The submillimeter opacity of dust in the diffuse interstellar medium in the Galactic plane has been quantified using a pixel-by-pixel correlation of images of continuum emission with a proxy for column density .
We used multi-wavelength continuum data : three BLAST bands at 250 , 350 , and 500 µm and one IRAS at 100 µm .
The proxy is the near-infrared color excess , E ( J - K _ { s } ) , obtained from 2MASS .
Based on observations of stars , we show how well this color excess is correlated with the total hydrogen column density for regions of moderate extinction .
The ratio of emission to column density , the emissivity , is then known from the correlations , as a function of frequency .
The spectral distribution of this emissivity can be fit by a modified blackbody , whence the characteristic dust temperature T and the desired opacity \sigma _ { e } ( 1200 ) at 1200 GHz or 250 µm can be obtained .
We have analyzed 14 regions near the Galactic plane toward the Vela molecular cloud , mostly selected to avoid regions of high column density ( N _ { H } > 10 ^ { 22 } cm ^ { -2 } ) and small enough to ensure a uniform dust temperature .
We find \sigma _ { e } ( 1200 ) is typically 2 to 4 \times 10 ^ { -25 } cm ^ { 2 } H ^ { -1 } and thus about 2 to 4 times larger than the average value in the local high Galactic latitude diffuse atomic interstellar medium .
This is strong evidence for grain evolution .
There is a range in total power per H nucleon absorbed ( and re-radiated ) by the dust , reflecting changes in the strength of the interstellar radiation field and/or the dust absorption opacity .
These changes in emission opacity and power affect the equilibrium T , which is typically 15 K , colder than at high latitudes .
Our analysis extends , to higher opacity and lower temperature , the trend of increasing \sigma _ { e } ( 1200 ) with decreasing T that was found at high latitudes .
The recognition of changes in the emission opacity raises a cautionary flag because all column densities deduced from dust emission maps , and the masses of compact structures within them , depend inversely on the value adopted .