Metallicity is a crucial tracer of galaxy evolution, providing insights into gas accretion, star formation, and feedback. At high redshift, these processes reveal how early galaxies assembled and enriched their interstellar medium. In this work, we present rest-frame optical spectroscopy of 12 massive (log(M-*/M-circle dot)>9) galaxies at z similar to 6-8 from the REBELS ALMA large program, observed with JWST NIRSpec/IFU in the prism mode. These observations span emission lines from [O II]lambda 3727,9 to [S II]lambda 6716,31, providing key information on nebular dust attenuation, ionisation states, and chemical abundances. We find lower O32 ratios (average similar to 3.7) and [OIII]lambda 5007 equivalent widths (median EW[OIII] similar to 480 angstrom) than are generally found in existing large spectroscopic surveys at z > 6, indicating less extreme ionising conditions. Strong-line diagnostics suggest that these systems are some of the most metal-rich galaxies observed at z > 6 (average Z(gas) similar to 0.4Z(circle dot)), including sources with near-solar oxygen abundances, in line with their high stellar masses (average logM(*)/M-circle dot similar to 9.5). Supplementing with literature sources at lower masses, we investigate the mass-metallicity and fundamental metallicity relations (MZR and FMR, respectively) over a 4 dex stellar mass range at 6 < z < 8. In contrast to recent studies of lower-mass galaxies, we find no evidence for negative offsets to the z = 0 FMR for the REBELS galaxies. This work demonstrates the existence of chemically-enriched galaxies just similar to 1 Gyr after the Big Bang, and indicates that the MZR is already in place at these early times, in agreement with other recent z > 3 studies.

Neutral gas present in galaxies during the Epoch of Reionisation (EoR) is responsible for regulating star formation, dust growth, and the escape of ionising photons, making it a key ingredient in our understanding of both galaxy assembly and reionisation. However, direct constraints on the H I content of galaxies at z > 6 have been scarce thus far. With JWST , it is possible to obtain a direct probe of this neutral component by surveying Ly α damping wings in galaxy spectra. We analysed JWST /NIRSpec prism spectra of 12 UV-luminous galaxies from the REBELS-IFU program at z ∼ 6.5–7.7, deriving H I column densities by modelling Ly α damping wings. Significant damped Ly α absorption from local H I gas was detected in six galaxies, with N H I ≳ 10 21 cm −2 . We use the column densities and sizes derived for these sources to estimate their H I masses and compare with L [C II ] – M H I calibrations. The resulting H I masses show a tentative correlation with those inferred from [C II ], although the [C II ]-based estimates are systematically larger, suggesting that the H I reservoirs might extend beyond the [C II ]-emitting gas. We also combined measurements taken from damped Lyman- α (Ly α ) systems (DLAs) with far-infrared (FIR) derived dust-to-gas ratios (DTGs), dust attenuation, and gas-phase metallicities. No correlation was found between DLA-based and FIR-based DTGs; however, combining the REBELS-IFU sample with literature samples at lower metallicities revealed a strong correlation between A V / N H I and metallicity. These findings suggest that by z ∼ 7 massive galaxies already have the capacity to host substantial, enriched reservoirs of neutral gas and dust, consistent with A V / N H I –metallicity trends at lower redshifts. Finally, we also see tentative evidence of a redshift evolution of the A V / N H I –metallicity relation, with lower A V / N H I at fixed metallicity with increasing redshift, which might point to pristine gas accretion or more efficient dust destruction and/or expulsion.

We present Keck/MOSFIRE H-band spectroscopic measurements covering the [CIII]1907, CIII]1909 doublet for a sample of 8 z 7 spectroscopically-confirmed star-forming galaxies drawn from the Reionization Era Bright Emission Line Survey (REBELS). This sample is notable for its bright median UV luminosity (Muv=-22.5 AB) and large median stellar mass (log(Mstar/Msun)=9.2). Although three sources show tentative evidence of a CIII] detection, we obtain no confident detections for any of the 8 REBELS sources. The median [CIII]1907+CIII]1909 3-sigma upper limit in equivalent width (EW) for the REBELS-MOSFIRE sample is 6.5 AA, and a stack of their H-band MOSFIRE spectra yields a non-detection with an associated 3-sigma upper limit of 2.6 AA. These upper limits fall significantly below the CIII] EW measured in a composite spectrum of representative z 7 star-forming galaxies, as well as those measured for notable early star-forming galaxies such as GN-z11, GHZ2, GS-z12, and RXCJ2248-ID. The lack of strong CIII] emission can be understood within the context of the stellar populations of the REBELS galaxies, as well as the ionization conditions and gas-phase metallicity implied by rest-frame optical spectroscopic properties ([OIII]+Hb EWs, and [OIII]5007/[OII]3727 and [NeIII]3869/[OII]3727 line ratios). The REBELS-MOSFIRE sample represents the higher-mass, higher-metallicity, lower-excitation tail of the z 7 galaxy population, whose ionizing properties must be fully characterized to constrain the role of star-forming galaxies during cosmic reionization.

