[论文解读] The dark matter profile of the Milky Way inferred from its circular velocity curve

Xiaowei Ou, Anna–Christina Eilers+2 more|arXiv (Cornell University)|Mar 22, 2023

Galaxies: Formation, Evolution, Phenomena被引用 13

一句话总结

本研究利用 APOGEE DR17、Gaia DR3 及其他光度测量的光谱-视差，计算银河系圆速度曲线至约 30 kpc，并给出一个最佳拟合的带核的 Einasto 暗物质分布，具有相对较低的 virial 质量。

ABSTRACT

In this paper, we construct the circular velocity curve of the Milky Way out to $\sim 30$ kpc, providing an updated model of the dark matter density profile. We derive precise parallaxes for 120,309 stars with a data-driven model, using APOGEE DR17 spectra combined with Gaia DR3, 2MASS, and WISE photometry. At outer galactic radii up to 30 kpc, we find a significantly faster decline in the circular velocity curve compared to the inner parts. This decline is better fit with a cored Einasto profile with a slope parameter $0.91^{+0.04}_{-0.05}$ than a generalized Navarro-Frenk-White (NFW) profile. The virial mass of the best-fit dark matter halo profile is only $1.81^{+0.06}_{-0.05} imes10^{11}$ $M_{\odot}$, significantly lower than what a generalized NFW profile delivers. We present a study of the potential systematics, affecting mainly large radii. Such a low mass for the Galaxy is driven by the functional forms tested, given that it probes beyond our measurements. It is found to be in tension with mass measurements from globular clusters, dwarf satellites, and streams. Our best-fit profile also lowers the expected dark matter annihilation signal flux from the galactic centre by more than an order of magnitude, compared to an NFW profile-fit. In future work, we will explore profiles with more flexible functional forms to more fully leverage the circular velocity curve and observationally constrain the properties of the Milky Way's dark matter halo.

研究动机与目标

• 在较高精度的视差下，重新确定银河系在较大半径（至约 30 kpc）上的圆速度曲线。
• 通过拟合含重 baryonic+暗物质模型到观测曲线，推断暗物质密度剖面。
• 评估系统性不确定性并比较 DM 剖面拟合（Einasto vs. gNFW）。
• 评估对局部 DM 密度、湮灭信号以及银河系质量估计的影响。

提出的方法

• 利用 APOGEE DR17 光谱与 Gaia DR3 及其光度数据（Gaia、2MASS、WISE）构建一个数据驱动的光谱-视差模型。
• 选择 APOGEE log g 在 0.0 与 2.2 之间的红巨星分支示踪体，并与 Gaia DR3 进行交叉匹配。
• 在对数视差的线性模型中，以光度特征和 8575 个 APOGEE 光谱特征为自变量，使用 L1 正则化训练，以识别有信息量的特征。
• 将光谱光度视差传播到银心坐标系下的坐标与速度，并在轴对称假设下通过 Jeans 方程推导圆速度。
• 将示踪密度建模为指数分布，并将径向速度色散也同样估计为指数形式，然后通过公式 vc(R)^2 = <v_phi^2> - <v_R^2> [1 + dln nu/dlnR + dln< v_R^2>/dlnR] 计算 vc(R)。
• 使用 emcee MCMC 将两种 DM 暗物质晕模型（gNFW 与 Einasto）拟合到圆速度数据，并通过卡方和后验收敛性比较拟合效果。

实验结果

研究问题

• RQ1 What does the Milky Way circular velocity curve look like out to ~30 kpc with updated parallaxes and tracer selection?
• RQ2 Which dark matter density profile (Einasto vs. generalized NFW) best reproduces the outer decline of the measured circular velocity curve?
• RQ3 What are the implied Milky Way halo properties (M200, r200, c200, local DM density) under the preferred DM model, and how do they compare with previous estimates?
• RQ4 How significant are the systematic uncertainties (e.g., tracer density, solar parameters, asymmetric drift) in shaping the outer Galactic rotation curve?
• RQ5 What are the implications of the inferred DM profile for DM detection signals from the Galactic center and for Galactic formation history?

主要发现

• The circular velocity declines steadily with radius, from ~234 km/s at R≈7.9 kpc to ~173 km/s at R≈27.3 kpc, with a noticeably faster outer decline.
• An Einasto DM profile provides a substantially better fit to the data than a generalized NFW profile, with a median alpha ≈ 0.91 and a reduced chi-squared ≈ 2.97.
• The best-fit DM halo implies a virial mass M200 ≈ 1.81 × 10^11 Msun and a virial radius r200 ≈ 119 kpc, significantly lower than typical NFW-based inferences.
• The local DM density is inferred to be ρ_DM,⊙ ≈ 0.447 GeV cm^-3 for Einasto, and the corresponding J-factor is ~15.8 × 10^22 GeV^2 cm^-5 for θ<15°, both reflecting a cored inner profile.
• Systematic uncertainties are modest (1–5%) up to R≈22 kpc but can reach ~15% at larger radii due to neglected asymmetric drift corrections; nonetheless the outer velocity decline persists across systematics.
• The inferred cored DM profile reduces the expected dark matter annihilation flux from the Galactic center by more than an order of magnitude relative to an NFW-based fit.