狮子座II的大视野--使用SDSS进行结构分析
A Wide-Field View of Leo II -- A Structural Analysis Using the SDSS
摘要：
利用SDSS I观测资料，分析了狮子座II矮球状星系（距离233千秒差距）的恒星分布，以寻找潮汐形变的证据。现有的SDSS光度计目录包含高恒星拥挤区域的空白，因此我们使用应用于SDSS图像的DAOPHOT算法填充了Leo II中心的区域。合并后的DAOPHOT-SDSS数据集包含以Leo II为中心的4x 4平方度区域内的三个滤光片测光。通过定义一个面具在三个过滤器的颜色幅度空间，我们删除了大部分的前景领域的明星。我们测量了Leo II的以下结构参数：核心半径r_c = 2.64 +/- 0.19弧分（178 +/- 13件），潮汐半径r_t = 9.33 +/- 0.47弧分（632 +/- 32 pc），V带总光度L_V =（7.4 +/- 2.0）乘以10^5 L_sun（M_V = -9.9 +/- 0.3）。我们对狮子座II结构的全面分析没有发现任何潮汐扭曲的明显迹象。这个物体的内部结构只包含轻微的等照度线扭曲。在距离狮子座II中心约4.5个潮汐半径处发现了一个小的超密度，但是我们得出结论，根据狮子座II的恒星星族，它不太可能是潮汐剥离的物质，并且很可能是恒星的前景超密度。我们的结果表明，银河系引力场对狮子座II的结构的影响相对温和。我们使用现有的运动学数据结合我们改进的结构测量重新推导了这个系统的质光比，并支持狮子座II强烈主导的暗物质与（M/L）_V ~ 100太阳单位的情况。
英文摘要：
Using SDSS I data, we have analysed the stellar distribution of the Leo II dwarf spheroidal galaxy (distance of 233 kpc) to search for evidence of tidal deformation. The existing SDSS photometric catalogue contains gaps in regions of high stellar crowding, hence we filled the area at the centre of Leo II using the DAOPHOT algorithm applied to the SDSS images. The combined DAOPHOT-SDSS dataset contains three-filter photometry over a 4x4 square degree region centred on Leo II. By defining a mask in three-filter colour-magnitude space, we removed the majority of foreground field stars. We have measured the following Leo II structural parameters: a core radius of r_c = 2.64 +/- 0.19 arcmin (178 +/- 13 pc), a tidal radius of r_t = 9.33 +/- 0.47 arcmin (632 +/- 32 pc) and a total V-band luminosity of L_V = (7.4 +/- 2.0) times 10^5 L_sun (M_V = -9.9 +/- 0.3). Our comprehensive analysis of the Leo II structure did not reveal any significant signs of tidal distortion. The internal structure of this object contains only mild isophotal twisting. A small overdensity was discovered appoximately 4.5 tidal radii from the Leo II centre, however we conclude it is unlikely to be material tidally stripped from Leo II based on its stellar population, and is most likely a foreground overdensity of stars. Our results indicate that the influence of the Galactic graviational field on the structure of Leo II has been relatively mild. We rederived the mass-to-light ratio of this system using existing kinematic data combined with our improved structural measurements, and favour the scenario in which Leo II is strongly dominated by dark matter with (M/L)_V ~ 100 in solar units.
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