REBELS-IFU: Steeply rising star formation histories and the importance of dust obscuration in massive $z \simeq7$ galaxies revealed by multi-wavelength observations

TL;DR

This study demonstrates that massive $z\sim7$ galaxies harbor substantial dust-obscured star formation and that their star formation histories rise more steeply than those of lower-mass systems. By combining JWST/NIRSpec IFU spectroscopy with ALMA FIR data, the authors derive SFRs from rest-UV, H$\alpha$, FIR, and non-parametric SFHs, showing that conventional constant-SFH luminosity-to-SFR conversions overestimate long-timescale SFRs by about a factor of $\sim3$ for rest-UV tracers. They compute new, SFH-aware luminosity-to-SFR calibrations and confirm that differential dust attenuation remains significant at $z\simeq7$, with $f=E(B-V)_{stellar}/E(B-V)_{gas}=0.50\pm0.08$. The work further finds that the H$\alpha$-to-UV ratio is an unreliable burstiness proxy in these sources, underscoring the value of multi-wavelength data and non-parametric SFHs for accurate SFR and SFH inferences in the early universe.

Abstract

Reliable star formation rate (SFR) measurements are essential for understanding early galaxy evolution, yet derived values rely on several assumptions. To address this problem, we investigate the SFRs of 12 massive ($9~<~\log(M_{\star}/{\rm M}_{\odot})~<~10$) Lyman-break galaxies at $z=6.5-7.7$, drawn from the Atacama Large Millimeter/submillimeter Array (ALMA) Reionization Era Bright Emission Line Survey (REBELS) program. The multi-wavelength data, including JWST NIRSpec IFU spectroscopy and ALMA observations, make this a unique sample for investigating SFR tracers at this epoch. We compare SFRs derived from the rest-UV, H$α$, and far-infrared emission, and from spectral energy distribution (SED) fits. We apply robust dust attenuation corrections, which are crucial since between $50-80$ per cent of the star formation is obscured, and find a stellar-to-nebular attenuation ratio of $f=0.50\pm0.08$, consistent with local star-forming galaxies. The majority of the derived total SFRs (medians $25-120$ ${\rm M}_{\odot}$ yr$^{-1}$) place the REBELS galaxies systematically above $z=7$ literature star-forming main-sequence relations, and our best-fit star formation histories (SFHs) rise more steeply than lower-mass galaxies at the same redshift. We show that these rising SFHs mean commonly used luminosity-to-SFR conversion factors, derived assuming a constant SFH over given timescales, overestimate the SFRs averaged over these timescales for our galaxies. We provide updated luminosity-to-SFR calibrations for $z\simeq7$ galaxies with rising SFHs, showing that commonly assumed rest-UV conversion factors overestimate the $100$ Myr average SFR by a factor of $\simeq3$. Finally, we investigate burstiness indicators in the REBELS-IFU galaxies, finding that the rising SFHs imply that the H$α$-to-UV luminosity ratio is an unreliable probe of bursty star formation.

Figures (13)

• Figure 1: We show with coloured rectangles the wavelength ranges used to infer star formation rates in this work. The full NIRSpec PRISM spectrum for one of the REBELS-IFU galaxies (REBELS-05) is shown by the solid black line in the left panel. The intrinsic, dust-free SED model is shown by the dashed line, demonstrating the wavelength-dependent effect of dust attenuation, with the rest-UV flux being impacted more than the H$\alpha$ flux. The right panel shows a modified blackbody model of the rest-frame far-infrared emission. The ALMA Band 6 flux continuum measurement is shown by the black point. The integrated flux under this curve traces the dust-obscured star formation rate.
• Figure 2: The Balmer decrements, H$\alpha$/H$\beta$, are plotted against stellar mass, $M_{\star}$, in red for the 8 of the 12 massive REBELS-IFU galaxies at $6.5\leq~z~<~7.0$ for which H$\alpha$ lies within the NIRSpec wavelength coverage. The H$\alpha$/H$\beta$ ratios all exceed the intrinsic Case B recombination value of 2.86, shown by the horizontal dashed line, indicative of dust attenuation affecting the nebular emission of these galaxies. The results from stacked CEERS galaxy spectra Shapley2023b are shown in purple, and JADES galaxies at the same redshift as the REBELS-IFU sample are shown in blue. The REBELS-IFU galaxies are consistent with local galaxies of similar stellar masses from the SDSS survey, shown by the grey histogram, suggesting no significant evolution in the Balmer decrement against stellar mass relation with redshift.
• Figure 3: The colour excess measured for the ionised gas, $E(B-V)_{\text{gas}}$, derived from the Balmer decrement, compared to the colour excess measured for the stellar continuum, $E(B-V)_{\text{stellar}} = A_{V, \text{stellar}}/{R_V}$, for the 8 of the 12 massive galaxies in the REBELS-IFU sample at $6.5\leq z < 7.0$ for which H$\alpha$ lies within the NIRSpec wavelength coverage. The average stellar-to-nebular attenuation ratio of $f = (0.50 \pm 0.08)$ is shown by the red line, indicating on average the nebular regions in these galaxies are approximately $2$ times more dust-obscured than the stellar continuum. This is consistent with the $f = 0.44$ relation (dashed line) derived for local star-forming galaxies by Calzetti1997b, and the relations for the ALPINE galaxies at $z\simeq5$Tsujita2025 and the MOSDEF galaxies at $z\simeq2$Shivaei2020, shown by the dash-dot and dotted lines, respectively. The points are coloured by their gas-phase metallicities Rowland2025.
• Figure 4: The ${\rm IRX}$--$\beta$ points for the 12 REBELS-IFU galaxies at $z\simeq7$. The rest-frame UV continuum slope, $\beta$, and luminosity, $L_{\rm UV}$, values are measured directly from the NIRSpec spectra. In red we show the points using $L_{\rm IR}$ derived by Bowler2023, which assume $T_{\rm d} = 46$ K and $\beta_{\rm d} = 2.0$, but with rest-UV quantities re-derived from the NIRSpec spectra. The orange points use the $L_{\rm IR}$ values of Algera2023 and Algera2024 derived from multi-band ALMA observations. The red line shows the best-fit ${\rm IRX}$--$\beta$ relation with the intrinsic rest-UV slope fixed to the median value from Fisher2025: $\beta_{0} = -2.43$. This lies between the expected relations for the Calzetti-like attenuation and SMC extinction curves, shown by the solid and dashed black lines, respectively.
• Figure 5: The obscured SFR fractions, $f_{\text{obs}} =$ SFR$_{\text{IR}}$/SFR$_{\text{UV+IR}}$, versus stellar mass for the 12 REBELS-IFU galaxies at $z\simeq7$ (blue). The REBELS-IFU galaxies have significant dust-obscured SFR fractions, demonstrating the necessity of robust dust corrections. The small grey points show the obscured fractions for the other REBELS galaxies from Bowler2023. The $f_{\text{obs}}$--$M_{\star}$ relation derived from a mass-complete sample of galaxies at $0<z<2.5$ by Whitaker2017 is shown by the black line.