1 Introduction

Carbon monoxide (CO), the most common molecule after $\,{\rm H_{2}}$, is now routinely detected in external galaxies. However, when exposed to energetic radiation, CO is readily photodissociated turning atomic carbon into an important constituent of the interstellar medium. As the ionization potential of neutral carbon is quite close to the dissociation energy of CO, neutral carbon subsequently may be ionized rather easily. As a consequence, [CI] emission primarily arises from interface regions between zones emitting in [CII] and CO respectively (see e.g. Israel et al. 1996; Bolatto et al. 2000). It requires column densities sufficiently high for shielding against ionizing radiation, but not so high that CO selfshielding allows most gas-phase carbon to be bound in molecules. In principle, observations of emission from CO, C$^{\circ}$ and C⁺ provide significant information on the physical condition of the cloud complexes from which the emission arises (Israel et al. 1998; Gerin & Phillips 2000; Israel & Baas 2001). Even though the far-infrared continuum and the [CII] line are much more efficient coolants, the various CO and [CI] lines are important coolants for relatively cool, dense molecular gas, contributing about equally to its cooling (Israel et al. 1995; Gerin & Phillips 2000). In galaxies, however, studies of the dense interstellar medium are complicated by the effectively very large linear observing beams which incorporate whole ensembles of individual, mutually different clouds. The clumpy nature of the interstellar medium allows UV radiation to penetrate deeply into cloud complexes, so that the CO, [CI] and [CII] emitting volumes appear to coincide when observed with large beamsizes. The physics and structure of such photon-dominated regions (PDR's) has been reviewed most recently by Hollenbach & Tielens (1999), whereas their consequent observational parameters have been modelled by e.g. Kaufman et al. (1999).

[CII] emission has been observed towards numerous galaxies, both from airborne (the now defunct NASA Kuiper Airborne Observatory) and from spaceborne (the equally defunct Infrared Space Observatory) platforms. In contrast to these [CII] observations, observations of [CI] emission can be performed on the ground, at least in the ${\rm ^{3}P_{1}}{-}{\rm ^{3}P_{0}\,[CI]}$ transition at 492 GHz. However, atmospheric transparency is poor at such high frequencies and weather conditions need to be unusually favourable for observations of the often weak extragalactic [CI] emission to succeed, even at the excellent high-altitude site of telescopes as the JCMT and the CSO. Consequently, the number of published results is relatively limited. Beyond the Local Group, i.e. at distances larger than 1 Mpc, [CI] has been mapped in bright galaxies such as IC 342 (Büttgenbach et al. 1992), M 82 (Schilke et al. 1993; White et al. 1994; Stutzki et al. 1997) and NGC 253 (Israel et al. 1995; Harrison et al. 1995), as well as M 83 (Petitpas & Wilson 1998; Israel & Baas 2001) and NGC 6946 (Israel & Baas 2001). A survey of 13 galaxies, including limited radial mapping of NGC 891 and NGC 6946 was recently published by Gerin & Phillips (2000). In this paper, we present a similar [CI] survey of 15 galaxies. We also obtained J= 2-1 ${\rm ^{13}CO}$ measurements for all galaxies, and J= 4-3 measurements for all but two. Taking overlap into account, this survey brings the total number of galaxies outside the Local Group, detected in [CI], to 26.

	Figure 1: [CI] and J= 2-1 ${\rm ^{13}CO}$ spectra observed towards sample galaxies. The vertical scale is $T_{\rm mb}$ in K; the horizontal scale is velocity $V_{\rm LSR}$ in $\,{\rm {km\,s^{-1}}}$. For all galaxies, the temperature range in [CI] is four times that in ${\rm ^{13}CO}$.
	Figure 1: continued.

   

Table 1: Line observations log.