Neutral gas in galaxies during the Epoch of Reionisation regulates star formation, dust growth, and the escape of ionising photons, making it a key ingredient in understanding both galaxy assembly and reionisation. Yet, direct constraints on the HI content of galaxies at z>6 have been scarce. With JWST, Lyα damping wings in galaxy spectra can now provide a direct probe of this neutral component. We analyse JWST/NIRSpec prism spectra of 12 UV-luminous galaxies from the REBELS-IFU program at z 6.5-7.7, deriving HI column densities by modelling Lyα damping wings. Significant damped Lyα absorption is detected in eight galaxies, with N_HI≳10^21 cm^-2. We use the column densities and sizes derived for these sources to estimate their HI mass and compare with L_[CII]-M_HI calibrations. The resulting HI masses show a tentative correlation with those inferred from [CII], although the [CII]-based estimates are systematically larger, suggesting that the HI reservoirs may extend beyond the [CII]-emitting gas. We also combine the DLA-based measurements with FIR-derived dust-to-gas ratios, dust attenuation, and gas-phase metallicities. No correlation is found between DLA-based and FIR-based dust-to-gas ratios, but combining the REBELS-IFU sample with literature samples at lower metallicities reveals a strong correlation between A_V/N_HI and metallicity. These findings suggest that by z∼7 massive galaxies can already host substantial, enriched reservoirs of neutral gas and dust, consistent with A_V/N_HI-metallicity trends at lower redshift. At the highest redshifts (z>8), however, we see tentative evidence for systematically lower A_V/N_HI at fixed metallicity, which may point to pristine gas accretion or more efficient dust destruction/expulsion.

With the advent of JWST, we can probe the rest-frame optical emission of galaxies at z> 3 with high sensitivity and spatial resolution, making it possible to accurately characterize red, optically faint galaxies and thus move towards a more complete census of the galaxy population at high redshifts. To this end, we present a sample of 148 massive, dusty galaxies from the JWST/Cosmic Evolution Early Release Science survey, colour-selected using solely JWST bands. With deep JWST/NIRCam data from 1.15 to 4.44 mu m and ancillary HST/ACS and WFC3 data, we determine the physical properties of our sample using spectral energy distribution fitting with bagpipes. We demonstrate that our selection method efficiently identifies massive ({log M-*/M-circle dot}similar to 10) and dusty ({A(V)} similar to 2,7 mag) sources, with a majority at z > 3 and predominantly lying on the galaxy main sequence. The main results of this work are the stellar mass functions (SMFs) of red, optically faint galaxies from redshifts between 3 < z < 8: these galaxies make up a significant relative fraction of the pre-JWST total SMF at 3 < z < 4 and 4< z < 6, and dominate the high-mass end of the pre-JWST SMF at 6 < z < 8, suggesting that our census of the galaxy population needs amendment at these epochs. While larger areas need to be surveyed in the future, our results suggest already that the integrated stellar mass density at log M-*/M circle dot >= 9.25 may have been underestimated in pre-JWST studies by up to similar to 15-20% at similar to 3 -6, and up to similar to 45% at z similar to 6-8, indicating the rapid onset of obscured stellar mass assembly in the early Universe.

