We present measurements of the morphology of star-forming gas in galaxies from the EAGLE simulations, and its alignment relative to stars and dark matter (DM). Imaging of such gas in the radio continuum enables weak lensing experiments that complement traditional optical approaches. Star-forming gas is typically more flattened than the stars and DM within halo centres, particularly for present-day structures of total mass $\sim 10^{12-12.5}\, {\rm M}_\odot$, which preferentially host star-forming galaxies with rotationally supported stellar discs. Such systems have oblate, spheroidal star-forming gas distributions, but in both less- and more-massive subhaloes the distributions tend to be prolate, and its morphology correlates positively and significantly with that of its host galaxy’s stars, both in terms of sphericity and triaxiality. The minor axis of star-forming gas most commonly aligns with the minor axis of its host subhalo’s central DM distribution, but this alignment is often poor in subhaloes with a prolate DM distribution. Star-forming gas aligns with the DM at the centre of its parent subhalo less strongly than is the case for stars, but its morphological minor axis aligns closely with its kinematic axis, affording a route to observational identification of the unsheared morphological axis. The projected ellipticities of star-forming gas in EAGLE are consistent with shapes inferred from high-fidelity radio continuum images, and they exhibit greater shape noise than is the case for images of the stars, owing to the greater characteristic flattening of star-forming gas with respect to stars.

中文翻译：

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EAGLE 模拟中恒星形成气体的形态及其与星系和暗物质晕的排列

我们展示了 EAGLE 模拟中星系中恒星形成气体形态的测量结果，以及它相对于恒星和暗物质 (DM) 的排列。在无线电连续体中对这种气体进行成像可以进行微弱的透镜实验，以补充传统的光学方法。恒星形成气体通常比晕中心内的恒星和 DM 更扁平，特别是对于总质量为 $\sim 10^{12-12.5}\, {\rm M}_\odot$ 的当今结构，它优先拥有旋转支撑的恒星盘的恒星形成星系。这样的系统具有扁圆形的恒星形成气体分布，但在质量较小和较大的亚晕中，分布往往是扁平的，其形态与其宿主星系的恒星的形态呈正相关且显着相关，无论是在球形度和三轴性。恒星形成气体的短轴通常与其宿主亚晕的中心 DM 分布的短轴对齐，但这种排列在具有长 DM 分布的亚晕中通常很差。恒星形成气体与其母子晕中心的 DM 对齐的强度不如恒星的情况，但其形态短轴与其运动轴紧密对齐，为观察识别未剪切形态轴提供了途径。EAGLE 中恒星形成气体的投影椭圆率与从高保真射电连续图像推断的形状一致，并且它们表现出比恒星图像更大的形状噪声，这是由于恒星形成气体具有更大的特征扁平化相对于明星。