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Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly α transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly α profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at z ∼ 7 remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [C ii]-based redshifts from ALMA. Here, we report MMT/Binospec Ly α spectroscopy of eight UV-bright (MUV ∼ −22) galaxies at z ≃ 7 selected from the ALMA REBELS survey. We detect Ly α in four of eight galaxies and use the [C ii] systemic redshifts to investigate the Ly α velocity profiles. The Ly α lines are significantly redshifted from systemic (average velocity offset = 223 km s–1) and broad (FWHM ≈ 300–650 km s−1), with two sources showing emission extending to ≈750 km s−1. We find that the broadest Ly α profiles are associated with the largest [C ii] line widths, suggesting a potential link between the Ly α FWHM and the dynamical mass. Since Ly α photons at high velocities transmit efficiently through the z = 7 IGM, our data suggest that velocity profiles play a significant role in boosting the Ly α visibility of the most UV-luminous reionization-era galaxies.

 

ABSTRACT Reionization-era galaxies tend to exhibit weak Ly α emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Ly α emission now known in four of the most massive z = 7–9 galaxies in the CANDELS fields, all of which also exhibit intense [O iii]+H β emission (EW > 800 Å). To better understand why Ly α is anomalously strong in a subset of massive z ≃ 7–9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N = 22) of similarly luminous (≃1–6 L$^{\ast }_{\mathrm{UV}}$) z ≃ 7 galaxies selected over very wide-area fields (∼3 deg2). We confidently (>7σ) detect Ly α in 78 per cent (7/9) of galaxies with strong [O iii]+H β emission (EW > 800 Å) as opposed to only 8 per cent (1/12) of galaxies with more moderate (EW = 200–800 Å) [O iii]+H β. We argue that the higher Ly α EWs of the strong [O iii]+H β population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs (≳30 Gyr−1). We also find evidence that Ly α transmission from massive galaxies declines less rapidly over 6 < z < 7 than in low-mass lensed systems. In particular, our data suggest no strong evolution in Ly α transmission, consistent with a picture wherein massive z ≃ 7 galaxies often reside in large ionized regions. We detect three closely separated (R = 1.7 physical Mpc) z ≃ 7 Ly α emitters in our sample, conceivably tracing a large ionized structure that is consistent with this picture. We detect tentative evidence for an overdensity in this region, implying a large ionizing photon budget in the surrounding volume. 

Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimetre wavelengths to resolve. Here, we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [C ii] and dust emission in UV-selected galaxies at z ∼ 7, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation fobs = SFRIR/SFRUV+IR depends on stellar mass, similar to what is observed at lower redshift, and ranges from fobs ≈ 0.3 − 0.6 for galaxies with log10(M⋆/M⊙) = 9.4–10.4. We further adopt the z ∼ 7 stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between 3 ≲ z ≲ 7, with dust-obscured star formation still contributing ${\sim}30{{\ \rm per\ cent}}$ at z ∼ 7. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization.

 

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.