The ALPINE-CRISTAL-JWST Survey: NIRSpec IFU Data Processing and Spatially-resolved Views of Chemical Enrichment in Normal Galaxies at z=4-6

TL;DR

The paper presents a comprehensive JWST/NIRSpec IFU study of 18 main-sequence galaxies at redshifts z = 4–6 to map spatially resolved gas-phase metallicity, dust attenuation, and star-formation properties at kiloparsec scales. Through optimized data reduction, targeted background subtraction, stripe removal, error rescaling, astrometric refinement, and PSF matching, the authors produce robust emission-line maps and pixel-by-pixel metallicities using R3 and O32 calibrations, tied to a pixel-based SED analysis via Prospector. They find generally flat metallicity gradients (Δlog(O/H)/Δr ≈ a few × 10^{-2} dex kpc^{-1}) and a strong resolved fundamental metallicity relation (rFMR) with metallicity dependent more strongly on Σ_SFR than on Σ_* (α ≈ 2.1), including a low-metallicity horizontal branch indicative of recent pristine-gas inflow. These results imply vigorous metal mixing and regulatory processes operating as early as z ≈ 4–6, signaling continuity of ISM physics from the early universe to the present and highlighting the role of gas inflows and feedback in shaping chemical enrichment at high redshift.

Abstract

We present a statistical study of spatially resolved chemical enrichment in 18 main-sequence galaxies at $z=4$--6, observed with \jwst/NIRSpec IFU as part of the ALPINE-CRISTAL-\jwst\ survey. Performing an optimized reduction and calibration procedure, including local background subtraction, light-leakage masking, stripe removal, and astrometry refinement, we achieve robust emission-line mapping on kiloparsec scales. Although line-ratio distributions vary across galaxies in our sample, we generally find mild central enhancements in [O\,\textsc{iii}]/H$β$, [O\,\textsc{ii}]/[O\,\textsc{iii}], [S\,\textsc{ii}]$_{6732}$/[S\,\textsc{ii}]$_{6718}$, H$α$/H$β$, and $L_{\rm Hα}/L_{\rm UV}$, consistent with elevated electron density, dust obscuration, and bursty star formation accompanied by reduced metallicity and ionization parameter. These features point to inside-out growth fueled by recent inflows of pristine gas. Nevertheless, the median metallicity gradient is nearly flat over a few kpc scale, $Δ\log({\rm O/H}) = 0.02 \pm 0.01$ dex kpc$^{-1}$, implying efficient chemical mixing through inflows, outflows, and mergers. From pixel-by-pixel stellar and emission-line characterizations, we further investigate the resolved Fundamental Metallicity Relation (rFMR). Metallicity is described by a fundamental plane with stellar mass and SFR surface densities, but with a stronger dependence on $Σ_{\rm SFR}$ than seen in local galaxies. Our results indicate that the regulatory processes linking star formation, gas flows, and metal enrichment were already vigorous $\sim$1 Gyr after the Big Bang, producing the nearly flat metallicity gradient and a stronger coupling between star formation and metallicity than observed in evolved systems in the local universe.

Figures (25)

• Figure 1: Overview of the ALPINE–CRISTAL JWST/NIRSpec IFU sample. Each panel shows a $5"\times5"$ RGB composite (F150W, F277W, F444W from JWST/NIRCam, except DC-417567 which uses HST ACS F814W, and WFC3 F125W, and F160W). The green dashed box indicates the footprint of the NIRSpec IFU in the two-point sparse cycling dither pattern, which is designed to place the primary target within the overlapped region so that it receives full exposure coverage. In several cases, galaxies show spatially distinct components at the same spectroscopic redshift within the IFU. These components are labeled "a" and "b" and their radial properties are analyzed separately (Section \ref{['sec:radial_grad']}).
• Figure 2: Example of the local background estimated for DC-630594. The color dots denote the median at the sky area in each channel of the IFU cubes. The red and black curves indicate the best-fit polynomial function and the predicted value at the target coordinate and observation date with JWST background tool, respectively. Among our targets, these two curves are generally consistent within $\lesssim10\%$ at $\simeq$ 2--4$\mu$m, while the gaps increase to $\simeq$ 20--30% at shorter and longer wavelengths. We adopt the best-fit polynomial function to subtract the local background in our IFU cubes.
• Figure 3: Systematic stripes emerged in the collapsed continuum map among our IFU cubes. Here we highlight DC-630594 as an example. From the top left, top right, and bottom panels, we show the continuum maps collapsing all channels of the G235M cube, the best-fit empirical stripe model cube (see Section \ref{['sec:stripe']}), and the residual cube by subtracting the best-fit empirical stripe model from the observed cube. We use the residual cube in our analyses.
• Figure 4: Astrometry correction procedure. Here we highlight VC-5101218326 as an example. The left panels present the actual NIRCam F277W and F444W maps, and the right panels represent the pseudo-F277W and F444W maps generated from the G235M and G395M cubes by convolving them with the NIRCam filter responses, respectively. We determine the source center by performing the 2D elliptical Gaussian fit to both images, evaluate the spatial offset between pseudo and actual F277W and F444W maps, and use the average $\Delta$R.A. and $\Delta$Decl. for the correction. The astrometry offset estimated by this procedure is summarized in the Appendix Table \ref{['tab:offset']}.
• Figure 5: Example of NIRSpec IFU products for the target DC-417567 at $z=5.67$. Top: Extracted 1D spectra with a $0 \hbox{. $^{\prime\prime}$}4$-radius aperture at the H$\alpha$ line peak from the G235M (green) and G395M (red) gratings, normalized to the maximum flux. Key emission lines are indicated by vertical dashed lines. Bottom: Moment-0 maps ($5^{\prime\prime}\times5^{\prime\prime}$) for strong rest-optical emission lines observed with NIRSpec IFU, together with the ALMA dust continuum and [C ii] 158 $\mu$m moment-0 maps. White contours show $3\sigma$, $5\sigma$, and $10\sigma$ significance levels.