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[论文解读] Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS

Samir Nepal, C. Chiappini|arXiv (Cornell University)|Feb 1, 2024
Astronomy and Astrophysical Research被引用 4
一句话总结

本研究利用高精度的Gaia-RVS数据与StarHorse恒星年龄模型，发现了银河系中最古老的薄盘，其形成时间早于宇宙大爆炸后10亿年。研究证实，处于冷、薄盘轨道上的金属贫乏恒星绝大多数年龄超过130亿年，其垂直速度弥散度（σ_Vz）约为20–25 km s⁻¹，与詹姆斯·韦伯太空望远镜（JWST）和阿塔卡马大型毫米波/亚毫米波阵列（ALMA）观测到的高红移盘星系一致，表明银河系盘在早期即已形成，并经历了持续的由内而外的生长过程。

ABSTRACT

JWST has recently detected numerous disc galaxies at high-redshifts and there have been observations of cold disc galaxies at z > 4 with ALMA. In the Milky Way, recent studies find metal-poor stars in cold disc orbits, suggesting an ancient disc. We investigated a sample of 565,606 stars from the hybrid-CNN analysis of the Gaia-DR3 RVS stars. The sample contains 8,500 stars with [Fe/H]<-1. For a subset of ~200,000 MSTO and subgiant stars we computed distances and ages using the StarHorse code with a mean precision of 1% and 12%, respectively. First, we confirm the existence of metal-poor stars in thin disc orbits - over 50% are older than 13 Gyr. Second, we report the discovery of the oldest thin disc of the Milky Way(MW), which extends across a wide range of metallicities, from metal-poor to super-solar stars. The metal-poor stars in disc orbits manifest as a readily visible tail of the metallicity distribution. The high-[α/Fe] thick disc exhibits a vertical velocity dispersion of 35 km/s, while the thin disc shows 10 to 15 km/s lower at similar ages. Our old thin disc $σ_{V_z}$ appears similar to those estimated for the high-z disc galaxies. Third, we extend the [Y/Mg] chemical clock to the oldest ages and estimate a slope of -0.038 dex/Gyr. Finally, we confirm our discovery by showing that the splash includes high- and low-[α/Fe] populations that are both old and extends to super-solar [Fe/H]. We find about 6 to 10% of the old thin disc was heated to thick disc orbits with the youngest splashed stars being 9 to 10 Gyrs. We conclude the MW thin disc forms <1 billion years from Big Bang, building up inside-out, preceding earlier estimates by about 4-5 billion years. Considering a massive merger event such as the GSE, a Splash is expected - we find a portion of the old thin disc is heated to thick disc velocities and the Splash extends to super-solar [Fe/H] regimes.

研究动机与目标

  • 解决银河系薄盘的形成 epoch，挑战早期估计的约80–90亿年前的结论。
  • 确定处于冷盘轨道中的金属贫乏恒星是否属于古老薄盘，还是星系晕/子结构的一部分。
  • 研究重大并合事件（如Gaia-Enceladus）在将恒星从薄盘动力学状态加热至厚盘动力学状态中的作用。
  • 通过[ Y/Mg ]化学钟验证最古老年龄区间的恒星年龄估计。
  • 绘制古老薄盘的完整金属丰度范围，包括超太阳系金属丰度。

提出的方法

  • 利用Gaia DR3 RVS中的202,384颗主序转折点及亚巨星支恒星，其具有六维相空间和高质量恒星参数。
  • 应用StarHorse代码，结合测光与光谱数据，分别以1%和12%的精度推导距离与年龄。
  • 将GALAH巡天的化学丰度数据与Gaia-RVS数据结合，计算[α/Fe]与[Fe/H]，用于运动学与化学动力学分析。
  • 使用[ Y/Mg ]丰度比作为化学钟验证年龄估计，发现其斜率为−0.038 dex Gyr⁻¹。
  • 分析垂直速度弥散度（σ_Vz），以区分薄盘（低σ_Vz）与厚盘（高σ_Vz）星族。
  • 追踪Splash星族在年龄与金属丰度上的分布，识别由并合事件加热至厚盘轨道的恒星。
Figure 1 : Properties of the selected samples: a) the Kiel diagram ( log ( g ) vs $\mathrm{\textit{T}_{\text{eff}}}$ ) for the full sample colour-coded by logarithm of stellar density. The dashed red lines represent the selection of MSTO+SGB stars (i.e. ”age sample”). The black dots represent the me
Figure 1 : Properties of the selected samples: a) the Kiel diagram ( log ( g ) vs $\mathrm{\textit{T}_{\text{eff}}}$ ) for the full sample colour-coded by logarithm of stellar density. The dashed red lines represent the selection of MSTO+SGB stars (i.e. ”age sample”). The black dots represent the me

实验结果

研究问题

  • RQ1银河系薄盘形成于何时？其是否早于GSE并合事件？
  • RQ2处于冷盘轨道中的金属贫乏恒星是否属于原始薄盘，还是星系晕/子结构的一部分？
  • RQ3古老薄盘的垂直速度弥散度是多少？与高红移盘星系相比如何？
  • RQ4GSE并合事件在多大程度上将古老薄盘加热至厚盘动力学状态？
  • RQ5Splash星族是否延伸至超太阳系金属丰度？这对早期盘形成意味着什么？

主要发现

  • 银河系薄盘形成于大爆炸后不足10亿年，比以往估计提前了40–50亿年。
  • 超过50%处于薄盘轨道的金属贫乏恒星（Z_max < 1 kpc，V_φ > 180 km s⁻¹）年龄超过130亿年，证实其古老起源。
  • 古老薄盘的垂直速度弥散度（σ_Vz）为20–25 km s⁻¹，比高[α/Fe]厚盘（σ_Vz ≈ 35 km s⁻¹）低10–15 km s⁻¹。
  • [ Y/Mg ]化学钟被拓展至最古老年龄区间，测得斜率为−0.038 dex Gyr⁻¹，验证了StarHorse年龄估计的可靠性。
  • 约6–10%的古老薄盘被加热至厚盘动力学状态，且在该速度范围内未发现较年轻的低[α/Fe]恒星，表明加热过程发生于90–100亿年前之前。
  • Splash星族包含年龄≥90亿年的高[α/Fe]与低[α/Fe]恒星，并延伸至超太阳系[Fe/H] ≈ 0.25，表明冷盘中存在早期且持续的恒星形成。
Figure 2 : Toomre diagrams ( $\sqrt{V^{2}_{R}+V^{2}_{\phi}}$ vs. $V_{\phi}$ ) for the age sample stars in bins of age and [ Fe / H ] . The plots are color-coded by logarithm of stellar density. Older to younger ages from top to bottom and metal-poor to metal-rich [ Fe / H ] from left to right. The a
Figure 2 : Toomre diagrams ( $\sqrt{V^{2}_{R}+V^{2}_{\phi}}$ vs. $V_{\phi}$ ) for the age sample stars in bins of age and [ Fe / H ] . The plots are color-coded by logarithm of stellar density. Older to younger ages from top to bottom and metal-poor to metal-rich [ Fe / H ] from left to right. The a

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