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Direct Abundance Maps and Radial Metallicity Gradients of two Galaxies at z~4-5 in the GARDEN Survey

L. Stanghellini, S. A. Kassin, C. Pacifici, J. E. Morrison, M. E. Dickinson, E. Sukay, C. R. Mulcahey, L. E. Bergeron, M. W. Regan, C. N. A. Willmer, B. J. Weiner, N. Dencheva, D. Law, A. de la Vega, A. M. Koekemoer, C. Conselice, J. P. Gardner, Y. Guo, F. Hammer, A. Henry, B. W. Holwerda, J. Kartaltepe, R. A. Lucas, M. Puech, M. Rafelski, I. Shivaei, C. Welker, X. Xu, L. Y. A. Yung

TL;DR

We present spatially resolved direct-abundance maps for two z~4–5 galaxies from the GARDEN survey using JWST/NIRSpec MSA auroral-line spectroscopy. Direct O/H is mapped in individual spaxels, revealing a significant negative radial gradient of $-0.111^{+0.026}_{-0.025}$ dex/kpc in CANDELS 8005 and an inconclusive gradient in CANDELS 7986, illustrating inside-out chemical enrichment at high redshift. Direct abundances are found to be broadly consistent with strong-line calibrations, validating high-z abundance techniques despite large uncertainties in some regions. The kinematic analysis hints at a merger in CANDELS 8005 and complex dynamics in CANDELS 7986, with no AGN activity detected in either galaxy. This study demonstrates the feasibility of 2D direct-abundance mapping at z>4 and sets the stage for expanding such measurements to larger samples with JWST.

Abstract

We investigate galaxies in the GARDEN (Galaxies at All Redshifts Deciphered and Explained with the NIRSpec MSA) survey that show auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Two galaxies have spectra suitable for this analysis: CANDELS 8005 at z=3.794 and CANDELS 7986 at z=4.702. For both, we measure auroral and key nebular emission-line fluxes across their full extent, allowing direct-method oxygen abundance determinations in individual spaxels. These observations demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses at high redshift, aided by weak nebular continua and low interstellar extinction. We derive global direct abundances of 12 + log(O/H) = 8.008 (+0.025, -0.027) for CANDELS 8005 and 7.89 (+0.027, -0.028) for CANDELS 7986. Emission-line diagnostics indicate neither galaxy hosts an active galactic nucleus. A first-order kinematic analysis suggests a potential merger in CANDELS 8005. The direct abundances agree with strong-line estimates from our data and recent high-redshift calibrations. We build emission line, radial velocity, strong-line abundance, electron temperature, and direct abundance maps for both galaxies. From these maps, we measure linear radial metallicity gradients of -0.111 (+0.026, -0.025) dex/kpc for CANDELS 8005 (statistically significant) and -0.093 +/- 0.088 dex/kpc for CANDELS 7986, where the large uncertainties limit significance. These results represent the first detection of a radial metallicity gradient from direct-method abundances with measurements taken in galaxies at z>0, supporting inside-out galaxy growth with feedback-regulated chemical enrichment.

Direct Abundance Maps and Radial Metallicity Gradients of two Galaxies at z~4-5 in the GARDEN Survey

TL;DR

We present spatially resolved direct-abundance maps for two z~4–5 galaxies from the GARDEN survey using JWST/NIRSpec MSA auroral-line spectroscopy. Direct O/H is mapped in individual spaxels, revealing a significant negative radial gradient of dex/kpc in CANDELS 8005 and an inconclusive gradient in CANDELS 7986, illustrating inside-out chemical enrichment at high redshift. Direct abundances are found to be broadly consistent with strong-line calibrations, validating high-z abundance techniques despite large uncertainties in some regions. The kinematic analysis hints at a merger in CANDELS 8005 and complex dynamics in CANDELS 7986, with no AGN activity detected in either galaxy. This study demonstrates the feasibility of 2D direct-abundance mapping at z>4 and sets the stage for expanding such measurements to larger samples with JWST.

