2025MNRAS.537.2232Z

The physical factors that influence the development of molecular cloud's density contrast are connected to those that affect star formation in the galaxy. For NGC 628 (M74), the proportion of high- and low-density contrast clouds initially increases with the distance to the galactic centre ($R_{G}$) and then keeps relatively stable. Spiral arms, bubbles, and magnetic fields are not responsible for the variations in density contrast observed among molecular clouds. The effects of shear and tides calculated from the galactic rotation curve consistently decrease as $R_{G}$ increases, and the shear effect can be neglected. We further studied the tidal effects of the neighbouring material on each cloud using the tidal tensor analysis and the pixel-by-pixel computation, after combining molecular gas, atomic gas, and stellar mass surface density maps. When $R_{\rm G} \lt $ 4 kpc, the tidal strengths derived from the pixel-by-pixel computation decrease as $R_{\rm G}$ increases, and then remains relatively constant when $R_{\rm G} \gt $ 4 kpc. This aligns well with the dependence of the proportion of high- and low-density contrast clouds on $R_{\rm G}$. Therefore, the tidal effects of neighbouring material have a significant impact on the development of molecular cloud's density contrast. A key factor contributing to the low star formation rate in the galactic centre is the excessive tidal influences from neighbouring material on molecular clouds, which hinder the gravitational collapse within these clouds, resulting in low density contrasts. The tidal effects from neighbouring material may also be a significant contributing factor to the slowing down of a pure free-fall gravitational collapse for gas structures on galaxy-cloud scales revealed in our previous works by velocity gradient measurements.