[论文解读] JADES: Discovery of extremely high equivalent width Lyman-alpha emission from a faint galaxy within an ionized bubble at z=7.3

Aayush Saxena, Brant Robertson+44 more|arXiv (Cornell University)|Feb 24, 2023

Advanced Optical Sensing Technologies参考文献 12被引用 13

一句话总结

论文报道在z=7.278的非常微弱的Lyα发射星系的发现与表征，具有极高的原始帧Lyα等效宽度(EW)和接近系统速度，暗示通过低柱密度通道逃逸以及存在扩展的电离泡。

ABSTRACT

We report the discovery of a remarkable Ly$α$ emitting galaxy at z = 7.2782, JADES-GS-z7-LA, with EW$_0$(Ly$α$) $= 388.0 \pm 88.8$Åand UV magnitude -17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$β$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$α$ line is detected in both lower resolution PRISM as well as medium resolution G140M grating spectra. The LSF-deconvolved Ly$α$ FWHM in the grating is $383.9 \pm 56.2$ km/s and the Ly$α$ velocity offset compared to the systemic redshift is $113.3 \pm 80.0$ km/s, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$α$ escape fraction to be >70%. JADES-GS-z7-LA has a O32 ratio of $11.1 \pm 2.2$ and a R23 ratio of $11.2 \pm 2.6$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by 0.23"), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$β$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a "bursty" star formation history, with a low stellar mass of $\approx 10^7$ $M_\odot$. Assuming that the Ly$α$ transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size > 1.5 pMpc is needed to explain the high Ly$α$ EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Ly$α$ emission. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z>7$.

研究动机与目标

• 研究z>7 Lyα发射星系中的ISM条件和电离状况。
• 约束Lyα的逃逸及对LyC逃逸和再电离的含义。
• 通过Lyα透射和附近结构来表征周围IGM的电离状态。

提出的方法

• 利用JWST NIRSpec光谱（PRISM与G140M）检测Lyα及原始波段的电观线；从[O III] 5007、4959和Hβ推导系统赤红移。
• 利用NIRCam成像获取休止光学/紫外光测光并分离一个邻近伴星源。
• 对发射线进行高斯拟合以测量通量和等效宽度；从PRISM数据推断线比(O32, R23)。
• 从PRISM连续谱和NIRCam测光估算UV星等与斜率。
• 在Case B再组合假设和金属性考虑下推断SFR（基于Hα）。
• 用多波段光度拟合SED以约束恒星质量、金属丰度和恒星形成历史。

Figure 1: NIRSpec PRISM (R100) spectrum of JADES-GS-z7-LA at $z=7.278$ , showing both 1D and 2D spectra. The error spectrum has been shown as the orange-shaded region. We additionally show zoom-ins around the Ly \textalpha line (top left) and the H \textbeta , [O iii ] $\lambda 4959$ and [O iii ] $\

实验结果

研究问题

• RQ1 在z>7的UV微弱Lyα发射星系中，ISM条件与电离参数是多少？
• RQ2 在具高EW且接近系统速度偏移的星系中Lyα发射如何逃逸？
• RQ3 Lyα透射对周围IGM及在z~7.3时电离泡大小有何含义？
• RQ4 是否存在邻近的电离区域或超密集区的证据，促成Lyα可见性？

主要发现

Parameter	Measurement
z_sys	7.2782± 0.0003
z_Lyα	7.2822± 0.0004
F([O ii] 3727) ×10^-20 erg s^-1 cm^-2	5.4±1.4
F(Hβ) ×10^-20 erg s^-1 cm^-2	7.9±2.2
F([O iii] 4959) ×10^-20 erg s^-1 cm^-2	23.1±6.0
F([O iii] 5007) ×10^-20 erg s^-1 cm^-2	59.9±4.0
EW0([O ii] 3727)	69.9±14.4
EW0(Hβ)	113.0±23.1
EW0([O iii] 4959)	330.3±62.9
EW0([O iii] 5007)	854.8±58.0
F(Lyα) (R1000) ×10^-20 erg s^-1 cm^-2	229.2±25.9
F(Hβ) (R1000) ×10^-20 erg s^-1 cm^-2	14.3±5.9
F([O iii] 5007) (R1000) ×10^-20 erg s^-1 cm^-2	64.6±6.2
EW0(Lyα)	388.0±88.8
EW0(Hβ) (R1000)	204.9±62.5
EW0([O iii] 5007)	1028.1±165.5

• 检测到Lyα发射在z Lyα = 7.2822，系统红移z_sys = 7.2782±0.0003，来自[O III]线。
• Lyα原始帧EW为388.0±88.8 Å（R1000），线宽FWHM去卷积为383.9±56.2 km/s，速度偏移Δv = 113.3±80.0 km/s。
• Lyα通量F(Lyα) = 229.2±25.9 ×10^-20 erg s^-1 cm^-2；连续谱微弱，得到M_UV ≈ -17.05且UV斜率β ≈ -2.06±0.14。
• O32 = 11.1±2.2，R23 = 11.2±2.6，以及R3 = 7.6±1.5，与ISM中低金属丰度和高电离参数一致。
• 强[O III] 5007支配 rests光学光谱，EW0([O III] 5007) = 1028.1±165.5 Å，EW0([O III] 4959) = 330.3±62.9 Å。
• 在相近红移处存在一个近邻伴星源，提示可能的超密集环境和重叠的电离泡。

Figure 2: Robust detection of the [O iii ] $\lambda 5007$ emission line in the $R\sim 1000$ G395M grating spectrum, which was used to derive an accurate systemic redshift of $z=7.2782\pm 0.0003$ . This redshift is consistent with the weaker detections of the [O iii ] $\lambda 4959$ and H \textbeta l