2021ApJ...922..106B

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 (CH₃OH) 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 (ΔV_1/2 ∼ 1-3 km s^–1). From a radiative transfer analysis, column densities for these clouds of N_tot = 0.1-1.5 x 10¹³ cm^–2 were derived, corresponding to fractional abundances, relative to H₂, of f (CH₃OH) ∼ 0.2-4.9 x 10^–9. The analysis also indicates that these clouds are cold (T_K ∼ 10-25 K) and dense (n(H₂) ∼ 10⁶ cm^–3), as found from previous H₂CO observations. The methanol abundances in the outer Galaxy are comparable to those observed in colder molecular clouds in the solar neighborhood. The abundance of CH₃OH 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.