Top panel. Region searched for maser counterparts to Hi-GAL sources. Methanol maser detections are denoted by “+” signs, and OH maser detections by red squares. Also shown are the UC H ii regions from the CORNISH catalog (green asterisks). The concentration of sources around ℓ ≃ 35° and b ≃ 0° is near the W44 region. Bottom panel. Longitude-velocity distribution of all masers detected at Arecibo. The colored loci represent the spiral arms and correspond to those shown in the inset: Norma (red), Scutum (black), Sagittarius (green), Perseus (blue). The locus of the tangent point is shown in purple.