The VLA-COSMOS 3 GHz Large Project : Evolution of Specific Star Formation Rates out to z similar to 5

We provide a coherent, uniform measurement of the evolution of the logarithmic star formation rate (SFR)-stellar mass (M-*) relation, called the main sequence (MS) of star-forming galaxies, for star-forming and all galaxies out to z similar to 5. We measure the MS using mean stacks of 3 GHz radio-continuum images to derive average SFRs for similar to 200,000 mass-selected galaxies at z > 0.3 in the COSMOS field. We describe the MS relation by adopting a new model that incorporates a linear relation at low stellar mass (log(M-*/M-circle dot) <10) and a flattening at high stellar mass that becomes more prominent at low redshift (z <1.5). We find that the SFR density peaks at 1.5 <z <2, and at each epoch there is a characteristic stellar mass (M-* = 1-4 x 10(10)M(circle dot)) that contributes the most to the overall SFR density. This characteristic mass increases with redshift, at least to z similar to 2.5. We find no significant evidence for variations in the MS relation for galaxies in different environments traced by the galaxy number density at 0.3 <z <3, nor for galaxies in X-ray groups at z similar to 0.75. We confirm that massive bulge-dominated galaxies have lower SFRs than disk-dominated galaxies at a fixed stellar mass at z <1.2. As a consequence, the increase in bulge-dominated galaxies in the local star-forming population leads to a flattening of the MS at high stellar masses. This indicates that "mass quenching" is linked with changes in the morphological composition of galaxies at a fixed stellar mass.Peer reviewe

Cold dust and low [O iii]/[C ii] ratios: an evolved star-forming population at redshift 7

We present new ALMA Band 8 (rest-frame 90 μm) continuum observations of three massive (M⋆ ≈ 1010 M⊙) galaxies at z ≈ 7 previously detected in [C II]158 μm and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect dust emission from two of our targets in Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through optically thin modified blackbody fitting, we determine dust temperatures of Tdust ≈ 30 − 35 K in both of the dual-band detected targets, indicating they are colder than most known galaxies at z ∼ 7. Moreover, their inferred dust masses are large (Mdust ≈ 108 M⊙), albeit still consistent with models of high-redshift dust production. We furthermore target and detect [O III]88 μm emission in both REBELS-12 and REBELS-25, and find L[O III]/L[C II] ≈ 1 − 1.5 – low compared to the L[O III]/L[C II] ≳ 2 − 10 observed in the known z ≳ 6 population thus far. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets, although the possibility of REBELS-12 being a merger of an [O III]-bright and [O III]-faint component prevents the unambiguous interpretation of its [O III]/[C II] ratio. Nevertheless, a low burstiness forms a natural explanation for the cold dust temperatures and low [O III]λλ4959, 5007 + Hβ equivalent widths of REBELS-25 and REBELS-38. Overall, these observations provide evidence for the existence of a massive, dust-rich galaxy population at z ≈ 7 which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner

Metal and dust evolution in ALMA REBELS galaxies: insights for future JWST observations

ALMA observations revealed the presence of significant amounts of dust in the first Gyr of Cosmic time. However, the metal and dust buildup picture remains very uncertain due to the lack of constraints on metallicity. JWST has started to reveal the metal content of high-redshift targets, which may lead to firmer constraints on high-redshift dusty galaxies evolution. In this work, we use detailed chemical and dust evolution models to explore the evolution of galaxies within the ALMA REBELS survey, testing different metallicity scenarios that could be inferred from JWST observations. In the models, we track the buildup of stellar mass by using non-parametric SFHs for REBELS galaxies. Different scenarios for metal and dust evolution are simulated by allowing different prescriptions for gas flows and dust processes. The model outputs are compared with measured dust scaling relations, by employing metallicity-dependent calibrations for the gas mass based on the [CII]158micron line. Independently of the galaxies metal content, we found no need for extreme dust prescriptions to explain the dust masses revealed by ALMA. However, different levels of metal enrichment will lead to different dominant dust production mechanisms, with stardust production dominant over other ISM dust processes only in the metal-poor case. This points out how metallicity measurements from JWST will significantly improve our understanding of the dust buildup in high-redshift galaxies. We also show that models struggle to reproduce observables such as dust-to-gas and dust-to-stellar ratios simultaneously, possibly indicating an overestimation of the gas mass through current calibrations, especially at high metallicities.Comment: 16 pages + appendices, 9 Figures, 1 Table. Resubmitted to MNRAS after moderate revisio

