2025ApJ...986..122K

Observations of the J_Ka, Kc_ = 4_1,4 → 3_0,3, 4_0,4 → 3_1,3, and 2_2,0 → 1_1,1 rotational lines of c-C₃H₂ and the J = 3/2 → 1/2, Ω = 1/2 transition of NO (X²Π_r) were conducted at 2 mm toward 20 Galactic edge clouds with R_GC = 10.8-23.5 kpc using the Arizona Radio Observatory 12 m telescope. The c-C₃H₂ molecule was detected in all 20 objects in the sample, based typically on 2-3 transitions. NO, which exhibits a distinct pattern of lambda doubling and hyperfine splitting in the measured line, was identified in 16 clouds, with distances as far as R_GC ∼ 23.5 kpc. While a 3 mm transition of c-C₃H₂ had been observed before in some of the sample clouds, NO had not previously been detected at such large distances from the Galactic center. These new identifications double the number of molecular clouds known to contain NO. From a radiative transfer analysis, fractional abundances, relative to H₂, were determined to be f(c-C₃H₂) ∼ 0.5-39.8 × 10^–10 and f(NO) ∼ 0.2-21.2 × 10^–8. These abundances are comparable to values observed in molecular clouds in the inner Galaxy (R_GC < 12 kpc). The abundances therefore appear to remain relatively constant with increasing galactocentric distance. These results suggest that elemental abundance gradients at R_GC ≥ 15 kpc in C, N, and O are not as severe as predicted. They also indicate that gas-phase chemistry in the outer Galaxy is quite robust, strengthening the case for widening the extent of the Galactic habitable zone.