A first look at a complete view of spatially resolved star formation at 1<z<1.8 with JWST NGDEEP+FRESCO slitless spectroscopy

TL;DR

This study uses JWST NGDEEP/NIRISS and FRESCO/NIRCam slitless spectroscopy to map Pa$\alpha$ (dust-insensitive) and H$\alpha$ (dust-affected) emission in 31 galaxies at $1<z<1.8$, enabling the first direct spatial comparison of obscured versus unobscured star formation on the main sequence. Through PSF matching and stacking in three stellar-mass bins, the authors extract radially resolved surface-brightness, equivalent width, and line-ratio profiles, revealing consistent inside-out growth for both tracers and mass-dependent dust attenuation trends. They find that the Pa$\alpha$/H$\alpha$ ratio declines with radius in the highest and lowest mass bins but remains flat in the intermediate bin, suggesting that centrally concentrated dust attenuation is not universal among main-sequence galaxies at cosmic noon. The results demonstrate the power of JWST slitless spectroscopy to study spatially resolved star formation and highlight the need for larger samples to understand the intrinsic scatter in resolved SF across the main sequence.

Abstract

[abridged] The previously inaccessible star formation tracer Pa$α$ can now be spatially resolved by JWST NIRCam slitless spectroscopy in distant galaxies up to cosmic noon. In the first study of its kind, we combine JWST NGDEEP NIRISS and FRESCO NIRCam slitless spectroscopy to provide the first direct comparison of spatially resolved dust-obscured (traced by Pa$α$) versus unobscured (traced by H$α$) star formation across the main sequence. We stack Pa$α$ and H$α$ emission-line maps, along with stellar continuum images at both wavelengths of 31 galaxies at 1<z<1.8 in three bins of stellar mass. Surface brightness profiles are measured and equivalent width (EW) profiles computed. Increasing Pa$α$ and H$α$ EW profiles with galactocentric radius across all stellar masses probed provide direct evidence for the inside-out growth of galaxies both via dust-obscured and unobscured star formation for the first time. For galaxies on the main sequence, a weakly positive ($0.1\pm0.1$) Pa$α$/H$α$ line profile as a function of radius is found at $8.8\leqslant\mathrm{log}(M_{*}/\mathrm{M}_{\odot})<9.9$ with a negative ($-0.4\pm0.1$) Pa$α$/H$α$ line profile found at $9.9\leqslant\mathrm{log}(M_{*}/\mathrm{M}_{\odot})<11.0$. Low mass galaxies ($7.7\leqslant\mathrm{log}(M_{*}/\mathrm{M}_{\odot})<8.8$) with high sSFRs are found to have a negative ($-0.5\pm0.1$) Pa$α$/H$α$ line profile gradient. Our results demonstrate that while inside-out growth via star formation is ubiquitous across the main sequence just after cosmic noon, centrally concentrated dust attenuation is not. Along with other recent work in the literature, our findings motivate future studies of resolved SFR profiles in large samples of individual cosmic noon galaxies across the main sequence, to understand the intrinsic scatter in spatially resolved star formation.

Figures (7)

• Figure 1: The FRESCO and NGDEEP NIRISS observing footprints on the sky.
• Figure 2: Star formation main sequence of our sample measured using SED fitting (see Section \ref{['sec:masses_sfms']}). The shaded grey regions delineate our stellar mass bins for the stacking analysis. SFRs and the Popesso2023 SFMS include dust corrections. The dotted-line indicates extrapolation. The brown line and orange region delineate the FRESCO Pa$\upalpha$ SFMS from Liu2024 and Neufeld2024 respectively.
• Figure 3: PSF surface brightness profiles using the same star in the F150W NGDEEP NIRISS and F444W FRESCO NIRCam imaging. The thin pink line shows the PSF profile after PSF-matching (see Section \ref{['sec:PSFs']}) the FRESCO PSF to the NGDEEP PSF. The amplitude (top panel) of the PSF is maintained and its shape (bottom panel) is appropriately altered.
• Figure 4: H$\upalpha$ (first column) and Pa$\upalpha$-to-NGDEEP convolved (second column) stacks for our sample in three stellar mass bins (rows). The number of galaxies is shown in the bottom left corner of each H$\upalpha$ stack thumbnail. Each thumbnail is $60\times60$ pixels with 1 pixel = 0.05$^{\prime\prime}$. Emission-line maps are generated such that the spectral axis is along the $x$-direction, while the $y$-direction is the spatial axis (see Section \ref{['sec:profiles']}).
• Figure 5: Peak-normalised H$\upalpha$ (green) and Pa$\upalpha$ (turquoise) surface brightness profiles for our stacks shown in Figure \ref{['fig:stacks']}. Both profiles are more extended than their PSFs, demonstrating they are well-resolved. At the lowest and highest masses, the H$\upalpha$ emission is more extended than the Pa$\upalpha$ emission, whereas their spatial profiles are similar for our middle stellar mass bin, at least out to $\sim2$ kiloparsecs after which the Pa$\upalpha$ emission is more extended than the H$\upalpha$ emission.