Abstract

We investigate galaxies in the GARDEN (Galaxies at All Redshifts Deciphered and Explained with the NIRSpec MSA) survey that show auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Two galaxies have spectra suitable for this analysis: CANDELS 8005 at z=3.794 and CANDELS 7986 at z=4.702. For both, we measure auroral and key nebular emission-line fluxes across their full extent, allowing direct-method oxygen abundance determinations in individual spaxels. These observations demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses at high redshift, aided by weak nebular continua and low interstellar extinction. We derive global direct abundances of 12 + log(O/H) = 8.008 (+0.025, -0.027) for CANDELS 8005 and 7.89 (+0.027, -0.028) for CANDELS 7986. Emission-line diagnostics indicate neither galaxy hosts an active galactic nucleus. A first-order kinematic analysis suggests a potential merger in CANDELS 8005. The direct abundances agree with strong-line estimates from our data and recent high-redshift calibrations. We build emission line, radial velocity, strong-line abundance, electron temperature, and direct abundance maps for both galaxies. From these maps, we measure linear radial metallicity gradients of -0.111 (+0.026, -0.025) dex/kpc for CANDELS 8005 (statistically significant) and -0.093 +/- 0.088 dex/kpc for CANDELS 7986, where the large uncertainties limit significance. These results represent the first detection of a radial metallicity gradient from direct-method abundances with measurements taken in galaxies at z>0, supporting inside-out galaxy growth with feedback-regulated chemical enrichment.
Paper Structure (20 sections, 4 equations, 22 figures, 4 tables)

This paper contains 20 sections, 4 equations, 22 figures, 4 tables.

Table of Contents

  1. Introduction
  2. GARDEN Data Inspection and Sample Selection
  3. Observations and data reduction
  4. Sample selection
  5. Data analysis
  6. Spectral inspection, flux continuum, and line fitting
  7. Reddening Correction, Plasma Diagnostics and Direct Abundance Analysis
  8. Detailed analysis of CANDELS 8005
  9. 1D spectral analysis
  10. 2D spectral analysis
  11. Radial Metallicity Gradient
  12. Detailed analysis of CANDELS 7986
  13. 1D spectral analysis
  14. 2D spectral analysis
  15. Radial Metallicity Gradient
  16. ...and 5 more sections

Figures (22)

  • Figure 1: CANDELS 8005 (top row) and 7986 (bottom row) were observed by repeating a single slitlet step on the galaxy six times. There are five unique locations for the slitlet that make up the dither pattern. The central slitlet is overlaid (in red) on the F277W image in all figures. The slitlets in the four dithers (in purple) are shown on the right images. The dithers move out to the corners, offset in the dispersion direction by $\pm$0.09 arcseconds, and by $\pm$0.13 arcseconds in the cross-dispersion direction.
  • Figure 2: Images of CANDELS-8005. Left and center: NIRCam images in F090W and F277W, corresponding to rest-frame central wavelengths of 1900 and 4700 Å respectively, for z=3.794. F090W samples rest-frame ultraviolet emission, while F277W measures rest-frame optical light. The [O3] $\lambda$ 5007Å and H$\beta$ emission lines fall into the low-transmission wings of the bandpass. Right: GARDEN NIRSpec slitlet-stepping map, summing emission from the H$\beta$ and [O3] 4959,5007 emission lines plus nearby continuum.
  • Figure 3: Images of CANDELS 7986: Left and center: NIRCam images of galaxy in F090W and F277W, or rest-frame central wavelengths of 1600 and 4000 Å respectively, for z=4.702. F090W samples rest-frame ultraviolet emission, while F277W measures rest-frame optical light, including the [O3] $\lambda$ 5007Å and H$\beta$ emission lines. Right: GARDEN NIRSpec slitlet-stepping map, summing emission from the H$\beta$ and [O3] $\lambda\lambda$ 4959,5007 emission lines plus nearby continuum.
  • Figure 4: 1D spectrum of CANDELS 8005, flux density versus wavelength, obtained by spatially collapsing the cube over all spaxels inside the defined box (see text). Top and bottom panels are the spectral component on either side of the NIRSpec detector gap. The y-axis has been cut to show all lines, and in each panel the strongest lines above the y limit are indicated with red arrows. The thin vertical lines and labels indicate the most prominent observed lines used for the analysis in this paper.
  • Figure 5: Same as Fig. \ref{['8005_onedspec']}, but for CANDELS 7986
  • ...and 17 more figures