The ALMA REBELS Survey: Specific Star-Formation Rates in the Reionization Era

TL;DR

This study measures specific star-formation rates in 40 galaxies at redshifts 7-8, revealing higher sSFRs than previous estimates and highlighting the importance of dust, star formation history assumptions, and dynamical mass considerations in early universe galaxy analysis.

Contribution

It provides improved sSFR measurements using FIR data, explores the impact of star formation history assumptions, and examines the relationship between sSFR and redshift in reionization-era galaxies.

Findings

Median sSFR is 18 Gyr$^{-1}$, higher than previous estimates.

Dust emission indicates larger obscured SFRs than UV-based estimates.

sSFR increases with redshift as (1+z)^{1.7 extpm0.3}.

Abstract

We present specific star-formation rates for 40 UV-bright galaxies at $z\sim7-8$ observed as part of the Reionization Era Bright Emission Line Survey (REBELS) ALMA large program. The sSFRs are derived using improved measures of SFR and stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [CII]-based spectroscopic redshifts. For each source in the sample, we derive stellar masses from SED fitting and total SFRs from calibrations of the UV and FIR emission. The median sSFR is $18_{-5}^{+7}$ Gyr$^{-1}$, significantly larger than literature measurements lacking constraints in the FIR. The increase in sSFR reflects the larger obscured SFRs we derive from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly-dependent on the assumed star formation history (SFH) in reionization-era galaxies. When large sSFR galaxies are modeled with non-parametric SFHs, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [CII] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses that are easily able to accommodate the dominant old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties in the derived parameters, we find that the sSFR increases rapidly toward higher redshifts for massive galaxies ($9.6<\log(\rm M_*/M_{\odot})<9.8$), with a power law that goes as $(1+z)^{1.7\pm0.3}$, broadly consistent with expectations from the evolving baryon accretion rates.