微弱M31卫星And XV和And XVI的Keck/DEIMOS光谱巡天
A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And XV and And XVI
摘要：
我们提出了最近发现的M31卫星和XV和和XVI，躺在从M31的中心的93和130千秒差距的投影距离的光谱调查的结果。这些卫星位于M31的南部，在恒星晕的区域，宽视场成像显示为相对空洞（与度尺度相干流状结构相比）。使用安装在凯克II望远镜上的深度成像多目标光谱仪，我们已经确定了这些卫星的可能成员，我们推导出的径向速度精确到~6 km/s到i~21.5。虽然到And XVI的距离与以前报告的相同（525 pm 50 kpc），我们已经证明，以前用来定义红巨星分支（TRGB）尖端的最亮的三颗恒星实际上是银河系，而And XV实际上可能更遥远，在770 pm 70 kpc（与之前的630 kpc相比），亮度从MV-9.4增加到MV~-9.8。计算得到的速度色散分别为vr =-339+7-6 km/s和sigma-v = 11 +7-5 km/s。在vr =-385+5-6 km/s和sigma-v = 0+10-indef km/s的1 σ处，And XVI色散不完全分辨。利用已确认成员星的测光结果，我们发现了And XV（中值[Fe/H]=-1.58，4分位距+-0.08）和And XVI（中值[Fe/H]=-2.23，4分位距+-0.12）的金属丰度。将成员星的光谱叠加，我们发现And XV（And XVI）的光谱[Fe/H]=-1.8（-2.1），两种情况下的不确定度都为~0.2 dex。我们对And XV的测量合理地解决了它的质量（~10^8 Msun），并提出了一个极轨道，而And XVI的速度表明，鉴于它与M31的中心距离很大，它正在接近M31的逃逸速度。
英文摘要：
We present the results of a spectroscopic survey of the recently discovered M31 satellites And XV and And XVI, lying at projected distances from the centre of M31 of 93 and 130 kpc respectively. These satellites lie to the South of M31, in regions of the stellar halo which wide field imaging has revealed as relative voids (compared to the degree-scale coherent stream-like structures). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ~6 km/s down to i~21.5. While the distance to And XVI remains the same as previously reported (525pm50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch (TRGB) in And XV are in fact Galactic, and And XV is actually likely to be much more distant at 770pm70 kpc (compared to the previous 630 kpc), increasing the luminosity from MV -9.4 to MV~-9.8. The And XV velocity dispersion is resolved with vr =-339+7-6 km/s and sigma-v = 11+7-5 km/s. The And XVI dispersion is not quite resolved at 1sigma with vr =-385+5-6 km/s and sigma-v = 0+10-indef km/s. Using the photometry of the confirmed member stars, we find metallicities of And XV (median [Fe/H]=-1.58, interquar- tile range +-0.08), and And XVI (median [Fe/H]=-2.23, interquartile range +-0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H]=-1.8 (-2.1) for And XV (And XVI), with a uncertainty of ~0.2 dex in both cases. Our measure- ments of And XV reasonably resolve its mass (~10^8 Msun) and suggest a polar orbit, while the velocity of And XVI suggests it is approaching the M31 escape velocity given its large M31-centric distance.
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