Galaxy	Position		Adopted	[CI]			J= 2-1 ${\rm ^{13}CO}$		J= 4-3 ${\rm ^{12}CO}$
	RA(1950)	Dec(1950)	Distance	No. [CI]	Date	$T_{\rm sys}$	Date	$T_{\rm sys}$	Date	$T_{\rm sys}$
	(h m s)	($^{\circ}$ ' '')	(Mpc)	Points		(K)		(K)		(K)
NGC 253	00:45:05.7	-25:33:38	2.5	20	12/93	3770	12/93	1695	11/94	9800
NGC 278	00:49:15.0	+47:16:46	12	1	07/96	3650	06/95	480	01/01	1325
NGC 660	01:40:21.6	+13:23:41	13	1	07/96	3065	05/01	350	08/99	3870
Maffei 2	02:38:08.5	+59:23:24	2.7	1	12/93	4885	01/96	550	07/96	3700
NGC 1068	02:40:07.2	-00:13:30	14.4	22	07/96	4000	01/96	455	07/96	3365
IC 342	03:41:36.6	+67:56:25	1.8	27	11/94	4485	02/89	1440	04/94	2170
M  82	09:51:43.9	+69:55:01	3.25	6	12/93	7200	04/93	335	10/93	9085
NGC 3079	09:58:35.4	+55:55:11	18.0	7	03/94	6240	06/95	310	03/94	5510
NGC 3628	11:17:41.6	+13:51:40	6.7	8	11/94	3450	06/95	325	03/94	2414
NGC 4826	12:54:17.4	+21:57:06	5.1	2	03/97	3520	12/93	535	12/93	2045
M 51	13:27:45.3	+47:27:25	9.6	4	11/94	6600	06/95	370	04/96	4065
M 83	13:34:11.3	-29:36:39	3.5	14	12/93	4590	06/95	430	12/93	4360
NGC 5713	14:37:37.6	-00:04:34	21.0	1	02/99	8000	12/00	515	...	...
NGC 6946	20:33:48.8	+59:58:50	5.5	17	07/96	3970	01/96	530	07/96	2900
NGC 7331	22:34:46.6	+34:09:21	14.3	1	11/96	1925	12/97	320	...	...

 

 

Table 2: [CI], J= 2-1 ${\rm ^{13}CO}$, J= 4-3 ${\rm ^{12}CO}$ intensities.

Galaxy	Offset	Center Position					Area-integrated
		$T_{\rm mb}$([CI])	I([CI])	$I({\rm ^{12}CO}~4$-3)	I([CI])	$I({\rm ^{13}CO})$	[CI] Luminosity
		(10'')			(22'')
	''	(mK)	( $\,{\rm {K\,km\,s^{-1}}}$)				( $\,{\rm {K\,km\,s^{-1}}}$ kpc2)
NGC 253	0, 0	2615	$486 \pm 60$	$1019 \pm 120$	$290 \pm 45$	$106 \pm 13$	$14 \pm 2.2$
NGC 278	0, 0	100	$7 \pm 3$	$9 \pm 2$	($5 \pm 1$)	$2.6 \pm 0.4$	( $3.1 \pm 0.7$)
NGC 660	0, 0	240	$50 \pm 8$	$85 \pm 12$	($31 \pm 8$)	$7.8 \pm 1.1$	($26 \pm 7$)
Maffei 2	0, 0	190	$37 \pm 7$	$405 \pm 50$	($20 \pm 7$)	$22 \pm 4$	( $0.9 \pm 0.3$)
NGC 1068	0, 0	560	$109 \pm 12$	$153 \pm 19$	$49 \pm 9$	$11 \pm 2$	$53 \pm 8.5$
IC 342	0, 0	1030	$54 \pm 6$	$209 \pm 21$	$27 \pm 7$	$24 \pm 3$	$1.1 \pm 0.3$
M 82	0, 0	2130	$224 \pm 35$	$591 \pm 95$	$180 \pm 30$	$60 \pm 9$	$39 \pm 6.9$
NGC 3079	0, 0	530	$111 \pm 18$	$115 \pm 20$	($70 \pm 15$)	$12 \pm 3$	$143 \pm 31$
NGC 3628	-17, +5	265	$84 \pm 11$	$110 \pm 15$	$38 \pm 8$	$10 \pm 2$	$28 \pm 5.7$
NGC 4826	0, 0	135	$11 \pm 2$	...	$17 \pm 4$	$7.8 \pm 1.6$	...
	-20, +5	270	$86 \pm 10$	$73 \pm 9$	($47 \pm 10$)	$15 \pm 2$	($13 \pm 3$)
M 51	0, 0	565	$28 \pm 5$	$24 \pm 4$	($13 \pm 4$)	$8.4 \pm 1.9$	($8 \pm 2.7$)
	-12, -12	340	$24 \pm 5$	...	...	...	...
	-12, -24	755	$55 \pm 9$	...	...	...	...
	-24, -24	470	$55 \pm 9$	...	...	...	...
M 83	0, 0	685	$83 \pm 14$	$270 \pm 20$	$55 \pm 8$	$29 \pm 3$	$3.6 \pm 0.5$
NGC 5713	0, 0	<90	$2 \pm 0.4$	...	...	$7.4 \pm 1.6$	1.7-2.6
NGC 6946	0, 0	465	$85 \pm 9$	$216 \pm 20$	$44 \pm 8$	$22 \pm 3$	$13 \pm 2.4$
NGC 7331	0, 0	30	$2 \pm 0.3$	...	...	$2.5 \pm 0.6$	0.8-1.7

Note: Offset positions (-17, +5) of NGC 3628 and (-20, +5) of NGC 4826 are actual nucleus positions.