2025A&A...693A..21S

The W3/4 Giant Molecular Cloud (GMC) was an ideal target to study the impact of HII regions onto the surrounding molecular gas and star formation. We utilized PMO CO (1-0) data from the Milky Way Imaging Scroll Painting (MWISP) survey to analyze the cloud structure and the feedback effect from the W4 HII region. Our observations showed that cold gas, traced by CO, mainly resided in the W3 GMC, with C¹⁸O concentrated in dense regions, while gas around W4 was dispersed. The ¹³CO position-position-velocity (PPV) distributions revealed a “C” shaped structure in the W3 cloud with more redshifted gas at higher galactic longitudes. A high density layer (HDL) region on the eastern side of the W3 region exhibited a flattened structure facing W4. Subdividing the area into 16 subregions, we found that regions 6-9 on the HDL layer exhibited the strongest radiation, while clouds at the W4 bubble boundary not facing W3 exhibited weak signals, possibly due to star formation triggering and subsequent molecular gas dispersal by the HII region. Analysis along four paths from the W4 HII region to the far side showed a consistent trend of sharply increasing intensity followed by a slow decrease, indicating the gas was effectively eroded and heated by the photon dominated region (PDR) near the boundary of the HII region. Clump identification based on ¹³CO emission revealed 288 structures categorized as “bubble,” “HDL,” and “quiescent” clumps. Analysis of mass-radius and Virial-mass relationships showed a potential for high-mass star formation in 29.5% (85/288) of the clumps, with 39.2% (113/288) being gravitationally bound. HDL clumps exhibited distinct L/M and velocity dispersion, suggesting an earlier evolutionary stage and gravitational instability compared to quiescent and bubble clumps. Clump parameter differences provided evidence for triggered and dispersed effects of the W4 HII region on the HDL and bubble regions, respectively.