arXiv subjects

R. Smit

Publications and source records attributed to R. Smit.

At least 19 recordsLinked to original sources

arXiv2025

REBELS-IFU: Steeply rising star formation histories and the importance of dust obscuration in massive $z \simeq7$ galaxies revealed by multi-wavelength observations

Reliable star formation rate (SFR) measurements are essential for understanding early galaxy evolution, yet derived values rely on several assumptions. To address this problem, we investigate the SFRs of 12 massive ($9~<~\log(M_{\star}/{\rm M}_{\odot})~<~10$) Lyman-break galaxies at $z=6.5-7.7$, drawn from the Atacama Large Millimeter/submillimeter Array (ALMA) Reionization Era Bright Emission Line Survey (REBELS) program. The multi-wavelength data, including JWST NIRSpec IFU spectroscopy and ALMA observations, make this a unique sample for investigating SFR tracers at this epoch. We compare SFRs derived from the rest-UV, H$\alpha$, and far-infrared emission, and from spectral energy distribution (SED) fits. We apply robust dust attenuation corrections, which are crucial since between $50-80$ per cent of the star formation is obscured, and find a stellar-to-nebular attenuation ratio of $f=0.50\pm0.08$, consistent with local star-forming galaxies. The majority of the derived total SFRs (medians $25-120$ ${\rm M}_{\odot}$ yr$^{-1}$) place the REBELS galaxies systematically above $z=7$ literature star-forming main-sequence relations, and our best-fit star formation histories (SFHs) rise more steeply than lower-mass galaxies at the same redshift. We show that these rising SFHs mean commonly used luminosity-to-SFR conversion factors, derived assuming a constant SFH over given timescales, overestimate the SFRs averaged over these timescales for our galaxies. We provide updated luminosity-to-SFR calibrations for $z\simeq7$ galaxies with rising SFHs, showing that commonly assumed rest-UV conversion factors overestimate the $100$ Myr average SFR by a factor of $\simeq3$. Finally, we investigate burstiness indicators in the REBELS-IFU galaxies, finding that the rising SFHs imply that the H$\alpha$-to-UV luminosity ratio is an unreliable probe of bursty star formation.

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arXiv2025

The ALPINE-CRISTAL-JWST Survey: JWST/IFU Optical Observations for 18 Main-Sequence Galaxies at z=4-6

To fully characterize the formation and evolution of galaxies, we need to observe their stars, gas, and dust on resolved spatial scales. We present the ALPINE-CRISTAL-JWST survey, which combines kpc-resolved imaging and spectroscopy from HST, JWST, and ALMA for 18 representative main-sequence galaxies at z=4-6 and log(M/$M_\odot$) > 9.5 to study their star formation, chemical properties, and extended gas reservoirs. The co-spatial measurements resolving the ionized gas, molecular gas, stars, and dust on 1-2 kpc scales make this a unique benchmark sample for the study of galaxy formation and evolution at $z\sim5$, connecting the Epoch of Reionization with the cosmic noon. In this paper, we outline the survey goals and sample selection, and present a summary of the available data for the 18 galaxies. In addition, we measure spatially integrated quantities (such as global gas metallicity), test different star formation rate indicators, and quantify the presence of H$\alpha$ halos. Our targeted galaxies are relatively metal rich (10-70% solar), complementary to JWST samples at lower stellar mass, and there is broad agreement between different star formation indicators. One galaxy has the signature of an active galactic nuclei (AGN) based on its emission line ratios. Six show broad H$\alpha$~emission suggesting type 1 AGN candidates. We conclude with an outlook on the exciting science that will be pursued with this unique sample in forthcoming papers.

