[Paper Review] 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 TechnologiesPhysics and Astronomy12 references13 citations

TL;DR

The paper reports the discovery and characterization of a very faint Lyα-emitting galaxy at z=7.278 with an exceptionally high rest-frame Lyα EW and a near-systemic velocity, suggesting escape through low-column-density channels and the presence of an extended ionized bubble.

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 $z>7$.

Motivation & Objective

• Investigate the conditions of the interstellar medium and ionization in a z>7 Lyα-emitting galaxy.
• Constrain the escape of Lyα and implications for LyC escape and reionization.
• Characterize the surrounding IGM ionization state via Lyα transmission and nearby structures.

Proposed method

• Utilize JWST NIRSpec spectroscopy (PRISM and G140M) to detect Lyα and rest-frame optical lines; derive systemic redshift from [O III] 5007, 4959, and Hβ.
• Use NIRCam imaging to obtain rest-optical/UV photometry and de-blend a close companion source.
• Perform Gaussian fits to emission lines to measure fluxes and equivalent widths; infer line ratios (O32, R23) from PRISM data.
• Estimate UV magnitude and slope from PRISM continuum and NIRCam photometry.
• Infer SFR from Hα including Case B recombination assumptions and metallicity considerations.
• Model SED with multi-band photometry to constrain stellar mass, metallicity, and star formation history.

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 ] $\

Experimental results

Research questions

• RQ1 What are the ISM conditions and ionization parameters in a UV-faint Lyα emitter at z>7?
• RQ2 How does the Lyα emission escape in a galaxy with high EW and near-systemic velocity offset?
• RQ3 What does the Lyα transmission imply about the surrounding IGM and ionized bubble size at z~7.3?
• RQ4 Is there evidence for neighboring ionized regions or overdensities contributing to Lyα visibility?

Key findings

• Lyα emission detected at z Lyα = 7.2822 with systemic redshift z_sys = 7.2782±0.0003 from [O III] lines.
• Lyα rest-frame EW is 388.0±88.8 Å (R1000), with a line FWHM deconvolved to 383.9±56.2 km/s and velocity offset Δv = 113.3±80.0 km/s.
• Lyα flux F(Lyα) = 229.2±25.9 ×10^-20 erg s^-1 cm^-2; the continuum is faint, yielding M_UV ≈ -17.05 and UV slope β ≈ -2.06±0.14.
• O32 = 11.1±2.2, R23 = 11.2±2.6, and R3 = 7.6±1.5, consistent with low metallicity and high ionization parameter in the ISM.
• Strong [O III] 5007 dominates the rest-optical spectrum with EW0([O III] 5007) = 1028.1±165.5 Å and EW0([O III] 4959) = 330.3±62.9 Å.
• Presence of a close companion source at similar redshift, suggesting a potential overdense environment and overlapping ionized bubbles.

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