1 Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
2 ESA Villafranca, P.O. Box 50727, Madrid, Spain
The [CII] fine structure line emission of the singly-ionized carbon C+at 158 micron is one of the most relevant cooling processes of the Interstellar Gas. Gas-heating is mainly due to photoelectric emission from dust grains. The [CII]-line intensity typically amounts to a few 10-3 of the total far-infrared (FIR) continuum intensity.
Before the ISO mission, the [CII]-emission could only be studied towards
relatively bright, compact Galactic regions (i.e., HII-regions,
OB associations, giant molecular clouds, planetary nebulae)
and in a number of starburst and gas-rich galaxies.
The emerging tight correlation between the [CII] and [CO(J:1-0)]
(
)
line intensities and the similarities in space
distribution
and line-profile between these two emissions, then suggested an association
of the origin of the [CII] emission with photodissociation regions (PDRs),
i.e. the atomic regions between HII-regions and molecular clouds.
This, in turn, led to the interpretation of [CII] emission
from external galaxies mainly in terms of PDR models.
Here we present ISO measurements of the [CII]-line intensity for a sample
of 12 normal late-type galaxies of the Virgo Cluster, i.e. spiral galaxies
with a low star-formation activity, selected from Leech et al.
(this Conference).
In order to study the properties of the [CII]-emission in external galaxies
with the largest range in star-formation activity available at present,
we supplemented the previous sample of 12 normal late-type galaxies
with 3 low-mass Virgo spiral galaxies, 3 gas-rich and 3 starburst galaxies
with multi-wavelength data available in the literature.
Thus, we define a sample of 21 late-type galaxies with [CII], [CO] and FIR
data and, especially,
(
)
equivalent width
(EW) data.
The latter is a well-known quantitative diagnostic of the current SFR.
Moreover, we define ``quiescent'' and ``star-forming'' non starburst galaxies
with
,
respectively, i.e. with a parameter
'b' of Scalo 
0.1, where 'b' is defined as the ratio of the massive SFR
to the SFR averaged on the age of the disk.
As a first result, we find that the ratio of the [CII]-to-[CO] line
intensities has a reasonably tight positive correlation with the
EW.
Therefore, we demonstrate that
![$\rm I_{[CII]}/I_{[CO]}$](img209.gif){kind=small}
is a reliable tracer
of the massive SFR below its saturation value, i.e. at least
up to
.
We find, for the first time in such studies, that the ratio
of the [CII] line intensity-to-the FIR continuum emission decreases
with decreasing star-formation activity of quiescent galaxies,
pointing towards a ``[CII]-quiet'' component of the FIR emission.
Such galaxies do not have any counterparts in the compact Galactic
[CII]-emission regions. The latter result can be interpreted in terms
of a dominant [CII]-emission from diffuse components of the ISM
of quiescent galaxies.
Moreover, combining our results with those of Nakagawa et al.
(1998, ApJS, 115, 259) on the Galaxy, we infer that, for normal galaxies,
the diffuse fraction of the [CII]-emission is at least as strong as
the compact fraction.