[论文解读] Clumpy star formation and an obscured nuclear starburst in the luminous dusty z = 4 galaxy GN20 seen by MIRI/JWST

Arjan Bik, Javier Álvarez-Márquez+29 more|arXiv (Cornell University)|Dec 5, 2023

Astrophysics and Star Formation Studies参考文献 12被引用 1

一句话总结

利用詹姆斯·韦布空间望远镜（JWST）的深红外MIRI/MRS积分场光谱，本研究解析了红移z=4.0548的GN20星暴星系中成团的、高度遮蔽的恒星形成活动。结果揭示了一个极端消光的核区星暴（A_V = 17.2 ± 0.4 mag），其中仅7.7%的红外恒星形成率由Paα发射追踪，表明中心星暴被引力不稳定盘所深度掩埋，这一现象可能由晚期阶段的主并合驱动。

ABSTRACT

Dusty star-forming galaxies emit most of their light at far-IR to mm wavelengths as their star formation is highly obscured. Far-IR and mm observations have revealed their dust, neutral and molecular gas properties. The sensitivity of JWST at rest-frame optical and near-infrared wavelengths now allows the study of the stellar and ionized gas content. We investigate the spatially resolved distribution and kinematics of the ionized gas in GN20, a dusty star forming galaxy at $z$=4.0548. We present deep MIRI/MRS integral field spectroscopy of the near-infrared rest-frame emission of GN20. We detect spatially resolved \paa, out to a radius of 6 kpc, distributed in a clumpy morphology. The star formation rate derived from \paa\ (144 $\pm$ 9 \msunperyear) is only 7.7 $\pm 0.5 $\% of the infrared star formation rate (1860 $\pm$ 90 \msunperyear). We attribute this to very high extinction (A$_V$ = 17.2 $\pm$ 0.4 mag, or A$_{V,mixed}$ = 44 $\pm$ 3 mag), especially in the nucleus of GN20, where only faint \paa\ is detected, suggesting a deeply buried starburst. We identify four, spatially unresolved, clumps in the \paa\ emission. Based on the double peaked \paa\ profile we find that each clump consist of at least two sub-clumps. We find mass upper limits consistent with them being formed in a gravitationally unstable gaseous disk. The UV bright region of GN20 does not have any detected \paa\ emission, suggesting an age of more than 10 Myrs for this region of the galaxy. From the rotation profile of \paa\ we conclude that the gas kinematics are rotationally dominated and the $v_{rot}/σ_{m} = 3.8 \pm 1.4$ is similar to low-redshift LIRGs. We speculate that the clumps seen in GN20 could contribute to building up the inner disk and bulge of GN20.

研究动机与目标

• 解析GN20（z=4.0548的明亮尘埃星系）中电离气体的空间分布与运动学特征。
• 通过Paα发射测量恒星形成率，并与红外推导的恒星形成率比较，以评估消光效应的影响。
• 判断观测到的团块与运动学特征是否与主并合或引力不稳定盘一致。
• 研究UV明亮区域的性质及其相对于电离气体发射的年龄。

提出的方法

• 采用深度MIRI/MRS积分场光谱，获取GN20的Rest-frame近红外发射线数据。
• 对Paα发射线进行空间分辨，以映射电离气体分布与运动学，最远达6 kpc。
• 利用标准校准从Paα光度推导恒星形成率，并通过观测到的Paα/Hβ比值校正消光。
• 应用动力学建模以推断旋转支持与速度 dispersion，v_rot/σ_m = 3.8 ± 1.4 表明动力学以旋转为主导。
• 利用巴尔末减量（Paα/Hβ）估算消光，得到核区A_V = 17.2 ± 0.4 mag，A_V,mixed = 44 ± 3 mag。
• 分析团块形态与双峰线轮廓，以推断亚团块结构与引力不稳定性。

实验结果

研究问题

• RQ1 GN20在z=4.0548时电离气体的空间分布与运动学结构如何？
• RQ2 观测到的Paα基恒星形成率与红外推导的恒星形成率相比如何？这说明了什么消光效应？
• RQ3 Paα发射中观测到的团块是否可分辨为亚团块？这对其形成机制有何启示？
• RQ4 运动学轮廓是否与旋转盘或并合残骸一致？
• RQ5 UV明亮区域的年龄是多少？其与电离气体及星暴活动的关系如何？

主要发现

• Paα发射呈团块状，延伸至6 kpc半径，共识别出四个空间上未分辨的团块。
• Paα推导的恒星形成率为144 ± 9 M⊙yr⁻¹，仅占红外恒星形成率（1860 ± 90 M⊙yr⁻¹）的7.7 ± 0.5%，表明极端消光。
• 核区表现出A_V = 17.2 ± 0.4 mag 与 A_V,mixed = 44 ± 3 mag，与深度掩埋、高度遮蔽的核区星暴一致。
• 每个团块均显示双峰Paα轮廓，表明每个团块至少包含两个亚团块，可能由引力不稳定性引起。
• 气体动力学以旋转为主导，v_rot/σ_m = 3.8 ± 1.4，与低红移LIRGs相似，但无法区分大尺度盘与晚期并合残骸。
• UV明亮区域无Paα发射，表明其年龄超过10 Myr，暗示该区域UV与电离气体发射之间存在时间延迟。