[Paper Review] Spatially resolved emission lines in galaxies at $4\leq z < 10$ from the JADES survey: evidence for enhanced central star formation

Roberta Tripodi, Francesco D’Eugenio+19 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Mar 13, 2024

Astronomy and Astrophysical Research4 citations

TL;DR

This study presents the first spatially resolved spectroscopic analysis of 63 high-redshift galaxies (4 ≤ z < 10) using JWST/NIRSpec data from the JADES survey, revealing enhanced central star formation through negative Hβ equivalent width gradients and increasing ionization parameters with redshift. The results suggest inverted metallicity gradients and strong central starbursts, likely driven by recent pristine gas accretion or radial flows.

ABSTRACT

We present the first statistical investigation of spatially resolved emission-line properties in a sample of 63 low-mass galaxies at $4\leq z&lt;10$, using JWST/NIRSpec MSA data from the JWST Advanced Deep Extragalactic (JADES) survey focusing on deep, spatially resolved spectroscopy in the GOODS-S extragalactic field. By performing a stacking of the 2D spectra of the galaxies in our sample, we find an increasing or flat radial trend with increasing radius for [OIII]$\lambda5007$/H$β$ and a decreasing one for [NeIII]$\lambda3869$/[OII]$\lambda3727$ (3--4 $σ$ significance). These results are still valid when stacking the sample in two redshift bins (i.e., $4\leq z&lt;5.5$ and $5.5\leq z&lt;10$). The comparison with star-formation photoionization models suggests that the ionization parameter increases by $\sim 0.5$ dex with redshift. We find a tentative metallicity gradient that increases with radius (i.e., 'inverted') in both redshift bins. Moreover, our analysis reveals strong negative gradients for the equivalent width of \Hbeta (7$σ$ significance). This trend persists even after removing known AGN candidates, therefore, it is consistent with a radial gradient primarily in stellar age and secondarily in metallicity. Taken all together, our results suggest that the sample is dominated by active central star formation, with possibly inverted metallicity gradients sustained by recent episodes of accretion of pristine gas or strong radial flows. Deeper observations and larger samples are needed to confirm these preliminary results and to validate our interpretation.

Motivation & Objective

• To investigate spatially resolved emission-line properties in low-mass galaxies at 4 ≤ z < 10 using deep, high-resolution spectroscopy.
• To determine radial gradients in ionization conditions, metallicity, and star formation activity in high-redshift galaxies.
• To assess whether inverted metallicity gradients and central starbursts are common features in early galaxies.
• To test whether observed trends reflect physical evolution or selection effects in high-redshift galaxy samples.
• To evaluate the role of pristine gas accretion or radial flows in shaping the chemical and ionization structure of early galaxies.

Proposed method

• Stacking 2D NIRSpec MSA spectra of 63 galaxies in the JADES GOODS-S field to enhance signal-to-noise for radial profile analysis.
• Measuring radial gradients in key emission-line ratios: [O iii]λ5007/Hβ, [Ne iii]λ3869/[O ii]λ3727, and Hβ equivalent width.
• Comparing observed line ratios with photoionization models to infer ionization parameter and metallicity evolution.
• Splitting the sample into two redshift bins (4 ≤ z < 5.5 and 5.5 ≤ z < 10) to assess redshift-dependent trends.
• Removing known AGN candidates to isolate intrinsic star formation-related gradients in Hβ equivalent width.
• Using established strong-line calibrations (e.g., Curti et al. 2020) to estimate metallicity from observed line ratios.

Experimental results

Research questions

• RQ1 Do high-redshift galaxies at 4 ≤ z < 10 exhibit radial gradients in ionization conditions, and how do they evolve with redshift?
• RQ2 Is there evidence for inverted metallicity gradients in early galaxies, and what physical mechanisms could sustain them?
• RQ3 Are negative gradients in Hβ equivalent width indicative of central starbursts, and do they persist after removing AGN contamination?
• RQ4 How do ionization parameters and metallicities in high-redshift galaxies compare to local and cosmic noon analogs?
• RQ5 To what extent do selection effects or physical evolution shape the observed emission-line properties in early galaxies?

Key findings

• The stacked [O iii]λ5007/Hβ ratio shows a flat or increasing radial trend, indicating higher ionization in outer regions or enhanced central excitation.
• The [Ne iii]λ3869/[O ii]λ3727 ratio exhibits a decreasing radial trend with 3–4σ significance, suggesting stronger ionization in the center.
• The ionization parameter increases by approximately 0.5 dex from z ≈ 4 to z ≈ 10, indicating stronger ionizing radiation in the most distant galaxies.
• Tentative evidence for inverted metallicity gradients is found in both redshift bins, with metallicity increasing with radius.
• A strong negative gradient in Hβ equivalent width is detected with 7σ significance, persisting even after removing known AGN candidates.
• The results collectively suggest that the sample is dominated by central starbursts, likely fueled by recent accretion of pristine gas or strong radial flows.