[论文解读] Critical take on "Mass models of disk galaxies from the DiskMass Survey in MOND"

Mordehai Milgrom|arXiv (Cornell University)|Nov 25, 2015

Astronomy and Astrophysical Research参考文献 2被引用 3

一句话总结

本文认为，盘星系中MOND预测与观测的垂直速度弥散剖面之间的差异——MOND预测值比观测值高约30%——很可能是由于DiskMass Survey在综合谱线轮廓中过度代表了冷的、年轻的恒星种群，导致弥散速度系统性低估所致。利用银河系数据校正此偏差后，差异减小至接近1，使MOND预测与观测结果完全一致。

ABSTRACT

Angus et al. (2015) have recently faulted MOND as follows: Studying thirty disc galaxies from the DiskMass survey, they derive the profiles of velocity dispersion perpendicular to the discs as predicted by MOND, $σ_M(r)$, and compare them with $σ(r)$, measured as part of the DiskMass project. This is a new test of MOND, different from rotation-curve analysis. A nontrivial accomplishment of MOND -- not discussed by Angus et al. -- is that the shapes of $σ_M$ and $σ$ agree very well, i.e, $η(r)\equivσ_M(r)/σ(r)$ is well consistent with being $r$-independent (while $σ$ and $σ_M$ are strongly $r$ dependent). The fault found with MOND was that $η$ is systematically above 1 (with an average of about 1.3). I have suggested to Angus et al. that the fault may lie with the DiskMass dispersions, which may well be too low for the purpose at hand: Being based on population-integrated line profiles, they may be overweighed by younger populations, known to have much smaller dispersions, and scale heights, than the older populations, which weigh more heavily on the light distributions. I discuss independent evidence that supports this view, and show, besides, that if the DiskMas dispersions are underestimates by only 25 %, on average, the MOND predictions are in full agreement with the data, in shape and magnitude. Now, Aniyan et al. (2015) have questioned the DiskMass $σ$ on the same basis. They show for the solar column in the Milky Way that: " Combining the (single) measured velocity dispersion of the total young + old disc population... with the scale height estimated for the older population would underestimate the disc surface density by a factor of $\sim 2.$" If this mismatch found for the Milky Way is typical, correcting for it would bring the measured DiskMass $σ(r)$ to a remarkable agreement with the predicted MOND $σ_M(r)$.

研究动机与目标

• 解决来自DiskMass Survey的盘星系中MOND预测与观测的垂直速度弥散剖面之间的明显差异问题。
• 挑战‘群体综合速度弥散准确代表主导的、较老恒星种群（贡献K波段光）弥散’的假设。
• 评估DiskMass弥散测量中的系统误差是否可解释MOND预测与观测之间平均比值η ≈ 1.3的现象。
• 利用观测约束评估MOND建模中的关键假设（如与半径无关的恒星质量光度比ΥK和标高hz）的有效性。
• 证明对DiskMass弥散进行25–30%的向上修正，可使MOND预测在形状与幅度上与数据完全一致。

提出的方法

• 使用QUMOND（MOND的非相对论形式），从棒状物质面密度Σb(r)、标高hz和通过旋转曲线拟合得到的恒星质量光度比ΥK，计算预测的垂直速度弥散剖面σM(r)。
• 应用MOND修正因子ζM(r) ≈ 1/μ(V²(r)/(r a₀))，以考虑非线性引力，其中μ为插值函数，a₀为MOND加速度常数。
• 将MOND预测的σM(r)与DiskMass Survey观测的σ(r)进行比较，量化比值η(r) = σM(r)/σ(r)。
• 利用Aniyan等人（2015）的银河系数据，估算将群体综合弥散转换为代表较老、主导恒星种群弥散所需的校正因子。
• 评估hz（通过hz–hR相关性）和ΥK不确定性对预测σM(r)的影响，注意到hz约25%的离散度对观测η(r)离散度有贡献。
• 通过与侧向星系的直接观测及颜色梯度研究对比，评估与半径无关的ΥK和hz假设的稳健性。

实验结果

研究问题

• RQ1 为何在DiskMass Survey中，比值η(r) = σM(r)/σ(r)系统性大于1（约30%），尽管剖面形状吻合良好？
• RQ2 DiskMass Survey中的群体综合速度弥散是否因过度代表了冷的、年轻的恒星种群而系统性低估？
• RQ3 使用基于银河系的估计对DiskMass弥散进行校正后，能多大程度上消除与MOND预测的差异？
• RQ4 假设的标高hz和恒星质量光度比ΥK的不确定性如何影响预测的σM(r)和观测的η(r)比值？
• RQ5 观测到的η(r)与半径无关，是否与假设的与半径无关的ΥK和hz一致？还是暗示更深层次的物理一致性？

主要发现

• MOND预测与DiskMass测量之间观测到的平均比值η ≈ 1.3，很可能是由于DiskMass Survey中速度弥散系统性低估所致。
• Aniyan等人（2015）发现，银河系中群体综合弥散比真实弥散低约30%，对应校正因子约1.4，与观测到的η比值一致。
• 对DiskMass弥散剖面进行平均25%的向上校正，可使MOND预测在幅度和形状上与观测完全一致。
• η(r)的与半径无关性表明，与半径无关的ΥK和hz假设大致成立，尽管可能存在径向变化。
• hz从hz–hR相关性中获得的约25%离散度，对观测η(r)的离散度有显著贡献，解释了部分差异。
• MOND预测即使在幅度上存在偏差，其形状仍与数据高度吻合，支持该理论的稳健性，幅度偏差可能源于观测偏差而非理论失效。