[论文解读] Extreme emission line galaxies detected in JADES JWST/NIRSpec I: inferred galaxy properties

Kristan Boyett, Andrew J. Bunker+34 more|arXiv (Cornell University)|Jan 30, 2024

Astronomy and Astrophysical Research被引用 11

一句话总结

该研究分析了来自 JWST/JADES 的在 3<z<9.5 的 85 个恒星形成星系样本，通过 [OIII] 5007 rest-frame EW >750 Å 识别出 36 个极端发射线星系（EELGs），并探索它们的性质及红shift 演化。

ABSTRACT

Extreme emission line galaxies (EELGs) exhibit large equivalent widths (EW) in their rest-optical emission lines ([OIII]$\lambda5007$ or H$α$ rest-frame EW$ &gt; 750Å$) which can be tied to a recent upturn in star formation rate, due to the sensitivity of the nebular line emission and the rest-optical continuum to young ($&lt;10$Myr) and evolved stellar populations, respectively. By studying a sample of 85 star forming galaxies (SFGs), spanning the redshift and magnitude interval $3 $ M$_{UV}&gt;-21$, in the JWST Advanced Deep Extragalactic Survey (JADES) with NIRSpec/prism spectroscopy, we determine that SFGs initiate an EELG phase when entering a significant burst of star formation, with the highest EWs observed in EELGs with the youngest luminosity-weighted ages ($&lt;5$ Myr old) and the highest burst intensity (those with the greatest excess between their current and long-term average SFR). We spectroscopically confirm that a greater proportion of SFGs are in an EELG phase at high redshift in our UV-selected sample ($61\pm4\%$ in our $z&gt;5.7$ high-redshift bin, compared to $23^{+4}_{-1}\%$ in our lowest-redshift bin $35.7$ have observed Lyman-$α$ emission, potentially lying within large ionised regions. The high detection rate of Lyman-$α$ emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Lyman-$α$ photons.

研究动机与目标

• Identify and assemble a sample of high-redshift star-forming galaxies (3<z<9.5) with reliable rest-optical emission lines.
• Define and select EELGs based on [OIII] 5007 rest-frame EW thresholds to quantify the EELG population.
• Characterise physical properties of EELGs (ionisation, metallicity, SFH, Ly-alpha) and compare to non-EELGs.
• Investigate how the fraction and properties of EELGs evolve with redshift and selection effects.
• Infer implications for ionising photon production and escape in high-z galaxies.

提出的方法

• Use JWST-allocated NIRSpec prism and medium-resolution (R~1000) spectra from the JADES DEEP program to detect emission lines (notably [OIII] 5007 and H-beta) at z>3.
• Measure line fluxes from Gaussian fits to emission lines and derive rest-frame EWs by combining line flux with continuum flux density from apodised NIRCam photometry, corrected for filter transmission and line contamination.
• Adopt [OIII] 5007 EW >750 Å as the EELG threshold (with one additional source within 1-sigma) to define the EELG sample.
• Stack spectra to study average properties by redshift (split at z=5.7) and EW, and compare EELGs to non-EELGs.
• Discuss selection biases arising from UV-selected high-z targets and optical-selected low-z targets, and address potential continuum estimation systematics.

Figure 1: Redshift distribution of the JADES targeted galaxies (prism in orange, R1000 grating in green) and our defined full sample (red, $z>3$ galaxies with multiple emission lines, including [OIII] $\lambda 5007$ , detected at $>5\sigma$ ). Galaxies in our full sample with coverage in the prism a

实验结果

研究问题

• RQ1 What is the prevalence of EELGs among UV-selected high-z galaxies and how does it evolve with redshift?
• RQ2 What physical conditions (ionisation, metallicity, SFH) accompany entry into the EELG phase, and how do they differ from non-EELGs?
• RQ3 Do EELGs at high redshift produce ionising photons more efficiently (xi_ion) than non-EELGs, and what are the implications for reionisation?
• RQ4 Is there evidence that EELG phases facilitate Ly-alpha escape, and how does Ly-alpha emission correlate with EELG properties?

主要发现

• 35 galaxies meet the [OIII] 5007 EW >750 Å threshold, with one additional source within 1-sigma, making 36/85 (42%) of the z>3 emission-line sample an EELG.
• The fraction of EELGs in the UV-selected high-z bin (z>5.7) is 61±4%, higher than the lower-z bin (3<z<4.1) at 23+4−1%.
• EELGs show higher average ionisation efficiency, with log10(xi_ion,HII / erg−1 Hz) = 25.5±0.2 compared to non-EELGs.
• Among EELGs at z>5.7, 53% (9/17) exhibit observed Ly-alpha emission, suggesting large ionised regions and enhanced Ly-alpha escape in this phase.
• Stacked spectra of high-z EELGs yield [OIII]5007 EW ~1524±201 Å and H-alpha EW ~1129±158 Å, with O32 ≈25.8±4.7, indicating highly ionised, compact systems compared to non-EELGs.
• The study provides a dataset of EELG properties across a wide redshift range, highlighting the link between bursty SFHs, metallicity evolution, and the EELG phase.

Figure 2: Distribution of equivalents widths. Left panel presents the [OIII] $\lambda 5007$ EW distribution for the whole sample (black) and split into a high redshift ( $z>5.7$ , blue) and low redshift ( $z<5.7$ , orange) sub-samples. Right panel shows the same distributions for the H $\alpha$ EW.