Wide-Area Mapping of 155 Micron Continuum Emission from the Orion Molecular Cloud Complex

We present the results of a wide-area mapping of the far-infrared continuum emission toward the Orion complex by using a Japanese balloon-borne telescope. The 155-$\, {\mu \mathrm {m}}$continuum emission was detected over a region of $1.5 \,\mathrm{deg}^2$around the KL nebula with ${3{}^{\mathrm {\prime }}}$resolution similar to that of the IRAS 100-$\, {\mu \mathrm {m}}$map. Assuming a single-temperature model of the thermal equilibrium dust, maps of the temperature and the optical thickness were derived from the 155-$\, {\mu \mathrm {m}}$intensity and the IRAS 100-$\, {\mu \mathrm {m}}$intensity. The derived dust temperature is 5–15 K lower and the derived dust optical depth is 5–300 times larger than those derived from the IRAS 60 and 100-$\, {\mu \mathrm {m}}$intensities due to the significant contribution of the statistically heated very small grains to the IRAS 60-$\, {\mu \mathrm {m}}$intensity. The optical-thickness distribution shows a filamentary dust ridge that has a ${1\rlap {.}{}^{\mathrm {\circ }}5}$extent in the north-south direction and well resembles the Integral-Shaped Filament (ISF) molecular gas distribution. The gas-to-dust ratio derived from the CO molecular gas distribution along the ISF is in the range 30–200, which may be interpreted as being an effect of CO depletion due to the photodissociation and/or the freezing on dust grains.