I data, estimate the background brightness being absorbed, and compute its physical properties from a limited set of assumptions. I emission lying behind the HISA feature. We measure T ON , T OFF , and T C , and assume a likely value for p , but T S and tau remain unknown. We use a canonical ISM pressure of 4000 cm -3 K and consider two extreme values for f n of 1.0 and 0.01. We find most properties cluster around common values.
Most features have very narrow linewidths, indicating quiescent gas.
Though it often spatially anticorrelates with T S , the optical depth also drops significantly with T S for low f n . The second mass peak represents Local arm gas, with the main peak corresponding to Perseus arm gas. Our investigation of the spatial distribution of HISA is ongoing. I velocity channel with self-absorption. I spectra, from the cross and boxes respectively, are given for one strong but compact HISA feature marked in the intensity map.