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

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.