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A PROPOSED REVISION OF THE HUBBLE SEQUENCE FOR ELLIPTICAL GALAXIES

机译:拟议的椭圆星系序列修订

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The Hubble classification scheme has a well-known drawback for elliptical galaxies: the sequence E0-E6 correlates primarily with inclination and not with fundamental properties. In contrast, later-type galaxies are ordered by fundamental physical parameters. We propose to revise the Hubble sequence so that it orders ellipticals by isophote shape. Specifically, we suggest that the Im-spiral-S0 sequence be connected to disky ellipticals and thence to boxy ellipticals. The sequence is continuous from S0's to disky E's. However, boxy E's may be unrelated to other ellipticals: global and core properties both show signs of a dichotomy between (1) normal- and low-luminosity ellipticals that rotate rapidly, that are nearly isotropic and oblate-spheroidal, that are coreless, and that have disky-distorted isophotes, and (2) giant ellipticals that are essentially nonrotating, that are anisotropic and moderately triaxial, that have cuspy cores, and that are boxy-distorted. In the classification, we use isophote shape as an implicit indicator of velocity anisotropy. Two observations support this interpretation: (1) ellipticals with disky isophote distortions rotate rapidly while boxy ellipticals tend to rotate slowly; (2) disky ellipticals show little minor-axis rotation while boxy ellipticals often have large amounts of minor-axis rotation. An approximate classification by velocity anisotropy extends the Im-spiral-S0 sequence through the ellipticals in a physically reasonable way, and it is diagnostic of formation mechanisms. Care is required to recognize unfundamental complications, such as the building by accretion of disky substructure inside a boxy elliptical. However, we believe that the proposed scheme distills the essential physics that should be embodied in a relevant morphological classification.
机译:哈勃(Hubble)分类方案对于椭圆星系有一个众所周知的缺点:序列E0-E6主要与倾斜相关,而与基本属性无关。相反,后一类星系由基本物理参数排序。我们建议修改哈勃序列,使其按等视线形状对椭圆进行排序。具体而言,我们建议将Im-spiral-S0序列连接到盘状椭圆形,然后将其连接到矩形椭圆形。从S0到磁盘E的顺序是连续的。但是,盒形E可能与其他椭圆形无关:整体和核心属性都显示出(1)快速旋转的正光和低光度椭圆形(几乎各向同性和扁球形,无芯)之间的二分法迹象,以及具有圆盘形变形的等距线,和(2)基本上不旋转的各向异性的椭圆形,具有各向异性且中等三轴性,具有弯曲的核,并且呈盒形变形。在分类中,我们使用等渗线形状作为速度各向异性的隐含指标。有两个观察结果支持这种解释:(1)具有盘状等张线变形的椭圆形快速旋转,而盒形椭圆形倾向于缓慢旋转; (2)盘状椭圆形的短轴旋转很少,而盒形椭圆形的短轴旋转通常很多。通过速度各向异性的近似分类以物理上合理的方式将Im-spiral-S0序列扩展到了椭圆形中,这是对形成机理的诊断。需要注意识别基本的并发症,例如通过在椭圆形椭圆形内部堆积盘状下部结构来建造建筑物。但是,我们认为,提出的方案提炼了应在相关形态学分类中体现的基本物理学。

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