Structures of GMC W 37

We carried out observations toward the giant molecular cloud W 37 with the $J = 1 - 0$ transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O using the 13.7 m single-dish telescope at the Delingha station of Purple Mountain Observatory. Based on the three CO lines, we calculated the column densities, cloud masses for the molecular clouds with radial velocities at around $+20 \mathrm{km s}^{-1}$. The gas mass of W 37, calculated from $^{13}$CO emission, is $1.7\times10^5 M_\odot$, above the criteria of giant molecular cloud. The dense ridge of W 37 is a dense filament, which is supercritical in linear mass ratio. Dense clumps found by C$^{18}$O emission are aligned along the dense ridge with a regular interval about 2.8 pc, similar to the clump separation caused by large-scale `sausage instability'. We confirm the identification of the giant molecular filament (GMF) G 18.0-16.8 by \cite{2014A&A...568A..73R} and find a new giant filament, G16.5-15.8, located in the west 0.8 degree of G 18.0-16.8. Both GMFs are not gravitationally bound, as indicated by their low linear mass ratio ($\sim80 M_\odot \mathrm{pc}^{-1}$). We compared the gas temperature map with the dust temperature map from \emph{Herschel} images, and find similar structures. The spatial distributions of class I objects and the dense clumps is reminiscent of triggered star formation occurring in the northwestern part of W 37, which is close to NGC 6611.