Star-forming galaxies (SFGs) adhere to a surprisingly tight scaling relation of dust attenuation parameterized by the infrared excess (IRX=$L_{\rm IR}/L_{\rm UV}$), being jointly determined by the star formation rate (SFR), galaxy size ($R_{\rm e}$), metallicity ($Z$/Z$_\odot$) and axial ratio ($b/a$). We examine how these galaxy parameters determine the effective dust attenuation and give rise to the universal IRX relation, utilizing a simple two-component star-dust geometry model in which dust in the dense and diffuse interstellar medium (ISM) follows exponential mass density profiles, connected with but not necessarily identical to the stellar mass profiles. Meanwhile, empirical relations are adopted to link galaxy properties, including the gas--star formation relation, the dust-to-stellar size relation, as well as the dust-to-gas ratio versus metallicity relation. By fitting a large sample of local SFGs with the model, we obtain the best-fitting model parameters as a function of metallicity, showing that the two-component geometry model is able to successfully reproduce the dependence of IRX on SFR, $R_{\rm e}$, $b/a$ at given $Z$/Z$_\odot$, as well as the dependence of power-law indices on metallicity. Moreover, we also retrieve constraints on the model geometry parameters, including the optical depth of birth clouds (BCs), BC-to-total dust mass fraction, BC covering factor of UV-emitting stars, and star-to-total dust disc radius ratio, which all evolve with galaxy metallicity. Finally, a consistent picture of how the star-dust geometry in SFGs evolves with galaxy metallicity is discussed.

We present a new method to determine the star formation rate density (SFRD) of the Universe at z greater than or similar to 5 that includes the contribution of dust-obscured star formation. For this purpose, we use a [C II] (158 mu m) selected sample of galaxies serendipitously identified in the fields of known z greater than or similar to 4.5 objects to characterise the fraction of obscured star formation rate (SFR). The advantage of a [C II] selection is that our sample is SFR-selected, in contrast to an ultraviolet (UV)-selection that would be biased towards unobscured star formation. We obtain a sample of 23 [C II] emitters near star-forming (SF) galaxies and quasi-stellar objects (QSOs) - three of which we identify for the first time - using previous literature and archival Atacama Large Millimeter/submillimeter Array data. 18 of these serendipitously identified galaxies have sufficiently deep rest-UV data and are used to characterise the obscured fraction of the star formation in galaxies with SFRs greater than or similar to 30 M-circle dot yr(-1). We find that [C II] emitters identified around SF galaxies have approximate to 63 per cent of their SFR obscured, while [C II] emitters around QSOs have approximate to 93 per cent of their SFR obscured. By forward modelling existing wide-area UV luminosity function (LF) determinations, we derive the intrinsic UV LF using our characterisation of the obscured SFR. Integrating the intrinsic LF to MUV = -20, we find that the obscured SFRD contributes to >3 per cent and >10 per cent of the total SFRD at z similar to 5 and z similar to 6 based on our sample of companions galaxies near SF galaxies and QSOs, respectively. Our results suggest that dust obscuration is not negligible at z greater than or similar to 5, further underlining the importance of far-infrared observations of the z greater than or similar to 5 Universe.

A key to understanding the formation of the first galaxies is to quantify the content of the molecular gas as the fuel for star formation activity through the epoch of reionization. In this paper, we use the 158$\mu$m [CII] fine-structure emission line as a tracer of the molecular gas in the interstellar medium (ISM) in a sample of $z=6.5-7.5$ galaxies recently unveiled by the Reionization Era Bright Line Emission Survey, REBELS, with the Atacama Large Millimeter/submillimeter Array. We find substantial amounts of molecular gas ($\sim10^{10.5}\ M_\odot$) comparable to those found in lower redshift galaxies for similar stellar masses ($\sim10^{10}\ M_\odot$). The REBELS galaxies appear to follow the standard scaling relations of molecular gas to stellar mass ratio ($\mu_{\rm mol}$) and gas depletion timescale ($t_{\rm dep}$) with distance to the star-forming main-sequence expected from extrapolations of $z\sim1-4$ observations. We find median values at $z\sim7$ of $\mu_{\rm mol}=2.6_{-1.4}^{4.1}$ and $t_{\rm dep}=0.5_{-0.14}^{+0.26}$ Gyr, indicating that the baryonic content of these galaxies is gas-phase dominated and little evolution from $z\sim7$ to 4. Our measurements of the cosmic density of molecular gas, log$(\rho_{\rm mol}/(M_\odot {\rm Mpc}^{-3}))=6.34^{+0.34}_{-0.31}$, indicate a steady increase by an order of magnitude from $z\sim7$ to 4.

