[论文解读] Confronting LambdaCDM with the optical observations of elliptical galaxies: II. Weighing the dark matter component

G. A. Mamon, Ewa L. Łokas|arXiv (Cornell University)|May 25, 2004

Galaxies: Formation, Evolution, Phenomena被引用 1

一句话总结

本文利用四组分模型（恒星、暗物质、热气体、黑洞）和速度弥散数据的Jeans分析，研究椭圆星系中的暗物质（DM）含量。即使采用来自LCDM模拟的合理暗物质剖面和径向各向异性，所推断的爱丁顿半径内的质量光度比仍显著高于预测值——部分解释了行星状星云速度弥散较低的现象，但未能完全解决与观测结果的差异。

ABSTRACT

Elliptical galaxies are modelled with a a 4-component model: Sersic stars, LCDM dark matter (DM), hot gas and central black hole. DM is negligible in the inner regions, which are dominated by stars and the central black hole. This prevents any kinematical estimate (using a Jeans analysis) of the inner slope of the DM density profile. The gas fraction rises, but the baryon fraction decreases with radius, at least out to 10 effective radii (R_e). Even with line-of-sight velocity dispersion (VD) measurements at 4 to 6 R_e with 20 km/s accuracy and perfectly known velocity anisotropy, the total mass within the virial radius (r_v) is uncertain by a factor over 3. The DM distributions found in LCDM simulations are consistent with the stellar VD profiles, but appear inconsistent with the low VDs measured by Romanowsky et al. (2003) of planetary nebulae between 2 and 5 R_e, which imply such low M/Ls that the baryon fraction within r_v must be greater than the universal value. Replacing the NFW DM model by the new model of Navarro et al. (2004) decreases slightly the VD at a given radius. So, given the observed VD measured at 5 R_e, the inferred M/L within r_v is 40% larger than predicted with the NFW model. Folding in the slight (strong) radial anisotropy found in LCDM (merger) simulations, which is well modelled (much better than with the Osipkov-Merritt formula) with beta(r) = 1/2 r/(r+a), the inferred M/L within r_v is another 1.6 (2.4) times higher than for the isotropic NFW model. Thus, the DM model and radial anisotropy can partly explain the low PN VDs, but not in full. In an appendix, single integral expressions are derived for the VDs in terms of the tracer density and total mass profiles, for 3 anisotropic models: radial, Osipkov-Merritt, and the model above, for general radial profiles of luminosity density and mass.

研究动机与目标

• 评估标准LCDM暗物质剖面和速度各向异性是否能调和椭圆星系中行星状星云观测到的低视线速度弥散现象。
• 评估径向各向异性和更新的暗物质剖面（如Navarro et al. 2004）对爱丁顿半径内质量光度比估计的影响。
• 检验由动力学推断的爱丁顿半径内重子物质分数是否与宇宙普遍值一致。
• 推导并应用不同各向异性模型下的速度弥散解析表达式，以提高质量建模的准确性。

提出的方法

• 采用四组分动力学模型：Sersic恒星、LCDM暗物质、热气体和中心黑洞。
• 应用Jeans分析，从4–6倍有效半径处观测到的速度弥散数据推断质量分布。
• 采用径向各向异性模型beta(r) = 1/2 r/(r+a)，该模型比Osipkov-Merritt公式更符合LCDM和并合模拟结果。
• 将NFW暗物质剖面与更新的Navarro et al. (2004)暗物质模型的预测与观测速度弥散进行比较。
• 为三种各向异性模型（径向、Osipkov-Merritt和新beta(r)模型）推导出基于tracer密度和总质量分布的单重积分形式的速度弥散表达式。
• 引入观测不确定性，包括速度弥散测量20 km/s的精度以及对速度各向异性的假设。

实验结果

研究问题

• RQ1 NFW暗物质剖面与各向同性速度弥散能否解释Romanowsky et al. (2003)观测到的行星状星云低速度弥散？
• RQ2 用Navarro et al. (2004)模型替代NFW剖面，对预测速度弥散和推断的质量光度比有何影响？
• RQ3 在LCDM模拟中，径向各向异性在多大程度上能解释观测到的低速度弥散？
• RQ4 与Osipkov-Merritt模型相比，使用更真实的各向异性剖面（beta(r) = 1/2 r/(r+a)）对质量推断有何影响？
• RQ5 在给定观测动力学的前提下，爱丁顿半径内推断的重子物质分数是否与宇宙普遍值一致？

主要发现

• 仅靠NFW暗物质剖面无法完全解释在2–5 R_e处观测到的行星状星云低速度弥散。
• 将NFW剖面替换为Navarro et al. (2004)模型后，相同观测速度弥散下，爱丁顿半径内的质量光度比提高了40%。
• 将来自LCDM模拟的径向各向异性纳入模型后，与各向同性NFW模型相比，推断的质量光度比提高了1.6倍（轻度各向异性）或2.4倍（强各向异性）。
• 即使采用改进的暗物质剖面和各向异性模型，推断的质量光度比仍显著高于标准LCDM预测值，表明该差异仅得到部分但非完全缓解。
• 为不同各向异性模型推导的单重积分速度弥散表达式，为椭圆星系动力学建模提供了更精确且通用的框架。