We have mapped the CO(3-2) line emission from several nearby galaxies, usin
g the Heinrich-Hertz-Telescope on Mt. Graham, Arizona. Unlike earlier obser
vations, our investigation is not restricted to starburst galaxies, but inc
ludes twelve galaxies of various types and in different stages of star form
ing activity. Furthermore, we have not only observed the central positions
of these objects, but have obtained maps of the extended CO(3-2) emission,
with a typical map extent of 2 to 3 arcmin in each direction. Our observati
ons show that this extended mapping is necessary to reveal spatial changes
of the ISM properties within the galaxies. In this paper we present the dat
a sets and some data analysis. We compare the galaxies in view of their mor
phology and excitation conditions, using line ratios, luminosities and othe
r properties, like the extent of the CO(3-2) emission. The main results of
this CO(3-2) survey are: 1. In none of the observed objects the emission is
confined to the nucleus, as claimed in some earlier publications. CO(3-2)
emission can be detected for some objects to the same extent as the CO(2-1)
and the CO(1-0) lines. 2. The emission is more concentrated to the vicinit
y of star forming structures (nuclear regions and spiral arms) than the low
er CO transitions for most of the observed objects. This is shown by decrea
sing (3-2)/(1-0) line intensity ratios from the very centres towards larger
radii. The (deconvolved) sizes of the central emission peaks in the CO(3-2
) line vary from about 300 pc up to 3 kpc. 3. The CO(3-2) luminosity is str
onger in objects that contain a nuclear starburst or morphological peculiar
ities. The total power emitted in the CO(3-2) line from the central regions
(i.e. excluding spiral arms/outer disk) is highest in the starburst galaxi
es NGC2146, M82, NGC3628, and in the spiral galaxy M 51. When comparing the
total power normalized to the size of the emission region, the starburst g
alaxies M82 and NGC253 show the highest values (about three times higher th
an most other objects), while NGC278 and NGC4631 show the lowest. 4. With t
he present spatial resolution, the line ratios R-3,R-1 seem to be independe
nt of Hubble type, color or luminosity. Most galaxies with enhanced central
star formation ("starbursts") show line ratios of the integrated intensiti
es of R-3,R-1 similar to 1.3 in the very centre and similar to1.0 at a radi
us of about 1 kpc. Objects with a ring-like (or double-peak if seen edge-on
) molecular gas distribution (NGC253, M82, and NGC4631) show lower ratios.
The two galaxies that have CO(3-2) emission distributed over their spiral a
rms (NGC891 and M51) show very low line ratios despite their high infrared
luminosities. This result suggests that CO emission in these objects reflec
ts a large amount of molecular gas, but not enhanced star forming activity.
5. Starburst galaxies show CO(3-2) emission also in their disks. The line
intensities are higher than that of normal galaxies. This suggests that eve
n if a starburst is a localized phenomenon, it is related to different prop
erties of the molecular gas over the whole galaxy.