The process of galaxy cluster formation likely leaves an imprint on the properties of its individual member galaxies. Understanding this process is essential for uncovering the evolutionary connections between galaxies and cosmic structures. Here we study a sample of ten protoclusters at z 2-3 in different dynamical states that we estimate based on spectroscopic data of their members. We combine the dynamical information with HST imaging to measure galaxy sizes and pair fractions. Our analysis reveals a clear anti-correlation between the velocity dispersion of the protocluster and its galaxy pair fractions (indicative of merger rates). The velocity dispersion also anti-correlates with the dispersion in size among of the member galaxies. These correlations may be explained by protoclusters in colder dynamical states maintaining a velocity dispersion and galaxy number density that boosts galaxy mergers, which in turn contributes to the structural expansion and compaction of galaxies. Our findings offer constraints for cosmological models regarding the evolution of galaxy morphology across different stages in the assembly of protoclusters.

Over the last few years, both ALMA and Spitzer/IRAC observations have revealed a population of likely massive galaxies at $z>3$ that was too faint to be detected in HST rest-frame ultraviolet imaging. However, due to the very limited photometry for individual galaxies, the true nature of these so-called HST-dark galaxies has remained elusive. Here, we present the first sample of such galaxies observed with very deep, high-resolution NIRCam imaging from the Early Release Science Program CEERS. 30 HST-dark sources are selected based on their red colours across 1.6 $\mu$m to 4.4 $\mu$m. Their physical properties are derived from 12-band multi-wavelength photometry, including ancillary HST imaging. We find that these galaxies are generally heavily dust-obscured ($A_{V}\sim2$ mag), massive ($\log (M/M_{\odot}) \sim10$), star-forming sources at $z\sim2-8$ with an observed surface density of $\sim0.8$ arcmin$^{-2}$. This suggests that an important fraction of massive galaxies may have been missing from our cosmic census at $z>3$ all the way into the Reionization epoch. The HST-dark sources lie on the main sequence of galaxies and add an obscured star formation rate density (SFRD) of $\mathrm{3.2^{+1.8}_{-1.3} \times 10^{-3} M_{\odot}/yr/Mpc^{3}}$ at $z\sim7$ showing likely presence of dust in the Epoch of Reionization. Our analysis shows the unique power of JWST to reveal this previously missing galaxy population and to provide a more complete census of galaxies at $z=2-8$ based on rest-frame optical imaging.

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-)millimeter 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 [CII] and dust emission in UV-selected galaxies at $z\sim7$, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation $f_\mathrm{obs} = \mathrm{SFR}_\mathrm{IR} / \mathrm{SFR}_\mathrm{UV + IR}$ depends on stellar mass, similar to what is observed at lower redshift, and ranges from $f_\mathrm{obs} \approx 0.3 - 0.6$ for galaxies with $\log_{10}\left(M_\star / M_\odot\right) = 9.4 - 10.4$. We further adopt the $z\sim7$ 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 $3\lesssim z \lesssim 7$, with dust-obscured star formation still contributing $\sim30\%$ at $z\sim7$. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization.

ABSTRACT We present Atacama Large Millimeter/Submillimeter Array (ALMA) [C ii] and ∼158 $\rm \mu m$ continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; LIR = $1.5^{+0.8}_{-0.5} \times 10^{12}$ L⊙) at z = 7.31, spectroscopically confirmed by the Reionization Era Bright Emission Line Survey (REBELS) ALMA Large Programme. REBELS-25 has a significant stellar mass of $M_{*} = \mbox{$ 8^{+4}_{-2} \times 10^{9} $}{} ~\mbox{M$_\odot $}{}$. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR $= \mbox{$199^{+101}_{-63}$}{} ~ \mbox{M$_\odot $}~ \mbox{${\rm yr}$}^{-1}$. This is about four times the SFR estimated from an extrapolated main sequence. We also infer a [C ii]-based molecular gas mass of $M_{{\rm H}_{2}} = \mbox{$5.1^{+5.1}_{-2.6} \times 10^{10}$}{} ~\mbox{M$_\odot $}{}$, implying a molecular gas depletion time of $t_{\rm depl, {\rm H}_{2}} = \mbox{$0.3^{+0.3}_{-0.2} $}{}$ Gyr. We observe a [C ii] velocity gradient consistent with disc rotation, but given the current resolution we cannot rule out a more complex velocity structure such as a merger. The spectrum exhibits excess [C ii] emission at large positive velocities (∼500 km s−1), which we interpret as either a merging companion or an outflow. In the outflow scenario, we derive a lower limit of the mass outflow rate of 200 $\mbox{M$_\odot $}~ \mbox{${\rm yr}$}^{-1}$, which is consistent with expectations for a star-formation-driven outflow. Given its large stellar mass, SFR, and molecular gas reservoir ∼700 Myr after the big bang, we explore the future evolution of REBELS-25. Considering a simple, conservative model assuming an exponentially declining star formation history, constant star formation efficiency, and no additional gas inflow, we find that REBELS-25 has the potential to evolve into a galaxy consistent with the properties of high-mass quiescent galaxies recently observed at z ∼ 4.

