N₂H⁺ in the Orion ambient ridge: Cloud clumping versus rotation

Spectra of the J = 1 → 0 transition of N₂H⁺ have been obtained over a 2′ × 2′ area toward the Orion-KL/ IRc2 star-forming region with 26″ angular resolution using the IRAM 30 m telescope. The N₂H⁺ emission, which exclusively traces the ridge gas, exhibits multiple radial velocities which appear to arise from the presence of at least four clouds of quiescent material. One cloud, ∼75″ in extent, lies to the N and NE of OrionKL, with a velocity of K_LSR ∼ 9.5 km s^-1. A second cloud, ∼60″ in extent, lies to the NW and W, with an average velocity of V_LSR ∼ 7.2 km s^-1. There appears to be such a sharp velocity discontinuity between these two clouds that both velocity components are present within a 15″ region centered near IRc2 and other infrared sources. A third region, ∼35″ in extent, with K_LSR ∼ 8.1 km s^-1, is seen ∼ 15″ S of IRc2 and is likely to arise from the compact ridge. A fourth velocity component of V_LSR ∼ 6.4 km s^-1 lies 15″-30″ SW of IRc2 and is attributed to Orion-S. The velocity structure of N₂H⁺ , therefore, does not uniformly change across OMC-1 and, consequently, is inconsistent with the presence of large-scale (∼ 1′-2′) differential rotation of the extended ridge along the SW-NE axis about IRc2. The coincidence of the two larger clouds with star-forming activity in Orion-KL suggests that either the quiescent gas is being pushed apart or that the star formation may have been triggered by a cloud-cloud interaction.