Formaldehyde in the Far Outer Galaxy: Constraining the Outer Boundary of the Galactic Habitable Zone

We present results from an initial survey of the 2₁₂–1₁₁ transition of formaldehyde (H₂CO) at 140.8 GHz in giant molecular clouds in the far outer Galaxy (R_G ≥ 16 kpc). Formaldehyde is a key prebiotic molecule that likely plays an important role in the development of amino acids. Determining the outermost extent of the H₂CO distribution can constrain the outer limit of the Galactic Habitable Zone, the region where conditions for the formation of life are thought to be most favorable. We surveyed 69 molecular clouds in the outer Galaxy, ranging from 12 to 23.5 kpc in galactocentric radius. Formaldehyde emission at 140.8 GHz was detected in 65% of the clouds. The H₂CO spectral line was detected in 26 of the clouds with R_G > 16 kpc (detection rate of 59%), including 6 clouds with R_G > 20 kpc (detection rate of 55%). Formaldehyde is readily found in the far outer Galaxy—even beyond the edge of the old stellar disk. Determining the relatively widespread distribution of H₂CO in the far outer Galaxy is a first step in establishing how favorable an environment this vast region of the Galaxy may be toward the formation of life. Astrobiology 8, 59–73.