The ALMA REBELS Survey: Dust Continuum Detections at z > 6.5

We report 18 dust continuum detections ($\geq 3.3\sigma$) at $\sim88{\rm \mu m}$ and $158{\rm \mu m}$ out of 49 ultraviolet(UV)-bright galaxies ($M_{\rm UV} 6.5$, observed by the Cycle-7 ALMA Large Program, REBELS and its pilot programs. This has more than tripled the number of dust continuum detections known at $z>6.5$. Out of these 18 detections, 12 are reported for the first time as part of REBELS. In addition, 15 of the dust continuum detected galaxies also show a [CII]$_{\rm 158{\rm \mu m}}$ emission line, providing us with accurate redshifts. We anticipate more line emission detections from six targets (including three continuum detected targets) where observations are still ongoing. The dust continuum detected sources in our sample tend to have a redder UV spectral slope than the ones without a dust continuum detection. We estimate that all of the sources have an infrared (IR) luminosity ($L_{\rm IR}$) in a range of $3-8 \times 10^{11} L_\odot$, except for one with $L_{\rm IR} = 1.5^{+0.8}_{-0.5} \times 10^{12}\,L_{\odot}$. Their fraction of obscured star formation is significant at $\gtrsim 50\%$. Some of the dust continuum detected galaxies show spatial offsets ($\sim 0.5-1.5''$) between the rest-UV and far-IR emission peaks. These separations appear to have an increasing trend against an indicator that suggests spatially decoupled phases of obscured and unobscured star formation. REBELS offers the best available statistical constraints on obscured star formation in UV-bright, massive galaxies at $z > 6.5$.Comment: 17 pages, 9 figures, submitted to MNRA

The VLA-COSMOS 3 GHz Large Project: Evolution of Specific Star Formation Rates out to z ~ 5

We provide a coherent, uniform measurement of the evolution of the logarithmic star formation rate(SFR)-stellarmass(M*)relation, called the main sequence(MS)of star-forming galaxies , for star-forming and all galaxies outto~z5. We measure the MS using mean stacks of 3 GHz radio-continuum images to derive average SFRs for∼200,000 mass-selected galaxies at z>0.3 in the COSMOSfield. We describe the MS relation by adopting anew model that incorporates a linear relation at low stellar mass(log(M*/Me)<10)and aflattening at high stellarmass that becomes more prominent at low redshift(z < 1.5).Wefind that the SFR density peaks at 1.5<z<2,and at each epoch there is a characteristic stellar mass(M*=1-4 x 1010Me)that contributes the most to theoverall SFR density. This characteristic mass increases with redshift, at least toz ∼ 2.5. Wefind no significant evidence for variations in the MS relation for galaxies in different environments traced by the galaxy numberdensity at 0.3<z<3, nor for galaxies in X-ray groups atz ∼ 0.75. We confirm that massive bulge-dominatedgalaxies have lower SFRs than disk-dominated galaxies at afixed stellar mass atz < 1.2. As a consequence, theincrease in bulge-dominated galaxies in the local star-forming population leads to aflattening of the MS at highstellar masses. This indicates that "mass quenching" is linked with changes in the morphological composition ofgalaxies at afixed stellar mass.B.G. acknowledges the support of the Australian Research Council as the recipient of a Future Fellowship (FT140101202). Y.P. acknowledges the National Key R&D Program of China, Grant 2016YFA0400702 and NSFC grant Nos. 11773001, 11721303, 1199105

Cold dust and low [O iii]/[C ii] ratios: an evolved star-forming population at redshift 7

