H I 21 centimeter absorption line study of the W51 complex

We have carried out an H I 21 cm absorption line study of the W51 complex, using the VLA and the Arecibo telescope. We present H I 21 cm absorption spectra for the compact sources in G49.5-0.4, G49.4-0.3, and W51B. The spectra show that the absorption features toward the W51 complex divide into two groups, one between nu(LSR) = 0 and nu(LSR) +35 km s(-1) and the other between +35 and +73 km s(-1). The former is probably due to the H I gas in the Local arm whereas the latter is probably due to the H I gas in the Sagittarius arm. We have decomposed each absorption spectrum into five to seven Gaussian components. We have found that the absorbing H I column density of the Local arm is similar for all sources, i.e., (3.4-5.0) x 10(21) cm(-2), whereas for the Sagittarius arm it varies by a factor of 3, i.e., (0.7-2.2)x 10(22) cm(-2). We discuss individual compact sources based upon the Gaussian parameters of their absorption spectra. For G49.5-0.4, we conclude that it is behind the high-velocity H I stream and that components f + g and components a, b, and e are located on opposite sides of a +62 km s(-1) H I cloud. For G49.4-0.3, we conclude that it is likely in front of 049.5-0.4 and that components a and b and components d and f are likely spatially close to each other, respectively.