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arXiv2025

The ALMA REBELS survey: [OIII]$_{88\mu \text{m}}$ line scans of UV-bright $z \gtrsim 7.6$ galaxies

We present the [OIII]$_{88\mu \text{m}}$ spectral scan results from the ALMA Large Program REBELS (Reionization Era Bright Emission Line Survey). The generally high luminosity of [OIII]$_{88\mu \text{m}}$ and ALMA's Band 7 efficiency motivated its use for line scans of REBELS targets at $z>8$. Spectral scans of four sources covered 326.4-373.0 GHz ($z=8.10$-9.39), reaching [OIII]$_{88\mu \text{m}}$ luminosities of $\mathrm{\sim7.6\times10^8\ L_{\odot}}$ ($5\sigma$) for a FWHM of 400 km s$^{-1}$. No credible lines are detected for the four targets. For REBELS-04, the non-detection is unexpected given the $\geq92\%$ coverage of the redshift likelihood distribution and its estimated SFR of 40 $\text{M}_{\odot}\ \text{yr}^{-1}$. Possible explanations for the faint [OIII]$_{88\mu \text{m}}$ emission (assuming a FWHM of 100 km s$^{-1}$) include high ISM densities ($>n_{\text{crit}} \approx 510\ \text{cm}^{-3}$) and low ionization parameters ($\mathrm{log_{10}\ U_{ion}\lesssim -2.5}$). For REBELS-37, a subsequent detection of [CII]$_{158\mu \text{m}}$ ($z=7.643$) confirmed it lay outside our scan range. For REBELS-11 and REBELS-13, it remains unclear if the non-detection is due to the depth of the line scan or redshift coverage. REBELS-04 and REBELS-37 show significant ($\geq3.8\sigma$) dust continuum emission in Band 7. If the photometric redshift of REBELS-04 is accurate, i.e., $z_{\mathrm{phot}}=8.57^{+0.10}_{-0.09}$ or $z_{\mathrm{phot}}=8.43^{+0.10}_{-0.10}$ accounting for additional neutral hydrogen in the circumgalactic medium, REBELS-04 would constitute the most distant dust-detected galaxy identified with ALMA to date. Additional Band 6 dust observations of REBELS-37 constrain the shape of the far-IR SED, ruling out cold dust temperatures ($\lesssim28$ K) at $3\sigma$. Further insight into these galaxies will require spectroscopic redshifts and deeper multi-band dust observations.

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arXiv2025

Beneath the Surface: >85% of z>5.9 QSOs in Massive Host Galaxies are UV-Faint

We use [CII] observations of a large QSO sample to segregate sources by host galaxy mass, aiming to identify those in the most massive hosts. [CII] luminosity, a known tracer of molecular gas, is taken as a proxy for host mass and used to rank 190 QSOs at z>5.9, spanning a 6-mag UV luminosity range (-22 -24.5) and 25 especially UV-faint (Muv>-23.5) QSOs, improving statistics by 5x and 6x, respectively. Taking massive host galaxies to be those where L[CII]>1.8x10^9 Lsol (median L[CII] of UV-bright QSOs), we identify 61 QSOs, including 13 which are UV-faint and 7 especially UV-faint. Using these selections and recent QSO luminosity functions (LFs), we present the first characterization of UV luminosity distribution for QSOs in massive host galaxies and quantify [CII] LFs for both UV-bright and UV-faint QSOs. While ~3% of massive-host QSOs are UV-bright (Muv<-26), >~85% are UV-faint (Muv>-24.5). This wide dispersion in UV luminosities reflects variations in dust obscuration, accretion efficiency, and black hole mass. Though spectroscopy is needed for definitive conclusions, black hole mass appears to be the dominant factor driving variations in the UV luminosity, based on 34 [CII]-luminous (L[CII]>1.8x10^9 Lsol) QSOs distributed across a ~3-mag baseline in UV luminosity and with measured MBH. At Muv~-23, the median extrapolated log10 (MBH/Msol) is 8.1+/-0.4, consistent with the local relation. SMBHs in UV-bright QSOs thus appear to be ~15(-9)(+25)x more massive than typical for massive host galaxies at z~6.

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arXiv2025

The ALMA-CRISTAL survey: Gas, dust, and stars in star-forming galaxies when the Universe was ~1 Gyr old I. Survey overview and case studies

