HST Proper Motions of NGC 147 and NGC 185: Orbital Histories and Tests of a Dynamically Coherent Andromeda Satellite Plane
HST Proper Motions of NGC 147 and NGC 185: Orbital Histories and Tests of a Dynamically Coherent Andromeda Satellite Plane
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DOI：
10.3847/1538-4357/abaf49
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发表时间：
2020-08
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通讯作者：
S. Sohn;E. Patel;M. Fardal;G. Besla;R. P. van der Marel;M. Geha;P. Guhathakurta
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作者：
S. Sohn;E. Patel;M. Fardal;G. Besla;R. P. van der Marel;M. Geha;P. Guhathakurta
利用哈勃空间望远镜（HST）的多历元成像数据，首次测量了M31的两个卫星星系NGC 147和NGC 185的自行（PM）。对于每个星系，我们对HST高级测量相机/WFC和宽视场相机3/UVIS的测量值进行误差加权平均，以确定PM。我们对PM的最终结果是NGC 147的（μ W，μ N）N147 =（−0.0232，0.0378）±（0.0143，0.0146）mas yr−1，NGC 185的（μ W，μ N）N185 =（−0.0242，0.0058）±（0.0141，0.0147）mas yr−1。发现NGC 147绕M31的二维运动方向与其潮汐尾一致。将两个星系的三维位置和速度转换到一个共同的M31中心坐标系中，通过数值轨道积分研究了M31+NGC 147+NGC 185组合系统的详细轨道历史。我们发现NGC 147（NGC 185）在过去的6个Gyr内，距离M31 0.3-0.5 Gyr（1.6 Gyr）最近，距离为70-260 kpc。NGC 147/NGC 185的近心时间与星系周围潮汐尾的存在/不存在有很好的定性相关。我们的PM显示，NGC 147和NGC 185的轨道极点，尽管在较小的程度上，在不确定性范围内与仙女座大平面（GPoA）的法线一致。这是对被确定为GPoA原始成员的任何卫星的3D角动量矢量的首次测量。我们的研究结果加强了假设，GPoA可能是一个动态连贯的实体。我们重新审视了之前关于NGC 147和NGC 185是双星系的说法，并得出结论，这两个星系不太可能相互引力束缚。
We present the first proper-motion (PM) measurements for the dwarf elliptical galaxies NGC 147 and NGC 185, two satellite galaxies of M31, using multiepoch Hubble Space Telescope (HST) imaging data with time baselines of ∼8 yr. For each galaxy, we take an error-weighted average of measurements from HST Advanced Camera for Surveys/WFC and Wide Field Camera 3/UVIS to determine the PMs. Our final results for the PMs are (μ W, μ N)N147 = (−0.0232, 0.0378) ± (0.0143, 0.0146) mas yr−1 for NGC 147 and (μ W, μ N)N185 = (−0.0242, 0.0058) ± (0.0141, 0.0147) mas yr−1 for NGC 185. The 2D direction of motion for NGC 147 about M31 is found to be aligned with its tidal tails. The 3D positions and velocities of both galaxies are transformed into a common M31-centric coordinate system to study the detailed orbital histories of the combined M31+NGC 147+NGC 185 system via numerical orbit integration. We find that NGC 147 (NGC 185) had its closest passage to M31 0.3–0.5 Gyr (≳1.6 Gyr) within the past 6 Gyr at distances of ∼70 kpc (70–260 kpc). The pericentric times of NGC 147/NGC 185 correlate qualitatively well with the presence/absence of tidal tails seen around the galaxies. Our PMs show that the orbital poles of NGC 147, as well as NGC 185, albeit to a lesser degree, agree within the uncertainties with the normal of the Great Plane of Andromeda (GPoA). These are the first measurements of the 3D angular momentum vector of any satellite identified as original GPoA members. Our results strengthen the hypothesis that the GPoA may be a dynamically coherent entity. We revisit previous claims that NGC 147 and NGC 185 are binary galaxies and conclude that it is very unlikely that the two galaxies were ever gravitationally bound to each other.