arXiv 0708.30872007-08astro-ph

Local Group矮椭球体的潮汐演化

The Tidal Evolution of Local Group Dwarf Spheroidals

Jorge Penarrubia, Julio F. Navarro, Alan W. McConnachie

备注：: 17 pages, 12 figs., accepted by ApJ
DOI：: 10.1086/523686

摘要：

本文利用N体模拟方法研究了星系潮汐驱动下矮球状星系的演化。我们采用了一个宇宙学动机的模型，其中dSphs近似嵌入NFW晕内的国王模型。我们发现，这些NFW嵌入国王模型是非常弹性的潮汐，恒星密度分布仍然类似于国王模型，即使失去了超过99%的恒星。当潮汐剥离星系时，恒星的光度、速度色散、中心表面亮度和核心半径都单调下降。值得注意的是，我们发现，这些参数的演变是完全控制的总质量损失的发光半径内。在所有参数中，核心半径受到的影响最小：在失去99%的恒星后，R_c仅下降了约2倍。有趣的是，潮汐倾向于使dSphs更暗物质为主，因为紧密结合的中央暗物质“尖端”比"核心“国王轮廓更能适应破坏。我们研究了新发现的超微弱dSphs的极大M/L比是否可能是由曾经更亮的系统的潮汐剥离引起的。虽然dSPH潮汐演变轨迹平行观测到的光度半径平面的标度关系，他们预测过于陡峭的速度色散的变化相比，迄今为止在文献中报道的观测估计。因此，超暗矮星不太可能是像天炉座、天龙座或人马座这样的系统的潮汐遗迹。尽管在光度上跨越了四十年，dSphs似乎居住在具有可比峰值圆速度的晕中，这支持了设想矮椭球体只能在一定质量阈值以上的晕中形成的情景。

英文摘要：

(Abridged) We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically-motivated model where dSphs are approximated by a King model embedded within an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius decrease monotonically. Remarkably, we find that the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius. Of all parameters, the core radius is the least affected: after losing 99% of the stars, R_c decreases by just a factor of ~2. Interestingly, tides tend to make dSphs more dark-matter dominated because the tightly bound central dark matter ``cusp'' is more resilient to disruption than the ``cored'' King profile. We examine whether the extremely large M/L ratios of the newly-discovered ultra-faint dSphs might have been caused by tidal stripping of once brighter systems. Although dSph tidal evolutionary tracks parallel the observed scaling relations in the luminosity-radius plane, they predict too steep a change in velocity dispersion compared with the observational estimates hitherto reported in the literature. The ultra-faint dwarfs are thus unlikely to be the tidal remnants of systems like Fornax, Draco, or Sagittarius. Despite spanning four decades in luminosity, dSphs appear to inhabit halos of comparable peak circular velocity, lending support to scenarios that envision dwarf spheroidals as able to form only in halos above a certain mass threshold.

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