NGC1052-DF2

• 号称没有暗物质的星系, ultra diffuse galaxy [1] Nature March 2018
  

  HST/ACS image

观测

• S´ersic index n = 0.6, axis ratio b/a = 0.85, the central surface brightness is µ(V606,0) = 24.4magarcsec−2, D=20Mpc.
• effective radius along the major axis is Re = 2.2kpc, M606 = −15.4 mag, Mstars ≈ 2×10^8 Msun.
• an extraordinary population of globular clusters, larger and more luminous than average GCs in the Milky Way, total mass is roughly 4 percent of the mass in stars, making DF2 one of the galaxies with the largest specific frequencies known.
• the line-of-sight velocity dispersion of the GCs(10km/s) suggests a total dynamical mass for DF2 that leaves little room for any dark matter
• 气体含量非常低 [2] [3]

模型

• 测量误差很大，如果有些球状星团是背景，不能说得那么肯定 [4]
• 如果没有暗物质，星系和球状星团质量的比靠近，动力学摩擦效应显著（时标1-2Gyr）,大部分星团团已经被并合了[5]
• 考虑NGC1052的tidal效应，改变了DF2中球状星团在相空间的分布函数 （真实空间外围的GC少了，GC在速度空间的弥散小了） [6]
• 没有暗物质的dynamical friction没那么简单, core-stalling and GC-GC interactions效应, 让星团掉不进去。反过来，如果有cuspy的NFW暗晕，里面的dynamical friction更强，会形成核心GC [7]

• MOND

• 不支持MOND，弱引力场（近似于有暗物质）[8]
• 你没考虑NGC1052作用，符合MOND！ [9] Nature March 2019

结局

• 你们距离算错了 [10]

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• NGC1052-DF4， same group，第二个号称没有暗物质的星系 [11]（还是球状星团算的速度弥散度）