Possible collision-induced outflows and triggered star formation in the molecular complex G34

ABSTRACT G34 is an active star-forming region with complex velocity components. Within the 38–63 km s$^{-1}$ velocity range, we identify a possible cloud–cloud collision at a distance of $\sim$3 kpc. Using the $\rm ^{12}CO$ (${\it J}$ = 1–0) line from the Purple Mountain Observatory 13.7-m millimeter telescope to trace the diffuse gas structures associated with the collision. The gas components at 38–50 and 53–63 km s$^{-1}$ exhibit a U-shaped complementary distribution and a bridge feature in the position–velocity diagram. At the collision interface, the velocity dispersion of $\rm ^{12}CO$ is significantly enhanced, which may result from the impact of the collision. We analyse the spatial distributions of 6.7 GHz CH$_3$OH masers, APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) clumps, H ii regions, young stellar objects, and O-type stars, finding that most are concentrated near the collision interface. This supports a strong coupling between cloud–cloud collisions and star formation. In addition, we detect H i self-absorption features and molecular outflows at the interface. Based on observations of 6 and 2 cm H$_2$CO lines from the Effelsberg 100 m and Tianma Radio Telescope 64 m telescopes, along with NH$_3$ lines from the Nanshan 26 m telescope, we derive an H$_2$ volume density of $10^4$–$10^5$ cm$^{-3}$ in the compressed region. Finally, we compare the collision time-scales ($\gtrsim$0.35 Myr), the dynamical age of the H ii region G34.26+0.15 ($\gtrsim$0.33 Myr), and the outflow time-scale ($\sim$7.5 Myr). The results suggest that gas at the base of the U-shaped structure was compressed during the collision and driven into the outflow. After millions of years of evolution, the gas density increased, potentially triggering star formation.