通过斯皮策银河系恒星结构巡天（S4G）观测到的NGC 4013的异常垂直质量分布
The unusual vertical mass distribution of NGC 4013 seen through the Spitzer Survey of Stellar Structure in Galaxies (S4G)
摘要：
NGC 4013是一个邻近的Sb侧星系，以其“惊人的”HI弯曲和“巨大的”潮汐流而闻名。以前对这个不寻常的物体的研究表明，它不能令人满意地拟合一个典型的薄+厚盘结构。我们对NGC 4013进行了一个新的分解，考虑了三个恒星扁平组成部分（薄+厚盘加上一个额外的更扩展的组成部分）和一个气体盘。所有四个组件被认为是重力耦合和等温的。为了做到这一点，我们使用了来自斯皮策银河系恒星结构巡天（S4G）的3.6微米图像。我们发现证据表明NGC4013确实有一个薄的和一个厚的磁盘和一个额外的扁平组件。这个平滑和扩展的组成部分（标度高度z_EC kpc）可以解释为一个厚盘或一个被压扁的椭球晕，包含所有三个恒星组成部分总质量的20%。我们认为这不太可能与正在进行的合并有关，也不太可能是由于其他两个磁盘组件中的翘曲导致的离面恒星。相反，我们倾向于这样一种情况，即厚盘和扩展组件是在两个阶段的过程中形成的，在这个过程中，最初的厚盘在星系历史上很快就被合并动态加热，从而在其中形成一个新的厚盘。
英文摘要：
NGC 4013 is a nearby Sb edge-on galaxy known for its "prodigious" HI warp and its "giant" tidal stream. Previous work on this unusual object shows that it cannot be fitted satisfactorily by a canonical thin+thick disk structure. We have produced a new decomposition of NGC 4013, considering three stellar flattened components (thin+thick disk plus an extra and more extended component) and one gaseous disk. All four components are considered to be gravitationally coupled and isothermal. To do so, we have used the 3.6micron images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We find evidence for NGC4013 indeed having a thin and a thick disk and an extra flattened component. This smooth and extended component (scaleheight z_EC kpc) could be interpreted as a thick disk or as a squashed ellipsoidal halo and contains ~20% of the total mass of all three stellar components. We argue it is unlikely to be related to the ongoing merger or due to the off-plane stars from a warp in the other two disk components. Instead, we favor a scenario in which the thick disk and the extended component were formed in a two-stage process, in which an initially thick disk has been dynamically heated by a merger soon enough in the galaxy history to have a new thick disk formed within it.
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