We present new ALMA Band 8 (rest-frame 90 μm) continuum observations of three massive (M ≈ 1010 M) galaxies at z ≈ 7 previously detected in [C II]158 μm and underlying dust continuum emission in the Reionization Era Bright Emission Line Survey (REBELS). We detect dust emission from two of our targetsin Band 8 (REBELS-25 and REBELS-38), while REBELS-12 remains undetected. Through optically thin modified blackbody fitting, we determine dust temperatures of Tdust ≈ 30 − 35 K in both of the dual-band detected targets, indicating they are colder than most known galaxies at z ∼ 7. Moreover, their inferred dust masses are large (Mdust ≈ 108 M), albeit still consistent with models of high-redshift dust production. We furthermore target and detect [O III]88 μm emission in both REBELS-12 and REBELS-25, and find L[O III]/L[C II] ≈ 1 − 1.5 – low compared to the L[O III]/L[C II] 2 − 10 observed in the known z 6 population thusfar. We argue the lower line ratios are due to a comparatively weaker ionizing radiation field resulting from the less starbursty nature of our targets, although the possibility of REBELS-12 being a merger of an [O III]-bright and [O III]-faint component prevents the unambiguous interpretation of its [O III]/[C II] ratio. Nevertheless, a low burstiness forms a natural explanation for the cold dust temperatures and low [O III]λλ4959, 5007 + H β equivalent widths of REBELS-25 and REBELS-38. Overall, these observations provide evidence for the existence of a massive, dust-rich galaxy population at z ≈ 7 which has previously experienced vigorous star formation, but is currently forming stars in a steady, as opposed to bursty, manner.This work was supported by NAOJ ALMA Scientific Research Grant Code 2021–19A (HSBA and HI). PD acknowledges support from the NWO grant 016.VIDI.189.162 (‘ODIN’) and from the European Commission’s and University of Groningen’s CO-FUND Rosalind Franklin program. MA acknowledges support from FONDECYT grant 1211951 and ANID BASAL project FB210003. IDL and MP acknowledge support from ERC starting grant 851622 DustOrigin. RS acknowledges support from a STFC Ernest Rutherford Fellowship (ST/S004831/1). MS acknowledges support from the CIDEGENT/2021/059 grant, from project PID2019-109592GB-I00/AEI/10.13039/501100011033 from the Spanish Ministerio de Ciencia e Innovación–Agencia Estatal de Investigación, and from Proyecto ASFAE/2022/025 del Ministerio de Ciencia y Innovación en el marco del Plan de Recuperación, Transformación y Resiliencia del Gobierno de España.Peer reviewe

ALMA as a Redshift Machine: Using [C ii ] to Efficiently Confirm Galaxies in the Epoch of Reionization

The [C ii]158μm line has long been proposed as a promising line to spectroscopically confirm galaxies in the epoch of reionization. In this paper, we present the results of new ALMA observations spectral scanning for [C ii] in six particularly luminous Lyman-break galaxies at z ∼ 7. The six sources were drawn from a sample of bright z ∼ 7 galaxies identified using the wide-area optical, near-IR, and Spitzer/IRAC data over the COSMOS/UltraVISTA field and were targeted on the basis of tight constraints on their redshifts from their IRAC [3.6]–[4.5] colors. We detect significant (>9σ) [C ii] lines in three of our six targets (50%) cospatial with the rest-UV emission from the ground/space-based near-IR imaging. The luminosities of the [C ii] lines lie in the range 5.6–8.8 × 108 L ⊙, consistent with the local [C ii]–SFR relation. Meanwhile, their [C ii]/L IR ∼ 1–3 × 10−3 ratios are slightly elevated compared to local (U)LIRGS. This could be due to lower dust-to-gas or dust-to-metal ratios. We also find that our sources display a large kinematic diversity, with one source showing signs of rotation, one source a likely major merger, and one dispersion-dominated source that might contain a bright star-forming clump. Our results highlight the effectiveness of spectral scans with ALMA in spectroscopically confirming luminous galaxies in the epoch of reionization, something that is being be applied on a significantly larger sample in the ongoing REBELS large program

COLDz: Probing Cosmic Star Formation With Radio Free–Free Emission

Radio free–free emission is considered to be one of the most reliable tracers of star formation in galaxies. However, as it constitutes the faintest part of the radio spectrum—being roughly an order of magnitude less luminous than radio synchrotron emission at the GHz frequencies typically targeted in radio surveys—the usage of free–free emission as a star formation rate tracer has mostly remained limited to the local universe. Here, we perform a multifrequency radio stacking analysis using deep Karl G. Jansky Very Large Array observations at 1.4, 3, 5, 10, and 34 GHz in the COSMOS and GOODS-North fields to probe free–free emission in typical galaxies at the peak of cosmic star formation. We find that z ∼ 0.5–3 star-forming galaxies exhibit radio emission at rest-frame frequencies of ∼65–90 GHz that is ∼1.5–2 times fainter than would be expected from a simple combination of free–free and synchrotron emission, as in the prototypical starburst galaxy M82. We interpret this as a deficit in high-frequency synchrotron emission, while the level of free–free emission is as expected from M82. We additionally provide the first constraints on the cosmic star formation history using free–free emission at 0.5 z 3, which are in good agreement with more established tracers at high redshift. In the future, deep multifrequency radio surveys will be crucial in order to accurately determine the shape of the radio spectrum of faint star-forming galaxies, and to further establish radio free–free emission as a tracer of high-redshift star formation