Giant molecular cloud G18.1-0.3+51 associated with H II regions and supernova remnant in the 3-kpc expanding ring

Analysing the high-resolution CO line survey of the Galactic plane with the Nobeyama 45-m telescope (FUGIN), we show that the star-forming complex G18.15-0.30+51 (G18) at radial velocity of 51 km s^-1 is a tight triple association of a giant molecular cloud (GMC), H II regions, and a supernova remnant (SNR). The radial velocity of G18 allows three possible kinematic distances of d = 3.9 ± 0.2 kpc for near solution or 12 ± 0.2 kpc for far solution, if we assume circular Galactic rotation, or d = 6.1 ± 0.1 kpc, if it is moving with the 3-kpc expanding ring at an expanding velocity of 50 km s^-1. The H I line absorption of radio continuum from the H II region constrains the distance to 5.6 ≲ d_SNR ≤ 7.6 kpc. The Σ-D (radio brightness-diameter) relation yields the distance to the SNR of $d_{\rm SNR}=10.1^{+11.5}_{-4.7}$ kpc, allowing for a minimum distance of 5.4 kpc. From these, we uniquely determined the distance of G18 to be 6.07 ± 0.13 kpc in the 3-kpc expanding ring with the SNR being physically associated. The molecular mass of the GMC is estimated to be M_mol ~ 3 × 10⁵ M_⊙. The ratio of virial to luminous molecular masses is greater than unity in the central region and decreases outward to ≲0.2 at the cloud edge, indicating that the central region is dynamic, while the entire cloud is stable. We discuss the origin of the G18 triple system and propose a sustainable GMC model with continuous star formation.