The Galactic distribution of infrared dark clouds

CS (2-1) measurements toward a large sample of fourth Galactic quadrant infrared dark clouds (IRDCs) were made with the Australia Telescope National Facility Mopra telescope in order to establish their kinematic distances and Galactic distribution. Due to its large critical density, CS unambiguously separates the dense IRDCs from more diffuse giant molecular clouds. The fourth-quadrant IRDCs show a pronounced peak in their radial galactocentric distribution at R = 6 kpc. The first-quadrant IRDC distribution (traced by (CO)-C-13 emission) also shows a peak, but at a galactocentric radius of R = 5 kpc rather than 6 kpc. This disparity in the peak galactocentric radius suggests that IRDCs trace a spiral arm which lies closer to the Sun in the fourth quadrant. Indeed, the deduced IRDC distribution matches the location of the Scutum-Centaurus arm in Milky Way models dominated by two spiral arms. Since, in external galaxies, OB stars form primarily in spiral arms, the association of IRDCs with a Milky Way spiral arm supports the idea that high-mass stars form in IRDCs. The first-quadrant IRDC distribution also reveals a second peak near the solar circle, possibly due to the fact that (CO)-C-13 could trace somewhat lower density IRDCs. The reliability of the MSX IRDC catalog by Simon and coworkers is estimated by using the CS detection rate of IRDC candidates. The overall reliability is at least 58%, and increases to near 100% for high contrasts, Galactic longitudes within similar to 30 degrees of the Galactic center, and large mid-IR backgrounds. A significant fraction of our IRDC sample (14%) showed two CS velocity components, which probably represent two distinct IRDCs along the same line of sight.