even in what we consider largely “empty” regions between the major, well–known subunits of the Taurus molecular cloud complex, there is molecular gas. This is discussed further in the following section. The overall composition of the mask 0 region, particularly the presence of atomic gas, is the subject of another study.

The mask 0, mask 1, and mask 2 regions have close to equal numbers of pixels. Their distribution, however, is very different. Figure \[mask\_regions\] shows the four mask regions. It is evident that the mask 1 predominantly surrounds mask 2, which is consistent with the expectation that both isotopologues are detected in the regions of highest column density (mask 2) while in the periphery of these regions we detect in individual pixels the  but not the  emission.

**\[!htbp\] ![image](f4.pdf)**

The pixels in mask 3 are unusual inasmuch as they exhibit detectable  emission but not . There are evidently very few such pixels ($\simeq$ 0.1% of the total), although this number is considerably larger than would be expected purely on the basis of Gaussian noise statistics. On close inspection of these spectra, it appears that the problem is due to very low level baseline imperfections partially canceling the  integrated intensity, 