We present the ALMA-CRISTAL survey, an ALMA Cycle 8 Large Program designed to investigate the physical properties of star-forming galaxies at $4 \lesssim z \lesssim 6$ through spatially resolved, multi-wavelength observations. This survey targets 19 star-forming main-sequence galaxies selected from the ALPINE survey, using ALMA Band 7 observations to study [CII] 158 $\mu$m line emission and dust continuum, complemented by JWST/NIRCam and HST imaging to map stellar and UV emission. The CRISTAL sample expanded to 39 after including newly detected galaxies in the CRISTAL fields, archival data, and pilot study targets. The resulting dataset provides a detailed view of gas, dust, and stellar structures on kiloparsec scales at the end of the era of reionization. The survey reveals diverse morphologies and kinematics, including rotating disks, merging systems, [CII] emission tails from potential interactions, and clumpy star formation. Notably, the [CII] emission in many cases extends beyond the stellar light seen in HST and JWST imaging. Scientific highlights include CRISTAL-10, exhibiting an extreme [CII] deficit similar to Arp 220; and CRISTAL-13, where feedback from young star-forming clumps likely causes an offset between the stellar clumps and the peaks of [CII] emission. CRISTAL galaxies exhibit global [CII]/FIR ratios that decrease with increasing FIR luminosity, similar to trends seen in local galaxies but shifted to higher luminosities, likely due to their higher molecular gas content. CRISTAL galaxies also span a previously unexplored range of global FIR surface brightness at high-redshift, showing that high-redshift galaxies can have elevated [CII]/FIR ratios. These elevated ratios are likely influenced by factors such as lower metallicity gas, the presence of significant extraplanar gas, and contributions from shock-excited gas.

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arXiv2024

GA-NIFS: ISM properties and metal enrichment in a merger-driven starburst during the Epoch of Reionisation probed with JWST and ALMA

We present deep JWST/NIRSpec integral-field spectroscopy (IFS) and ALMA [CII]$\lambda$158$\mu$m observations of COS-3018, a star-forming galaxy at z$\sim$6.85, as part of the GA-NIFS programme. Both G395H (R$\sim$ 2700) and PRISM (R$\sim$ 100) NIRSpec observations revealed that COS-3018 is comprised of three separate components detected in [OIII]$\lambda$5008, which we dub as Main, North and East, with stellar masses of 10$^{9.4 \pm 0.1}$, 10$^{9.2 \pm 0.07}$, 10$^{7.7 \pm 0.15}$ M$_{\odot}$. We detect [OIII]$\lambda$5008, [OIII]$\lambda\lambda$3727,29 and multiple Balmer lines in all three components together with [OIII]$\lambda$4363 in the Main and North components. This allows us to measure an ISM temperature of T$_{e}$= 1.27$\pm0.07\times 10^4$ and T$_{e}$= 1.6$\pm0.14\times 10^4$ K with densities of $n_{e}$ = 1250$\pm$250 and $n_{e}$ = 700$\pm$200 cm$^{-3}$, respectively. These deep observations allow us to measure an average metallicity of 12+log(O/H)=7.9--8.2 for the three components with the T$_{e}$-method. We do not find any significant evidence of metallicity gradients between the components. Furthermore, we also detect [NII]$\lambda$6585, one of the highest redshift detections of this emission line. We find that in a small, metal-poor clump 0.2 arcsec west of the North component, N/O is elevated compared to other regions, indicating that nitrogen enrichment originates from smaller substructures, possibly proto-globular clusters. [OIII]$\lambda$5008 kinematics show that this system is merging, which is probably driving the ongoing, luminous starburst.

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arXiv2024

The JWST-PRIMAL Legacy Survey. A JWST/NIRSpec reference sample for the physical properties and Lyman-$\alpha$ absorption and emission of $\sim 500$ galaxies at $z=5.5-13.4$

One of the surprising early findings with JWST has been the discovery of a strong "roll-over" or a softening of the absorption edge of Ly$\alpha$ in a large number of galaxies at ($z\gtrsim 6$), in addition to systematic offsets from photometric redshift estimates and fundamental galaxy scaling relations. This has been interpreted as damped Ly$\alpha$ absorption (DLA) wings from high column densities of neutral atomic hydrogen (HI), signifying major gas accretion events in the formation of these galaxies. To explore this new phenomenon systematically, we assemble the JWST/NIRSpec PRImordial gas Mass AssembLy (PRIMAL) legacy survey of 494 galaxies at $z=5.5-13.4$. We characterize this benchmark sample in full and spectroscopically derive the galaxy redshifts, metallicities, star-formation rates, and ultraviolet slopes. We define a new diagnostic, the Ly$\alpha$ damping parameter $D_{\rm Ly\alpha}$ to measure and quantify the Ly$\alpha$ emission strength, HI fraction in the IGM, or local HI column density for each source. The JWST-PRIMAL survey is based on the spectroscopic DAWN JWST Archive (DJA-Spec). All the software, reduced spectra, and spectroscopically derived quantities and catalogs are made publicly available in dedicated repositories. The fraction of strong galaxy DLAs are found to be in the range $65-95\%$ at $z>5.5$. The fraction of strong Ly$\alpha$ emitters (LAEs) is found to increase with decreasing redshift, in qualitative agreement with previous observational results, and are predominantly associated with low-metallicity and UV faint galaxies. By contrast, strong DLAs are observed in galaxies with a variety of intrinsic physical properties. Our results indicate that strong DLAs likely reflect a particular early assembly phase of reionization-era galaxies, at which point they are largely dominated by pristine HI gas accretion. [abridged]