We present the H α luminosity function (LF) derived from a large sample of Lyman break galaxies at z ∼ 4.5 over the GOODS-South and North fields. This study makes use of the new, full-depth Spitzer/IRAC [3.6] and [4.5] imaging from the GOODS Re-ionization Era wide-Area Treasury from the Spitzer program. The H α flux is derived from the offset between the continuum flux estimated from the best-fit spectral energy distribution, and the observed photometry in IRAC [3.6]. From these measurements, we build the H α LF and study its evolution providing the best constraints of this property at high redshift, where spectroscopy of H α is not yet available. Schechter parameterizations of the H α LF show a decreasing evolution of Φ ^∗ with redshift, increasing evolution in L ^∗ , and no significant evolution in the faint-end slope at high z . We find that star formation rates (SFRs) derived from H α are higher than those derived from the rest-frame UV for low SFR galaxies but the opposite happens for the highest SFRs. This can be explained by lower mass galaxies (also lower SFR) having, on average, rising star formation histories (SFHs), while at the highest masses the SFHs may be declining. The SFR function is steeper, and because of the excess SFR(H α ) compared to SFR(UV) at low SFRs, the SFR density estimated from H α is higher than the previous estimates based on UV luminosities.

We present the deepest constraints yet on the median rest-UV+optical spectral energy distribution (SED) of z ∼ 10 galaxies prior to James Webb Space Telescope science operations. We constructed stacks based on four robust J 125 dropouts, previously identified across the GOODS fields. We used archival Hubble Space Telescope/Wide Field Camera 3 data and the full-depth Spitzer/IRAC mosaics from the GREATS program, the deepest coverage at ∼3–5 μ m to date. The most remarkable feature of the SED is a blue IRAC [3.6]–[4.5] = −0.18 ± 0.25 mag color. We also find a nearly flat H 160 − [3.6] = 0.07 ± 0.22 mag color, corresponding to a UV slope β = −1.92 ± 0.25. This is consistent with previous studies and indicative of minimal dust absorption. The observed blue IRAC color and SED fitting suggest that z ∼ 10 galaxies have very young (few × 10 Myr) stellar populations, with 80% of stars being formed in the last ≲160 Myr (2 σ ). While an exciting result, the uncertainties on the SED are too large to allow us to place strong constraints on the presence of a nebular continuum in z ∼ 10 galaxies (as might be suggested by the blue [3.6]–[4.5] < 0 mag color). The resulting sSFR is consistent with the specific accretion rate of dark matter halos, indicative of a star formation efficiency showing quite limited evolution at such early epochs.

The emergence of massive, optically-faint galaxies in infrared observations has revealed that our view of the high-redshift Universe was previously incomplete. With the advent of JWST, we can for the first time probe the rest-frame optical emission of galaxies at $z>3$ with high sensitivity and spatial resolution, thus moving towards a more complete census of the galaxy population at high redshifts. To this end, we present a sample of 148 massive, dusty galaxies from the JWST/CEERS survey, colour-selected using solely JWST bands. With deep JWST/NIRCam data from 1.15$μ$m to 4.44$μ$m and ancillary HST/ACS and WFC3 data, we determine the physical properties of our sample using spectral energy distribution fitting with BAGPIPES. We demonstrate that our selection method efficiently identifies massive ($\mathrm{\langle \log M_\star/M_\odot \rangle \sim 10}$) and dusty ($\mathrm{\langle A_V\rangle \sim 2.7\ mag}$) sources, with a majority at $z>3$ and predominantly lying on the galaxy main-sequence. The main results of this work are the stellar mass functions (SMF) of red, optically-faint galaxies from redshifts between $39.25}$ may have been underestimated by $\sim$20-25% at $z\sim3-6$, and $\sim$110% at $z\sim6-8$.

