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arXiv 1412.3402astro-ph.GA
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场椭圆星系NGC 7507的暗物质探测

Dark Matter Deprivation in Field Elliptical Galaxy NGC 7507

Richard R. Lane, Ricardo Salinas, Tom Richtler

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9 pages, 7 figures. Accepted for publication in A&A
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摘要：

先前的研究表明，椭圆星系NGC 7507的运动学不一定需要暗物质。这是令人不安的，因为在LCDM宇宙学的背景下，所有星系都应该有一个大的暗物质组成部分。我们使用惩罚像素拟合软件来提取速度和速度色散GMOS狭缝掩模光谱。使用牛仔裤和MONDian建模，我们产生最佳拟合模型的速度分散。我们发现NGC 7507有两个组成部分的恒星晕，外晕和内晕反向旋转。速度色散分布在~70”（~7.9 kpc）处呈现出增加，让人想起其他几个椭圆星系。我们的最佳拟合模型是那些在轻度各向异性下的模型，其中包括比LCDM预测的少100倍的暗物质，尽管完全暗物质自由的轻度各向异性模型几乎同样拟合得很好。我们的MONDian模型，各向同性和各向异性，系统地无法重现测量的速度分散在几乎所有的半径。反向旋转的外晕意味着合并残留物，在~70”处速度弥散的增加也是如此。从模拟看来，似乎合理的合并，导致速度分散的增加是一个螺旋螺旋合并。我们的Jeans模型与无暗物质的情况完全一致，然而，可以容纳一些暗物质，尽管浓度比LCDM模拟预测的低得多。这表明NGC 7507可能是一个无暗物质的椭圆星系。无论NGC 7507是完全没有暗物质还是非常缺乏暗物质，这都与当前LCDM宇宙学模拟的预测不一致。如果星系在视线方向上明显变平（例如由于旋转），那么观测到的速度色散可能会重现，然而，调用这种变平是有问题的。

英文摘要：

Previous studies have shown that the kinematics of the field elliptical galaxy NGC 7507 do not necessarily require dark matter. This is troubling because, in the context of LCDM cosmologies, all galaxies should have a large dark matter component. We use penalised pixel fitting software to extract velocities and velocity dispersions from GMOS slit mask spectra. Using Jeans and MONDian modelling we produce best fit models to the velocity dispersion. We find that NGC 7507 has a two component stellar halo, with the outer halo and inner haloes counter rotating. The velocity dispersion profile exhibits an increase at ~70" (~7.9 kpc), reminiscent of several other elliptical galaxies. Our best fit models are those under mild anisotropy which include ~100 times less dark matter than predicted by LCDM, although mildly anisotropic models that are completely dark matter free fit almost equally well. Our MONDian models, both isotropic and anisotropic, systematically fail to reproduce the measured velocity dispersions at almost all radii. The counter rotating outer halo implies a merger remnant, as does the increase in velocity dispersion at ~70". From simulations it seems plausible that the merger that caused the increase in velocity dispersion was a spiral-spiral merger. Our Jeans models are completely consistent with a no dark matter scenario, however, some dark matter can be accommodated, although at much lower concentrations that predicted by LCDM simulations. This indicates that NGC 7507 may be a dark matter free elliptical galaxy. Whether NGC 7507 is completely dark matter free or very dark matter poor, this is at odds with predictions from current LCDM cosmological simulations. It may be possible that the observed velocity dispersions could be reproduced if the galaxy is significantly flattened along the line of sight (e.g. due to rotation), however, invoking this flattening is problematic.

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