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arXiv2023

The Quasar Feedback Survey: characterising CO excitation in quasar host galaxies

We present a comprehensive study of the molecular gas properties of 17 Type 2 quasars at $z <$ 0.2 from the Quasar Feedback Survey (L$_{[OIII]}$ > $10^{42.1}$ $\rm ergs^{-1}$), selected by their high [OIII] luminosities and displaying a large diversity of radio jet properties, but dominated by LIRG-like galaxies. With these data, we are able to investigate the impact of AGN and AGN feedback mechanisms on the global molecular interstellar medium. Using APEX and ALMA ACA observations, we measure the total molecular gas content using the CO(1-0) emission and homogeneously sample the CO spectral line energy distributions (SLEDs), observing CO transitions (J$_{up}$ = 1, 2, 3, 6, 7). We observe high $r_{21}$ ratios (r$_{21}$ = L'$_{CO(2-1)}$/L'$_{CO(1-0)}$) with a median $r_{21}$ = 1.06, similar to local (U)LIRGs (with $r_{21}$ $\sim$ 1) and higher than normal star-forming galaxies (with r$_{21}$ $\sim$ 0.65). Despite the high $r_{21}$ values, for the 7 targets with the required data we find low excitation in CO(6-5) & CO(7-6) ($r_{61}$ and $r_{62}$ < 0.6 in all but one target), unlike high redshift quasars in the literature, which are far more luminous and show higher line ratios. The ionised gas traced by [OIII] exhibit systematically higher velocities than the molecular gas traced by CO. We conclude that any effects of quasar feedback (e.g. via outflows and radio jets) do not have a significant instantaneous impact on the global molecular gas content and excitation and we suggest that it only occurs on more localised scales.

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arXiv2023

The ALMA REBELS survey: obscured star formation in massive Lyman-break galaxies at z = 4-8 revealed by the IRX-$\beta$ and $M_{\star}$ relations

We investigate the degree of dust obscured star formation in 49 massive (${\rm log}_{10}(M_{\star}/{\rm M}_{\odot})>9$) Lyman-break galaxies (LBGs) at $z = 6.5$-$8$ observed as part of the 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 UV, optical and far-infrared (FIR) properties which reveal a significant fraction ($f_{\rm obs} = 0.4$-$0.7$) 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 ($\beta \simeq -2.2$) 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 $\beta_{\rm d} = 2.0$), we find that the REBELS sources are in agreement with the local ''Calzetti-like'' starburst Infrared-excess (IRX)-$\beta$ relation. By reanalysing the data available for 108 galaxies at $z \simeq 4$-$6$ from the ALPINE ALMA large program using a consistent methodology and assumed FIR SED, we show that from $z \simeq 4$-$8$, massive galaxies selected in the rest-frame UV have no appreciable evolution in their derived IRX-$\beta$ relation. When comparing the IRX-$M_{\star}$ 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 $\gtrsim 3$ at a given a $M_{\star}$. Our IRX-$\beta$ results are in good agreement with the high-redshift predictions of simulations and semi-analytic models for $z \simeq 7$ galaxies with similar stellar masses and SFRs.

