The ALPINE-CRISTAL-JWST Survey: Stellar and nebular dust attenuation of main-sequence galaxies at z~4-6

TL;DR

This study addresses how dust attenuates stellar and nebular light in massive star-forming galaxies at $z\sim4$--$6$ by directly measuring the stellar-to-nebular reddening ratio $f=E(B{-}V)_{\rm star}/E(B{-}V)_{\rm neb}$ using spatially resolved JWST/NIRSpec IFU and NIRCam data, complemented by ALMA continuum imaging. By performing both pixel-by-pixel and spatially integrated SED fitting with Prospector and incorporating emission-line constraints, the authors derive $f=0.51^{+0.04}_{-0.03}$ with scatter, and find that $f$ correlates modestly with sSFR, younger ages, and burstiness. They show that assuming $f=1$ underestimates intrinsic H$\alpha$ luminosities by about $3$–$36\%$ and $\xi_{\rm ion}$ by $13$–$78\%$, which has meaningful implications for the high-redshift line-luminosity function, SFR estimates, and ionizing photon budgets. The analysis also reveals an outshining effect: spatially integrated fits can bias stellar masses and ages low relative to pixel-resolved fits, while pixel-by-pixel attenuation tends to be higher due to patchy dust. Overall, the work provides a robust $f$ benchmark at $z>4$, quantifies the impact of reddening assumptions on key observables, and underscores the value of spatially resolved, flexible SFH modeling for interpreting the earliest massive galaxies.

Abstract

Characterizing dust attenuation is crucial for revealing the intrinsic physical properties of galaxies. We present an analysis of dust attenuation in 18 spectroscopically confirmed star-forming main-sequence galaxies at $z = 4.4-5.7$ observed with JWST/NIRSpec IFU and NIRCam, selected from the ALPINE and CRISTAL ALMA large programs. We fit the emission line fluxes from NIRSpec and the broad-band photometry from NIRCam with Prospector, using both spatially integrated emission and $\sim0.6$ kpc pixel-by-pixel measurements. We derive the stellar-to-nebular dust attenuation ratio ($f=E(B-V)_{\mathrm{star}}/E(B-V)_{\mathrm{neb}}$) from the SED fits and the Balmer decrement with H$α$ and H$β$. Although individual galaxies show large scatter, the best-fit value is $f = 0.51^{+0.04}_{-0.03}$, slightly higher than that measured for local starburst galaxies. We find weak correlations of $f$ with galaxy properties, increasing with higher specific star-formation rates, younger stellar ages, and more recent star-formation. For the range of $E(B-V)_{\mathrm{star}} = 0.009-0.15$ mag for in our sample, assuming $f = 1$ (often adopted in high-redshift studies) instead of $f = 0.51$ underestimate line luminosities and ionizing photon production efficiency $ξ_\text{ion}$ by $\sim3-36\%$ and $\sim4-46\%$, respectively. We also find that the total stellar masses estimated from spatially-integrated SED fits with a delayed-$τ$ star-formation histories are systematically smaller than the sum of pixel-by-pixel SED fits, with a median offset of $\sim 0.26$ dex, likely because the integrated fits are biased toward luminous young stellar populations.

Figures (6)

• Figure 1: An example of the spatially-integrated spectrum and velocity-integrated emission-line maps for VC-5110377875 at $z=4.55$. (Top) Spectra around H$\beta$, [O iii]$_{4960,5008}$, and H$\alpha$+[N ii], with best-fit Gaussian models overlaid in red and blue. Shaded regions indicate the three-channel windows centered on the line peaks, which are used to construct temporary emission maps. (Bottom) Emission maps obtained by summing the continuum-subtracted cube over those shaded channels and rescaled to match the total line flux from the Gaussian fit. The red line is the photometric aperture.
• Figure 2: An example of spatially-integrated SED fit for the same galaxy shown in Figure \ref{['fig:specfit']}. (Top left) NIRCam images used for the SED fit, shown at their original resolution prior to PSF matching. The red lines show the mask to obtain photometry (see section \ref{['sec:mask']}). The scale bar at the lower left corner corresponds to $0.2\hbox{$^{\prime\prime}$}$. (Bottom left) Best-fit SED, including observed photometry (red filled circles), model photometry (black open squares) and the model spectrum (orange line). Shaded curves represent the filter transmissions. Orange squares show the waverange coverages of the NIRSpec spectroscopy. The bottom part of the plot indicates the residual between the observed and modeled photometry, normalized by the flux uncertainties. (Right) Ratios of the observed line fluxes to the best-fit model predictions. The red vertical line marks the perfect match.
• Figure 3: Spatially resolved maps of the same galaxy shown in Figure \ref{['fig:sed']}. Each panel shows a physical property derived from pixel-by-pixel SED fitting: stellar mass ($M_*$), SFR, mass-weighted stellar age, reduced $\chi^2$ of the SED fit, stellar reddening $E(B-V)_{\mathrm{star}}$, nebular reddening $E(B-V)_{\mathrm{neb}}$, and the differential reddening ratio $f=E(B-V)_{\mathrm{star}}/E(B-V)_{\mathrm{neb}}$. The scale bar at the lower left corner corresponds to $0.2\hbox{$^{\prime\prime}$}$.
• Figure 4: Comparison of (a) total stellar mass, (b) SFR averaged over the last 10 Myr, (c) mass-weighted stellar age, and (d) $V$-band attenuation for the stellar continuum derived from spatially integrated SED fitting and pixel-by-pixel SED fitting. For spatially integrated SED fitting, results are shown for two SFH models: the non-parametric SFH (red points) and the parametric delayed-$\tau$ SFH (blue points). Symbols marked with a cross indicate galaxies that appear morphologically smooth (i.e., not clumpy or interacting) based on visual inspection. The thick dashed line represents the 1:1 relation, while the thin dashed lines indicate $\pm0.5$ dex and $\pm1$ dex offsets in (a)-(c) and $\pm0.2$ mag and $\pm0.4$ mag offsets in (d).
• Figure 5: Comparison of the stellar continuum and nebular line reddening for our sample. The red markers with black edges represent spatially integrated measurements, while the faint red circles and faint red triangles indicate pixel-by-pixel measurements (723 pixels). The circles correspond to reliable measurements, whereas the triangles indicate lower limits where H$\beta$ is undetected. The blue squares are binned averages of the pixel-by-pixel measurements. The gray dotted line shows the 1:1 relation. The red solid line and shaded region indicate the best-fit linear relation obtained from the spatially integrated points together with the binned averages, and its associated uncertainty.