Properties of molecular clouds in the extreme outer Galaxy

Abstract

We present a study of two molecular clouds with kinematic Galactocentric distances of 22 and 28 kpc (R⊙=8.5 kpc). These are several kiloparsecs further from the Galactic center than any previously known and apparently lie beyond the outermost massive star‐forming regions. We compare the physical properties of the clouds with molecular clouds of similar size near the solar circle, e.g., those in Taurus and Orion. Observations with the CfA 1.2 m and NRAO 12 m telescopes indicate that these distant clouds have sizes of 40–70 pc, molecular masses of 4–7×104 M⊙, and velocity widths 2–3 km s−1. They contain dense molecular clumps of sizes 7–10 pc and kinetic temperatures 15–25 K. The distant clouds are similar to molecular clouds near the solar circle in all the properties we have determined. Their primary difference is an apparent lack of embedded IRAS infrared sources.