A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites
A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites
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DOI：
10.1086/590336
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发表时间：
2008-05
期刊：
影响因子：
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通讯作者：
N. Martin;J. D. de Jong;H. Rix
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文献类型：
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作者：
N. Martin;J. D. de Jong;H. Rix
我们通过应用于 SDSS 数据的最大似然算法推导了最近发现的极低光度银河系卫星的结构参数。对于每颗卫星，即使只有几十颗恒星低于 SDSS 通量限制，该算法也会对质心、位置角、椭圆率、指数半光半径和成员恒星数量（在 SDSS 内）产生稳健的估计和误差。然后将后一个参数与卫星的恒星种群模型结合使用，得出它们的绝对星等和恒星质量，从而解释彩色星等图散粒噪声。大多数参数与之前的确定非常一致，但我们现在正确地考虑了参数协方差。然而，我们发现微弱的卫星比最初想象的更椭圆，并将这种效果归因于之前使用的平滑地图，这可以由平滑（圆形）内核主导。因此，银河系矮星系最暗的一半 (MV ≳ −7.5) 比最亮的一半 (MV≲ − 7.5, ⟨ϵ⟩ = 0.32 ± 0.02) 显着平坦 (4 σ) (⟨ϵ⟩ = 0.47 ± 0.03)。根据我们最好的模型，我们还研究了卫星看似扭曲的形状（通常被认为是潮汐扭曲的迹象）是否可以量化。我们发现，除了 Canes Venatici I 和 Ursa Major II 中存在扭曲的初步证据外，这些都可以完全由稀疏采样系统中的泊松散射来解释。我们考虑了三种可以解释微弱卫星相当拉长形状的情况：旋转支持系统、遵循更多三轴暗物质子晕形状的恒星，或由于与银河系的潮汐相互作用而导致的拉长。尽管这些都不是完全令人满意，但最后一个似乎是问题最小的，但显然需要更深入的观察来追踪这种潮汐相互作用的证据。
We derive the structural parameters of the recently discovered very low luminosity Milky Way satellites through a maximum likelihood algorithm applied to SDSS data. For each satellite, even when only a few tens of stars are available down to the SDSS flux limit, the algorithm yields robust estimates and errors for the centroid, position angle, ellipticity, exponential half-light radius and number of member stars (within the SDSS). This latter parameter is then used in conjunction with stellar population models of the satellites to derive their absolute magnitudes and stellar masses, accounting for color-magnitude diagram shot noise. Most parameters are in good agreement with previous determinations, but we now properly account for parameter covariances. However, we find that faint satellites are somewhat more elliptical than initially thought, and ascribe this effect to the previous use of smoothed maps, which can be dominated by the smoothing (round) kernel. As a result, the faintest half of the Milky Way dwarf galaxies (MV ≳ −7.5) is significantly (4 σ) flatter (⟨ϵ⟩ = 0.47 ± 0.03) than its brightest half (MV≲ − 7.5, ⟨ϵ⟩ = 0.32 ± 0.02). From our best models, we also investigate whether the seemingly distorted shape of the satellites, often taken to be a sign of tidal distortion, can be quantified. We find that, except for tentative evidence of distortion in Canes Venatici I and Ursa Major II, these can be completely accounted for by Poisson scatter in the sparsely sampled systems. We consider three scenarios that could explain the rather elongated shape of faint satellites: rotation supported systems, stars following the shape of more triaxial dark matter subhalos, or elongation due to tidal interaction with the Milky Way. Although none of these is entirely satisfactory, the last one appears the least problematic, but obviously warrants much deeper observations to track evidence of such tidal interaction.