We investigate the degree of dust obscured star formation in 49 massive (log10(M⋆/M⊙) > 9) Lyman-break galaxies (LBGs) at z = 6.5–8 observed as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Reionization Era Bright Emission Line Survey (REBELS) large program. By creating deep stacks of the photometric data and the REBELS ALMA measurements we determine the average rest-frame ultraviolet (UV), optical, and far-infrared (FIR) properties which reveal a significant fraction (fobs = 0.4–0.7) of obscured star formation, consistent with previous studies. From measurements of the rest-frame UV slope, we find that the brightest LBGs at these redshifts show bluer (β ≃ −2.2) colours than expected from an extrapolation of the colour–magnitude relation found at fainter magnitudes. Assuming a modified blackbody spectral energy distribution (SED) in the FIR (with dust temperature of $T_{\rm d} = 46\, {\rm K}$ and βd = 2.0), we find that the REBELS sources are in agreement with the local ‘Calzetti-like’ starburst Infrared-excess (IRX)–β relation. By re-analysing the data available for 108 galaxies at z ≃ 4–6 from the ALMA Large Program to Investigate C+ at Early Times (ALPINE) using a consistent methodology and assumed FIR SED, we show that from z ≃ 4–8, massive galaxies selected in the rest-frame UV have no appreciable evolution in their derived IRX–β relation. When comparing the IRX–M⋆ relation derived from the combined ALPINE and REBELS sample to relations established at z < 4, we find a deficit in the IRX, indicating that at z > 4 the proportion of obscured star formation is lower by a factor of ≳ 3 at a given a M⋆. Our IRX–β results are in good agreement with the high-redshift predictions of simulations and semi-analytic models for z ≃ 7 galaxies with similar stellar masses and star formation rates.
We present specific star formation rates (sSFRs) for 40 ultraviolet (UV)-bright galaxies at z ∼ 7–8 observed as part of the Reionization Era Bright Emission Line Survey (REBELS) Atacama Large Millimeter/submillimeter Array (ALMA) large programme. The sSFRs are derived using improved star formation rate (SFR) calibrations and spectral energy distribution (SED)-based stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [C ii]-based spectroscopic redshifts. The median sSFR of the sample is $18_{-5}^{+7}$ Gyr−1, significantly larger than literature measurements lacking constraints in the FIR, reflecting the larger obscured SFRs derived 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 in reionization-era galaxies. When large sSFR galaxies (a population that is common at z > 6) are modelled with non-parametric star formation histories, 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 [C ii] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses capable of accommodating an old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties among derived parameters, we find that sSFRs increase rapidly toward higher redshifts for massive galaxies (9.6 < log (M*/M⊙) < 9.8), evolving as (1 + z)1.7 ± 0.3, broadly consistent with expectations from the evolving baryon accretion rates.
more » « less- NSF-PAR ID:
- 10370377
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 516
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 975-991
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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