Methanol at the Edge of the Galaxy: New Observations to Constrain the Galactic Habitable Zone

• Published in: The Astrophysical journal Vol. 922; no. 2; pp. 106 - 115
• Main Authors: Bernal, J. J., Sephus, C. D., Ziurys, L. M.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.12.2021; IOP Publishing
• Subjects: Abundance; Astrobiology; Astrochemistry; Astrophysics; Chemical abundances; Clouds; Galaxies; Metallicity; Methanol; Milky Way Galaxy; Molecular clouds; Organic chemistry; Pre-biotic astrochemistry; Radiative transfer; Solar neighborhood; Stars & galaxies
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac27a6

Abstract The Galactic Habitable Zone (GHZ) is a region believed hospitable for life. To further constrain the GHZ, observations have been conducted of the J = 2 → 1 transitions of methanol (CH 3 OH) at 97 GHz, toward 20 molecular clouds located in the outer Galaxy ( R GC = 12.9–23.5 kpc), using the 12 m telescope of the Arizona Radio Observatory. Methanol was detected in 19 out of 20 observed clouds, including sources as far as R GC = 23.5 kpc. Identification was secured by the measurement of multiple asymmetry and torsional components in the J = 2 → 1 transition, which were resolved in the narrow line profiles observed (Δ V 1/2 ∼ 1–3 km s −1 ). From a radiative transfer analysis, column densities for these clouds of N tot = 0.1–1.5 × 10 13 cm −2 were derived, corresponding to fractional abundances, relative to H 2 , of f (CH 3 OH) ∼ 0.2–4.9 × 10 −9 . The analysis also indicates that these clouds are cold ( T K ∼ 10–25 K) and dense (n(H 2 ) ∼ 10 6 cm −3 ), as found from previous H 2 CO observations. The methanol abundances in the outer Galaxy are comparable to those observed in colder molecular clouds in the solar neighborhood. The abundance of CH 3 OH therefore does not appear to decrease significantly with distances from the Galactic Center, even at R GC ∼ 20–23 kpc. Furthermore, the production of methanol is apparently not affected by the decline in metallicity with galactocentric distance. These observations suggest that organic chemistry is prevalent in the outer Galaxy, and methanol and other organic molecules may serve to assess the GHZ.