We present interferometric HI observations of six double-disc stellar counterrotator ("2$σ$") galaxies from the Atlas3D sample. Three are detected in HI emission; two of these are new detections. NGC 7710 shows a modestly asymmetric HI disc, and the atomic gas in PGC 056772 is centrally peaked but too poorly resolved to identify the direction of rotation. IC 0719, the most instructive system in this study, shows an extended, strongly warped disc of 43 kpc diameter, with a faint tail extending towards its neighbor IC 0718. The gas has likely been accreted from this external source during an encounter whose geometry directed the gas into misaligned retrograde orbits (with respect to the primary stellar body of IC 0719). In the interior, where dynamical time-scales are shorter, the HI has settled into the equatorial plane forming the retrograde secondary stellar disc. This is the first direct evidence that a double-disc stellar counterrotator could be formed through the accretion of retrograde gas. However, the dominant formation pathway for the formation of $2σ$ galaxies is still unclear. The Atlas3D sample shows some cases of the retrograde accretion scenario and also some cases in which a scenario based on an unusually well-aligned merger is more likely.
Asymmetries in Extragalactic Double Radio Sources: Clues from 3D Simulations of Jet - Disc Interaction
河外射电源的观测和理论研究表明，一个不均匀的环境可能是负责观测臂长不对称的喷流和扩展的发射线区域的高红移射电星系的属性。我们进行三维流体动力学模拟的相互作用的强大的河外双极射流与盘状clusius星际介质的对数正态密度分布和分析的不对称性。此外，我们计算射流的不对称性和ISM属性之间的关系，通过蒙特卡罗模拟的基础上通过致密介质的一维传播模型的射流。我们发现，ISM的属性可以与喷射臂长度不对称性的概率分布：磁盘密度和高度被发现有最大的影响不对称性的现实参数范围内，而傅立叶能谱的ISM和湍流马赫数只有较小的影响。流体动力学模拟表明，由于射流及其弓形激波与气体团块的复杂相互作用，不对称性通常可能比1D模型预期的更大，这远远超出了简单的能量处理。根据我们的结果，观测到的中等大小的局域射电星系的不对称性可以用大质量椭圆星系中10^9到10^10太阳质量的气体质量来解释。此外，模拟结果还为观测到的相关性提供了理论基础，即在高红移射电星系中，发射线星云通常在短叶一侧更亮（McCarthy et al. 1991）。喷流与冷气体相的这种相互作用表明，在不断发展的高红移星系中，恒星的形成可能会受到喷流活动的相当大的影响。
