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

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 (Delta V (1/2) similar to 1-3 km s(-1)). From a radiative transfer analysis, column densities for these clouds of N (tot) = 0.1-1.5 x 10(13) cm(-2) were derived, corresponding to fractional abundances, relative to H-2, of f (CH3OH) similar to 0.2-4.9 x 10(-9). The analysis also indicates that these clouds are cold (T (K) similar to 10-25 K) and dense (n(H-2) similar to 10(6) 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) similar to 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.