxies are summarized in Table 1.

 

 

Table 1: Galaxy parameters

	M 83	NGC 6946
Typea	SBc	Scd
Optical Centre:
RA (1950)b	13$^{\rm h}$34$^{\rm m}$11.6$^{\rm s}$	20$^{\rm h}$33$^{\rm m}$48.8$^{\rm s}$
Decl (1950)b	$-29^{\circ}$36'42''	+59$^{\circ }$58'50''
Radio Centre :
RA (1950)c	13$^{\rm h}$34$^{\rm m}$11.1s	20$^{\rm h}$33$^{\rm m}$49.1$^{\rm s}$
Decl (1950)c	$-29^{\circ}$36'34.9''	+59$^{\circ }$58'49''
$V_{\rm LSR}^{d}$	510 $\,{\rm {km\,s^{-1}}}$	55 $\,{\rm {km\,s^{-1}}}$
Distance De	3.5 Mpc	5.5 Mpc
Inclination id	24$^{\circ }$	38$^{\circ }$
Position angle Pd	45$^{\circ }$	60$^{\circ }$
Luminosity LBe	1.2   1010 $L_{B\odot}$	3   1010 $L_{B\odot}$
Scale	59''/kpc	38''/kpc

Notes to Table 1:
^a RSA (Sandage & Tammann 1987); ^b Dressel & Condon (1976); Rumstay & Kaufman (1983);
^c Turner & Ho (1994); van der Kruit et al. (1977); ^d Tilanus & Allen (1993); Handa et al. (1990);
Carignan et al. (1990); ^e Banks et al. (1999); Tully (1988).

Although a member of the NGC 6643 group, NGC 6946 (Arp 29) is relatively isolated. Its distance is variously estimated between 3 Mpc (Ables 1971) and 10 Mpc (Rogstad & Shostak 1972; Sandage & Tammann 1974); here we adopt D = 5.5 Mpc (Tully 1988; McCall 1982). It has been relatively well-studied in the lower CO transitions. In fact, it was one of the first galaxies mapped in J=1-0 CO at 65'' resolution (Morris $\&$ Lo 1978; Rickard & Palmer 1981). Higher-resolution maps at 17-23''were published by Sofue et al. (1988) and Weliachew et al. (1988). At a similar resolution, disk spiral arm regions were observed in J=1-0 and J=2-1 CO by Casoli et al. (1990). The central region was also observed in the J=3-2 and J=4-3 CO transitions, again at similar resolutions (Wall et al. 1993; Mauersberger et al. 1999; Nieten et al. 1999). Using ISO, Valentijn et al. (1996) obtained a direct detection of warm $\h2$ towards the center of NGC 6946. Early high-resolution (about 6.5'') J = 1-0 maps were obtained by Ball et al. (1985) and Ishizuki et al. (1990). Very good maps with a resolution of 3-4'' can be found in Regan & Vogel (1995) and Sakamoto et al. (1999). These maps show an elongated concentration of CO in the center, extending to the northwest with a position angle changing from 315$^{\circ }$ close to the nucleus to 0$^{\circ }$ at 15'' from the nucleus. Similarly high-resolution maps of nuclear HCN emission by Helfer $\&$ Blitz (1997) show only a compact source.

Figure 1: Full resolution emission spectra observed towards the centers of NGC 6946 and M 83. Top row: NGC 6946; bottom row: M 83. Columns from left to right: J=2-1 CO, J=3-2 CO, J=4-3 CO, [CI]. Vertical scale is actually in $T_{\rm mb}$. Whenever available, $\13co$ profiles are shown as the lower of the two profiles in the appropriate $\,{\rm ^{12}CO}$ box, but with brightness temperatures multiplied by three, i.e. on the same temperature scale as [CI]

 

 

Table 2: Observations log

Transition	Object	Date	Freq	$T_{\rm sys}$	Beam	$\eta _{\rm mb}$	t(int)	Map Parameters
					Size			Points	Size	Spacing	PA
		(MM/YY)	(GHz)	(K)	( $\hbox{$^{\prime\prime}$ }$)		(sec)		( $\hbox{$^{\prime\prime}$ }$)	( $\hbox{$^{\prime\prime}$ }$)	($^{\circ }$)
12CO J=2-1	NGC 6946	02-06/89	230	1100	21	0.63	600	36	$60\times60$	10	0
	M 83	02-89		1295		0.63	600	49	$70\times120$	10	45
12CO J=3-2	NGC 6946	12/93	345	1270	14	0.53	400	40	$54\times54$	6	70
	M 83	04/91		1985		0.53	400	55	$70\times100$	10	45
		04/93		765		0.53	300
		12/93		1335		0.53	1000
12CO J=4-3	NGC 6946	11/94	461	8500	11	0.51	840	22	$30\times30$	6	70
		07/96		2900		0.53	360
	M 83	12/93		4360		0.51	400	20	$30\times30$	6	45
13CO J=2-1	NGC 6946	02-89	220	1000	21	0.63	2640	2
		06-95		420		0.69	6330
		01-96		530		0.69	6000
	M 83	02/05-89		1200	21	0.63	6840	3
		06-95		430		0.69	1200
13CO J=3-2	NGC 6946	01-96	330	2020	14	0.58	6600	1
	M 83	06-00		644		0.62	2400	1
CI 3P1-3P0	NGC 6946	11-94	492	4710	10	0.43	1280	17	$30\times24$	6	70
		07-96		3115		0.53	600
	M 83	11-94		5000		0.43	800	14	$18\times36$	6	45

M 83 (NGC 5236) is likewise a large Sc galaxy. It is part of the Centaurus A group dominated by the giant elliptical NGC 5128 (the radio source Cen A) and containing the peculiar galaxies NGC 4945 and NGC 5253 among others. All main group members have disturbed morphologies suggesting recent interactions or mergers. The group contains a large number of dwarf galaxies (Banks et al. 1999). For M 83, we adopt the group distance D = 3.5 Mpc (cf. Israel 1998; Banks et al. 1999). Presumably because of its southern declination, M 83 has not been studied nearly as well as NGC 6946 at (sub)millimeter wavelengths. Early, relatively low-resolution J=1-0 CO measurements were obtained by Rickard et al. (1977), Combes et al. (1978) and Lord et al. (1987). At a higher resolution of 16'', a J=1-0 CO map was published by Handa et al. (1990), showing a compact central concentration superposed on a "ridge'' of CO extending over 2' in a 45$^{\circ }$ counterclockwise position angle. Measurements of the J=2-1 and J=3-2 transitions of $\,{\rm ^{12}CO}$ and $\13co$ at 22'' resolution were analyzed by Wall et al. (1993), whereas Petitpas & Wilson (1998) reported on J=3-2 and J=4-3 CO and 492 GHz CI maps at similar resolutions. High-resolution aperture synthesis maps have been published for M 83 in J=1-0 CO both at the center (Handa et al. 1994) and at spiral arm disk positions (Kenney & Lord 1991; Lord & Kenney 1991; Rand et al. 1999) as well as in HCN (Helfer & Blitz 1997; Paglione et al. 1997) - the center maps showing a compact, slightly extended source.