H i emission and absorption spectrum of the extragalactic continuum point source G31.388−0.384. The emission spectrum is shown in the first panel and is measured in an annulus around the point source with inner and outer radii of 60′′ and 120′′, respectively (corresponds to 3 and 6 times the restoring beam). The second panel presents the absorption spectrum toward the point source. The color shaded areas represent the approximate velocities of the Milky Way spiral arms (Vallée 2008) in blue, red, green, and yellow, the Scutum-Centaurus, Cygnus, Sagittarius, and Perseus arm, respectively; the dashed black line in the first panel indicates the tangential velocity. In the first two panels, the gray shaded area indicates the 5σ noise level. The third panel shows the optical depth computed using Eq. (4) and the gray shaded area indicates the saturated optical depth limit, computed using Eq. (7). In the fourth panel, the spin temperature is presented, which is computed using Eq. (10).
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