[论文解读] RUBIES Reveals a Massive Quiescent Galaxy at z=7.3

Andrea Weibel, Anna de Graaff+27 more|arXiv (Cornell University)|Sep 5, 2024

Astronomy and Astrophysical Research被引用 5

一句话总结

本研究报告了 RUBIES-UDS-QG-z7 的光谱确认，该天体在 z=7.29 是一个巨大的静止星系，详细描述了其恒星质量、早期快速形成、紧凑尺寸，以及对早期抑制与星系形成模型的影响。

ABSTRACT

We report the spectroscopic discovery of a massive quiescent galaxy at $z_{ m spec}=7.29\pm0.01$, just $\sim700\,$Myr after the Big Bang. RUBIES-UDS-QG-z7 was selected from public JWST/NIRCam and MIRI imaging from the PRIMER survey and observed with JWST/NIRSpec as part of RUBIES. The NIRSpec/PRISM spectrum reveals one of the strongest Balmer breaks observed thus far at $z>6$, no emission lines, but tentative Balmer and Ca absorption features, as well as a Lyman break. Simultaneous modeling of the NIRSpec/PRISM spectrum and NIRCam and MIRI photometry (spanning $0.9-18\,μm$) shows that the galaxy formed a stellar mass of log$(M_*/M_\odot)=10.23^{+0.04}_{-0.04}$ before $z\sim8$, and ceased forming stars $50-100\,$Myr prior to the time of observation, resulting in $\log( m{sSFR/Gyr}^{-1})<-1$. We measure a small physical size of $209_{-24}^{+33}\,{ m pc}$, which implies a high stellar mass surface density within the effective radius of $\log(Σ_{*, m e}/M_\odot\,kpc^{-2})=10.85_{-0.12}^{+0.11}$ comparable to the highest densities measured in quiescent galaxies at $z\sim2-5$. The 3D stellar mass density profile of RUBIES-UDS-QG-z7 is remarkably similar to the central densities of local massive ellipticals, suggesting that at least some of their cores may have already been in place at $z>7$. The discovery of RUBIES-UDS-QG-z7 has strong implications for galaxy formation models: the estimated number density of quiescent galaxies at $z\sim7$ is $>100 imes$ larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.

研究动机与目标

• 利用 JWST 数据在最高红移处寻找巨大的静止星系的动机。
• 表征 RUBIES-UDS-QG-z7 的恒星族史与星形成抑制时间尺度。
• 量化该星系的结构特征并与跨红移的已知致密静止系统进行比较。
• 评估对星系形成模型及早期静止星系的预计数密度的影响。

提出的方法

• 从 JWST/NIRCam 和 MIRI 成像（PRIMER 数据集）中选择高红移静止候选对象。
• 获取 JWST/NIRSpec PRISM 光谱并使用 msaexp 流程减少数据，包括通量标定和条影校正。
• 使用 Prospector（FSPS/MILES/MIST，Chabrier IMF）进行贝叶斯 SED 拟合，采用非参 SFH 和 Kriek–Conroy 灰尘衰减模型。
• 通过联合拟合 NIRSpec 光谱和 NIRCam/MIRI 光度测度（0.9–18 μm）来约束红移、恒星质量、金属丰度、尘埃和 SFH。
• 通过从去投影的 Sérsic 拟合对 rest-frame 光学/紫外成像的 Abel 变换，推导恒星质量密度和三维密度分布。
• 评估不确定性和系统性效应，包括金属丰度先验和潜在的 α-增强。

Figure 1: NIRSpec/PRISM Spectrum of RUBIES-UDS-QG-z7. Top: 2D SNR spectrum. Bottom: 1D spectrum of RUBIES-UDS-QG-z7 in red, with 1 $\sigma$ uncertainties in gray. The NIRCam photometry is shown as orange dots and the best-fitting SED from prospector in blue (see Section 3 ). A zoom-in to the region

实验结果

研究问题

• RQ1 RUBIES-UDS-QG-z7 是否为 z>7 的真正巨大的静止星系，其恒星质量和恒星形成历史为何？
• RQ2 此类早期静止系统的结构特征与恒星质量密度为何，与较晚时期相比有何不同？
• RQ3 推断的形成与抑制时间尺度对早期宇宙中巨质量静止星系的形成情景意味着什么？

主要发现

• 光谱红移为 z_spec = 7.29 ± 0.01，伴随强烈 Balmer 割断且无发射线，表明处于静止状态。
• log(M*/M_sun) = 10.23 ± 0.04（fiducial），对应恒星质量约为 1.7 × 10^10 M_sun。
• 星形成历史显示在 z ~ 8–9 时出现快速星暴 (~100–200 Myr)，SFR ~100 M_sun/yr，z ~ 8 时抑制，最近 ~50 Myr 内的 SFR 可忽略。
• 极为紧凑的尺寸，有效半径 R_e ≈ 209^{+33}_{-24} pc，导致高恒星质量表面密度 log(Σ*,e / M_sun kpc^-2) = 10.85^{+0.11}_{-0.12}。
• 三维质量密度分布与本地大型椭圆星系核心相当，表明在 z>7 时中央密度已部分形成。
• 数密度的含义：观测到的群体表明在 z ~ 7 时静止星系的数密度比当前模型预测高出>100×。

Figure 2: Top: NIRCam and MIRI imaging cutouts of RUBIES-UDS-QG-z7. Middle: The posterior median SED from prospector along with the NIRCam + MIRI photometry, which constrains the fit over the rest-frame optical to near-infrared. Bottom: Fitted non-parametric SFHs for the fiducial model ( $Z=0.11\,{\