标题： 边缘：最暗弱星系中暗物质晕的形状 显示英文标题

标题： EDGE: The shape of dark matter haloes in the faintest galaxies

摘要： 无碰撞暗物质仅（DMO）结构形成模拟预测，暗物质（DM）晕在其中心是长椭球形的，在外围则是三轴形的。 气体凝聚的加入使中心暗物质形状变得更为圆润和扁平。 然而，这种形状变化是否发生在“超微弱”矮星系中尚不清楚，这些星系的重子物质比例极低。 我们首次研究了具有气体质量分数为$f_{\rm gas}(r<R_{\rm half}) < 0.06$的超微弱矮星系的形状和速度各向异性。 这些矮星系来自“星系形成边缘的工程矮星系”（EDGE）项目，使用高分辨率模拟，使我们能够在半光半径内解析暗物质晕的形状（$\sim 100\,$pc）。 我们表明，缺乏气体的超微弱矮星系（$M_{\rm 200c} \leqslant 1.5\times10^9\,$M$_\odot$；$f_{\rm gas} < 10^{-5}$）即使包含气体、恒星形成和反馈，仍保留其原始的长椭球形暗物质晕形状。 这可能为暗物质模型提供一种新的且稳健的检验方法。 相比之下，富含气体的极暗矮星（$M_{\rm 200c} > 3\times10^9\,$M$_\odot$;$f_{\rm gas} > 10^{-4}$）在$\sim 10$半光半径内变得更圆且更扁平。最后，我们发现我们模拟的大多数矮星具有显著的径向速度各向异性，在$R \gtrsim 3 R_{\rm half}$处达到$\tilde{\beta} > 0.5$。唯一例外是一个由于平面主要合并形成旋转气体/恒星盘的矮星。当构建贫气极暗矮星的质量模型时应考虑这种强烈的各向异性。 显示英文摘要

摘要： Collisionless Dark Matter Only (DMO) structure formation simulations predict that Dark Matter (DM) haloes are prolate in their centres and triaxial towards their outskirts. The addition of gas condensation transforms the central DM shape to be rounder and more oblate. It is not clear, however, whether such shape transformations occur in `ultra-faint' dwarfs, which have extremely low baryon fractions. We present the first study of the shape and velocity anisotropy of ultra-faint dwarf galaxies that have gas mass fractions of $f_{\rm gas}(r<R_{\rm half}) < 0.06$. These dwarfs are drawn from the Engineering Dwarfs at Galaxy formation's Edge (EDGE) project, using high resolution simulations that allow us to resolve DM halo shapes within the half light radius ($\sim 100\,$pc). We show that gas-poor ultra-faints ($M_{\rm 200c} \leqslant 1.5\times10^9\,$M$_\odot$; $f_{\rm gas} < 10^{-5}$) retain their pristine prolate DM halo shape even when gas, star formation and feedback are included. This could provide a new and robust test of DM models. By contrast, gas-rich ultra-faints ($M_{\rm 200c} > 3\times10^9\,$M$_\odot$; $f_{\rm gas} > 10^{-4}$) become rounder and more oblate within $\sim 10$ half light radii. Finally, we find that most of our simulated dwarfs have significant radial velocity anisotropy that rises to $\tilde{\beta} > 0.5$ at $R \gtrsim 3 R_{\rm half}$. The one exception is a dwarf that forms a rotating gas/stellar disc because of a planar, major merger. Such strong anisotropy should be taken into account when building mass models of gas-poor ultra-faints.