We present the first observational infrared luminosity function (IRLF) measurement in the Epoch of Reionization (EoR) based on a UV-selected galaxy sample with ALMA spectroscopic observations. Our analysis is based on the ALMA large program Reionization Era Bright Emission Line Survey (REBELS), which targets 42 galaxies at $\mathrm{z=6.4-7.7}$ with [CII] 158$\micron$ line scans. 16 sources exhibit a dust detection, 15 of which are also spectroscopically confirmed through the [CII] line. The IR luminosities of the sample range from $\log L_{IR}/L_\odot=11.4$ to 12.2. Using the UVLF as a proxy to derive the effective volume for each of our target sources, we derive IRLF estimates, both for detections and for the full sample including IR luminosity upper limits. The resulting IRLFs are well reproduced by a Schechter function with the characteristic luminosity of $\log L_{*}/L_\odot=11.6^{+0.2}_{-0.1}$. Our observational results are in broad agreement with the average of predicted IRLFs from simulations at $z\sim7$. Conversely, our IRLFs lie significantly below lower redshift estimates, suggesting a rapid evolution from $z\sim4$ to $z\sim7$, into the reionization epoch. The inferred obscured contribution to the cosmic star-formation rate density at $z\sim7$ amounts to $\mathrm{log(SFRD/M_{\odot}/yr/Mpc^{3}) = -2.66^{+0.17}_{-0.14} }$ which is at least $\sim$10\% of UV-based estimates. We conclude that the presence of dust is already abundant in the EoR and discuss the possibility of unveiling larger samples of dusty galaxies with future ALMA and JWST observations.

We present the average [C ii] 158 mu m emission line sizes of UV-bright star-forming galaxies at z similar to 7. Our results are derived from a stacking analysis of [C ii] 158 mu m emission lines and dust continua observed by the Atacama Large Millimeter/submillimeter Array (ALMA), taking advantage of the large program Reionization Era Bright Emission Line Survey. We find that the average [C ii] emission at z similar to 7 has an effective radius r ( e ) of 2.2 +/- 0.2 kpc. It is greater than or similar to 2x larger than the dust continuum and the rest-frame UV emission, in agreement with recently reported measurements for z less than or similar to 6 galaxies. Additionally, we compared the average [C ii] size with 4 < z < 6 galaxies observed by the ALMA Large Program to INvestigate [C ii] at Early times (ALPINE). By analyzing [C ii] sizes of 4 < z < 6 galaxies in two redshift bins, we find an average [C ii] size of r (e) = 2.2 +/- 0.2 kpc and r (e) = 2.5 +/- 0.2 kpc for z similar to 5.5 and z similar to 4.5 galaxies, respectively. These measurements show that star-forming galaxies, on average, show no evolution in the size of the [C ii] 158 mu m emitting regions at redshift between z similar to 7 and z similar to 4. This finding suggests that the star-forming galaxies could be morphologically dominated by gas over a wide redshift range.

ABSTRACT 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.

We present an analysis of physical properties of 34 [O iii] emission-line galaxies (ELGs) at z = 3.254 +/- 0.029 in the Extended Chandra Deep Field South (ECDFS). These ELGs are selected from deep narrow H2S(1) and broad K (s) imaging of 383 arcmin(2) obtained with CFHT/WIRCam. We construct spectral energy distributions (SEDs) from U to K (s) to derive the physical properties of ELGs. These [O iii] ELGs are identified as starburst galaxies with strong [O iii] lines of L (OIII ) similar to 10(42.6)-10(44.2) erg s(-1) and have stellar masses of M (*) similar to 10(9.0)-10(10.6) M (circle dot) and star formation rates of similar to 10-210 M (circle dot) yr(-1). Our results show that 24% of our sample galaxies are dusty with A (V) > 1 mag and EW([O iii])(rest) similar to 70-500 angstrom, which are often missed in optically selected [O iii] ELG samples. Their rest-frame UV and optical morphologies from HST/ACS and HST/WFC3 deep imaging reveal that these [O iii] ELGs are mostly multiple-component systems (likely mergers) or compact. And 20% of them are nearly invisible in the rest-frame UV owing to heavy dust attenuation. Interestingly, we find that our sample ELGs reside in an overdensity consisting of two components: one southeast (SE) with an overdensity factor of delta (gal) similar to 41 over a volume of 13(3) cMpc(3), and the other northwest (NW) with delta (gal) similar to 38 over a volume of 10(3) cMpc(3). The two overdense substructures are expected to be virialized at z = 0 with a total mass of similar to 1.1 x 10(15) M (circle dot) and similar to 4.8 x 10(14) M (circle dot) and probably merge into a Coma-like galaxy cluster.