2.3. Heating Sources
A long standing question has been the heating source for the IR
emission, and in particular, the extent to which this emission can be
considered a good star formation tracer (see e.g.
[Kennicutt1998]).
ISO now allows us to re-address this
question. Examining the global luminosity of disks of spiral
galaxies,
Roussel et
al. (1999b)
establish a clear correlation between
the MIR and H luminosity
of disks (see
fig. 1a). This correlation is linear and implies
that in
the disks of normal galaxies, the energy source for the MIR emission
is the young stellar population and therefore that it can be used as a
star formation tracer. There are however a few caveats: (1)
in the disk of normal spirals, the broad-band [6.75 µm] /
[15 µm] color is ~
1, as expected if there is little if any contribution from very small
hot grains such as observed in HII regions (see
sec. 1.2). This will not be the case in
stronger
star forming galaxies (see sec. 5). (2) The
central regions of spiral galaxies typically have a lower [6.75
µm] / [15 µm] ratio (e.g.
[Sauvage et
al. 1996];
[Roussel et
al. 1999a];
[Dale et al. 1999]),
possibly indicating again a higher local star
formation rate. Therefore, in the case of an unresolved galaxy,
Roussel et al
(1999b)
argue that their calibration should provide a
lower limit to the actual star formation rate.
The possibility to separate the nuclear region from the disk of the
galaxy in the MIR now allows a re-investigation of the
FIR-H
correlation. This correlation is known to be non-linear, implying
that more than one process is present and that the FIR cannot be
directly used to infer star-formation rates (e.g.
[Sauvage &
Thuan1992]).
Roussel et al
(1999b)
have selected in their sample those galaxies
with the smallest contribution of the central region in the MIR
maps and show that for this sample the
FIR-H
correlation is
very good and linear (fig. 1b). This indicates
that the non-linearity present in the general
FIR-H
relation is induced
by the nuclear component and that in the disk of normal galaxies, the
FIR emission collected by IRAS is mostly from dust heated by the
young stellar population.