  Dust emission in the far-infrared (FIR) characterizes the temperature and
quantity of interstellar dust in a spiral disk. The three Spitzer/MIPS bands
are well suited to measuring the gradient in temperature and the total optical
depth in the disk of a spiral galaxy. Another way to estimate the quantity of
dust in a spiral disk is the "Synthetic Field Method" (SFM, Gonzalez et al.
1998), which uses the number of distant field galaxies seen through the disk of
the nearby spiral. The optical depth estimated from this method can be compared
to the values derived from the FIR emission. Since the two techniques depend on
different assumptions regarding the dust geometry and emissivity, this
comparison between the optical depth profiles can potentially shed light on the
structure and quantity of the ISM in spiral disks, especially any colder
components. The dust responsible for the opacity from distant galaxy counts
appears to be predominantly cold (T < 20 K.). The differences between the
radial absorption profiles can be explained by spiral arms in the SFM
measurements. Taken over the same aperture, galaxy counts show higher
extinction values than the FIR derived ones. The implications for dust geometry
can hopefully be explored with a more rigorous estimate of dust mass from the
FIR fluxes.
