UGC 4599：最近Hoag型环星系的测光研究
UGC 4599: A Photometric Study of the Nearest Hoag-Type Ring Galaxy
摘要：
我们提出了一个测光研究的UGC 4599，低光度星系表面上类似于霍格的对象，在天空调查图像，它似乎是一个完整的环周围的圆形核心。霍格型星系外环的性质仍有争议，可能与缓慢的长期演化或环境过程有关，例如星系-星系相互作用。我们发现，在UGC 4599中，（a）近圆形的中心体几乎一直遵循r^1/4的光分布，（b）等照度线强烈扭曲，在半径r~6角秒处有45度的急剧转变，（c）蓝环似乎已经与中心体达到近平衡位形，（d）该环实际上是由一个1.5圈的螺旋特征组成的，（e）螺旋的一侧显示出明显的恒星形成，其形式为至少9个HII区，由它们的H_α发射揭示。基于光度数据，再加上HI信息从文献中，我们表征UGC 4599作为一个椭圆状的对象所包围的发光环和一个巨大的，非常扩展HI盘。考虑到它的观测特性，我们排除了UGC 4599代表早型棒星系演化的晚期阶段。我们讨论了UGC 4599的起源，并得出结论，这个星系可能是两个富含气体的螺旋星系之间发生至少5 Gyr前的主要相互作用的结果。然而，需要进行深度光学成像和详细的恒星星族分析，以确定大型气体库是否可以直接从星系际介质吸积到早期宇宙中预先存在的椭圆星系上。详细的运动学研究将揭示UGC 4599的中心体和环的确切性质。
英文摘要：
We present a photometric study of UGC 4599, a low-luminosity galaxy superficially resembling Hoag's Object in that on sky survey images it appears to be a complete ring surrounding a roundish core. The nature of the outer ring of Hoag-type galaxies is still debated and may be related either to slow secular evolution or to environmental processes, such as galaxy-galaxy interactions. we show that in UGC 4599 (a) the nearly round central body follows well an r^1/4 light profile almost all the way to the centre, (b) the isophotes are strongly twisted with a sharp 45 deg transition at a radius of r~6 arcsec, (c) the blue ring seems to have reached near-equilibrium configuration with the central body, (d) the ring is actually composed of a one-and-a-half turn spiral feature, and (e) one side of the spiral shows conspicuous star formation in the form of at least nine HII regions, revealed by their H_alpha emission. Based on the photometric data, together with HI information from the literature, we characterize UGC 4599 as an elliptical-like object surrounded by a luminous ring and a massive, extremely extended HI disc. Given its observed properties, we rule out UGC 4599 as representing a late phase in barred early-type galaxies evolution. We discuss the origin of UGC 4599 and conclude that this galaxy could be the result of a major interaction between two gas-rich spiral galaxies that took place at least 5 Gyr ago. However, deep optical imaging and a detailed stellar population analysis are required to determine whether the large gas reservoir could have been accreted directly from the intergalactic medium onto a pre-existing elliptical galaxy in the early Universe. A detailed kinematical study will shed light on the exact nature of the central body and the ring of UGC 4599.
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