大尺度星系效应对NGC 628分子云特性的研究：邻近物质潮汐效应对分子云演化的重大影响
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
AI总结：
本研究探讨NGC 628星系大尺度环境对分子云特性的影响，发现邻近物质的潮汐效应主导分子云密度对比度发展，是抑制星系中心恒星形成率及减缓气体自由落体坍缩的关键因素。
摘要：
影响分子云密度对比度发展的物理因素与影响星系内恒星形成的因素相关。对于NGC 628（M74），高、低密度对比度云的比例最初随到星系中心距离（$R_{G}$）的增加而上升，随后保持相对稳定。旋臂、气泡和磁场并不导致分子云间密度对比度的变化。由星系旋转曲线计算出的剪切和潮汐效应随$R_{G}$增加持续减弱，且剪切效应可忽略。我们结合分子气体、原子气体和恒星质量面密度图，利用潮汐张量分析和逐像素计算进一步研究了邻近物质对每个云的潮汐效应。当$R_{\ m G} <$ 4 kpc时，逐像素计算得出的潮汐强度随$R_{\ m G}$增加而减弱，并在$R_{\ m G} >$ 4 kpc时保持相对恒定。这与高、低密度对比度云比例对$R_{\ m G}$的依赖性高度一致。因此，邻近物质的潮汐效应对分子云密度对比度的发展有重大影响。星系中心恒星形成率低的一个关键因素是邻近物质对分子云的过度潮汐影响，这阻碍了云内的引力坍缩，导致低密度对比度。邻近物质的潮汐效应也可能是一个重要促成因素，减缓了我们先前通过速度梯度测量在星系-云尺度上揭示的气体结构纯自由落体引力坍缩。
英文摘要：
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.
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