Title: A Trail of the Invisible: Blue Globular Clusters Trace the Radial Density Distribution of the Dark Matter -- Case Study of NGC 4278 Show Chinese title

Title: 无形的踪迹：蓝色球状星团揭示暗物质的径向密度分布——NGC 4278 的案例研究

Abstract: We present new, deep optical observations of the early-type galaxy NGC 4278, which is located in a small loose group. We find that the galaxy lacks fine substructure, i.e., it appears relaxed, out to a radius of $\sim$70 kpc. Our $g$- and $i$-band surface brightness profiles are uniform down to our deepest levels of $\sim$28 mag arcsec$^{-2}$. This spans an extremely large radial range of more than 14 half-mass radii. Combined with archival globular cluster (GC) number density maps and a new analysis of the total mass distribution obtained from archival Chandra X-ray data, we find that the red GC subpopulation traces well the stellar mass density profile from 2.4 out to even 14 half-mass radii, while the blue GC subpopulation traces the total mass density profile of the galaxy over a large radial range. Our results reinforce the scenario that red GCs form mostly in-situ along with the stellar component of the galaxy, while the blue GCs are more closely aligned with the total mass distribution in the halo and were accreted along with halo matter. We conclude that for galaxies where the X-ray emission from the hot halo is too faint to be properly observable and as such is not available to measure the dark matter profile, the blue GC population can be used to trace this dark matter component out to large radii. Show Chinese abstract

Abstract: 我们对位于一个小型松散星系团中的早型星系NGC 4278进行了新的、深入的光学观测。我们发现，该星系在半径为$\sim$70 kpc的范围内缺乏精细的次结构，即它看起来是放松的。我们的$g$和$i$波段表面亮度轮廓在我们最深的水平$\sim$28 mag arcsec$^{-2}$处是均匀的。这涵盖了超过14个半质量半径的极大径向范围。结合档案中的球状星团(GC)数量密度图以及从档案Chandra X射线数据获得的总质量分布的新分析，我们发现红色GC子种群从2.4到甚至14个半质量半径很好地追踪了恒星质量密度轮廓，而蓝色GC子种群则在大径向范围内追踪了星系的总质量密度轮廓。我们的结果加强了这样的情景：红色GC主要是在星系恒星成分中就地形成的，而蓝色GC则更紧密地与晕中的总质量分布对齐，并随着晕物质被吸积。我们得出结论，对于X射线发射来自热晕太弱以至于无法正确观测的星系，因此无法用来测量暗物质轮廓的情况，蓝色GC种群可以用来追踪这一暗物质成分到大半径。