科研之友
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摘要
We study the globular cluster (GC) system of the nearby elliptical galaxy M87 using the newly available data set with accurate kinematics provided by Strader et al. We find evidence for three distinct subpopulations of GCs in terms of colours, kinematics and radial profiles. A decomposition into three populations - blue, intermediate and red GCs - is statistically preferred to one with two or four populations. The existence of three components has been suggested before, but here we are able to identify them robustly and relate them to the stellar profile. We exploit the subpopulations to derive dynamical constraints on the mass and dark matter (DM) content of M87 out to similar to 100 kpc. We deploy a class of global mass estimators, developed in Paper I, obtaining mass measurements at different locations. The DM fraction in M87 changes from approximate to 0.2 at the effective radius of the stellar light (0 degrees.02 or 6 kpc) to approximate to 0.95 at the distance probed by the most extended, blue GCs (0 degrees.47 or 135 kpc). We complete this analysis with virial decompositions, in which the dynamical model is used to produce velocity dispersions, which in turn are used to separate the GC populations. This ensures that the three subpopulations are simultaneously consistent with the same underlying mass profile. These yield the luminous mass as 5.5(-2.0)(+1.5) x 10(11) M-circle dot and the DM within 135 kpc as 8.0(-4.0)(+1.0) x10(12) M-circle dot. The inner DM density behaves as rho similar to r(-gamma) with gamma approximate to 1.6. This is steeper than the cosmologically preferred cusp of rho similar to r(-1) and may provide evidence of DM contraction. Finally, we combine the GC separation into three subpopulations with the Jeans equations, obtaining information on the orbital structure of the GC system. The centrally concentrated red GCs exhibit tangential anisotropy, consistent with the depletion of radial orbits by tidal shredding. The most extended blue GCs have an isotropic velocity-dispersion tensor in the central parts, which becomes more tangential moving outwards, consistent with adiabatic contraction scenarios of the DM halo.
- 出版日期2014-8-21
