An examination of large-scale galactic effects on molecular cloud properties in NGC 628 : The significant impact of tidal effects from neighboring material on the evolution of molecular clouds

TL;DR

This study investigates how tidal effects from neighboring material influence molecular cloud properties in NGC 628, revealing that tides significantly impact cloud density contrast and star formation rates, especially near the galactic center.

Contribution

It demonstrates the significant role of tidal effects from neighboring material on molecular cloud evolution, a factor previously underappreciated in star formation studies.

Findings

Tidal effects decrease with galactic radius beyond 4 kpc

Tides significantly hinder gravitational collapse in central regions

Density contrast of clouds correlates with tidal strength

Abstract

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 center ($R_{G}$) 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 neighboring 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 $R_{\rm G} <$ 4 kpc, the tidal strengths derived from the pixel-by-pixel computation decrease as $R_{\rm G}$ increases, and then remains relatively constant when $R_{\rm G} >$ 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 neighboring 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 center is the excessive tidal influences from neighboring material on molecular clouds, which hinder the gravitational collapse within these clouds, resulting in low density contrasts. The tidal effects from neighboring 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.