2015ApJ...806...57G

We report high-resolution near-infrared absorption spectroscopy of H₂, H₃⁺, and CO toward the young high mass object NGC 7538 IRS 1. The v = 1-0 H₂S(0) line and lines in the CO v = 2-0 band were detected; the v = 1-0 H₂S(1) line and the v = 1-0 H₃⁺ lines [R(1, 1)^l, R(1, 0), R(1, 1)^u] were not detected. The line of sight traverses two clouds, with temperatures 45 and 259 K and with roughly equal column densities of CO. Assuming that H₂ is at the same temperature as CO and that the two species are uniformly mixed, [H₂]/[CO] = 3600±1200. NGC 7538 is the most distant object from the Galactic center for which [H₂]/[CO] has been directly measured using infrared absorption spectroscopy.