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arXiv2023

NOEMA observations of GN-z11: Constraining Neutral Interstellar Medium and Dust Formation in the Heart of Cosmic Reionization at $z=10.6$

We present results of dust continuum and [CII]$\,158\,{\rm \mu m}$ emission line observations of a remarkably UV-luminous ($M_{\rm UV}=-21.6$) galaxy at $z=10.603$: GN-z11. Using the Northern Extended Millimeter Array (NOEMA), observations have been carried out over multiple observing cycles. We achieved a high sensitivity resulting in a $\lambda_{\rm rest}=160\,{\rm \mu m}$ continuum $1\,\sigma$ depth of $13.0\,\rm{\mu Jy/beam}$ and a [CII] emission line $1\,\sigma$ sensitivity of $31\,\rm{mJy/beam\,km/s}$ using $50\,\rm{km/s}$ binning with a $\sim 2\,{\rm arcsec}$ synthesized beam. Neither dust continuum nor [CII]$\,158\,{\rm \mu m}$ line emission are detected at the expected frequency of $\nu_{\rm [CII]} = 163.791\,\rm{GHz}$ and the sky location of GN-z11. The upper limits show that GN-z11 is neither luminous in $L_{\rm IR}$ nor $L_{\rm [CII]}$, with a dust mass $3\,\sigma$ limit of ${\rm log}(M_{\rm dust}/{\rm M_{\odot}}) < 6.5-6.9$ and with a [CII] based molecular gas mass $3\,\sigma$ limit of ${\rm log}(M_{\rm mol,[CII]}/{\rm M_{\odot}}) < 9.3$. Together with radiative transfer calculations, we also investigated the possible cause of the dust poor nature of the GN-z11 showed by the blue color in the UV continuum of GN-z11 ($\beta_{\rm UV}=-2.4$), and found that $\gtrsim3\times$ deeper observations are crucial to study dust production at very high-redshift. Nevertheless, our observations show the crucial role of deep mm/submm observations of very high redshift galaxies to constrain multiple phases in the interstellar medium.

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arXiv2023

The ALMA REBELS Survey: Discovery of a massive, highly star-forming and morphologically complex ULIRG at $z =7.31$

We present Atacama Large Millimeter/Submillimeter Array (ALMA) [CII] and $\sim158$ $\rm\mu m$ continuum observations of REBELS-25, a massive, morphologically complex ultra-luminous infrared galaxy (ULIRG; $L_{\rm IR}=1.5^{+0.8}_{-0.5}\times10^{12}$ L$_\odot$) 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 $M_{*}=8^{+4}_{-2}\times10^{9}$ M$_\odot$. From dust-continuum and ultraviolet observations, we determine a total obscured + unobscured star formation rate of SFR $=199^{+101}_{-63}$ M$_\odot$ yr$^{-1}$. This is about four times the SFR estimated from an extrapolated main-sequence. We also infer a [CII]-based molecular gas mass of $M_{\rm H_2}=5.1^{+5.1}_{-2.6}\times10^{10}$ $M_\odot$, implying a molecular gas depletion time of $ t_{\rm depl, H_2}=0.3^{+0.3}_{-0.2}$ Gyr. We observe a [CII] 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 [CII] emission at large positive velocities ($\sim500$ 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 M$_\odot$ yr$^{-1}$, which is consistent with expectations for a star formation-driven outflow. Given its large stellar mass, SFR and molecular gas reservoir $\sim700$ 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 $z\sim4$.

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arXiv2023

The JWST FRESCO Survey: Legacy NIRCam/Grism Spectroscopy and Imaging in the two GOODS Fields

We present the JWST Cycle 1 53.8hr medium program FRESCO, short for "First Reionization Epoch Spectroscopically Complete Observations". FRESCO covers 62 arcmin$^2$ in each of the two GOODS/CANDELS fields for a total area of 124 arcmin$^2$ exploiting JWST's powerful new grism spectroscopic capabilities at near-infrared wavelengths. By obtaining ~2 hr deep NIRCam/grism observations with the F444W filter, FRESCO yields unprecedented spectra at R~1600 covering 3.8 to 5.0 $\mu$m for most galaxies in the NIRCam field-of-view. This setup enables emission line measurements over most of cosmic history, from strong PAH lines at z~0.2-0.5, to Pa$\alpha$ and Pa$\beta$ at z~1-3, HeI and [SIII] at z~2.5-4.5, H$\alpha$ and [NII] at z~5-6.5, up to [OIII] and H$\beta$ for z~7-9 galaxies, and possibly even [OII] at z~10-12. FRESCO's grism observations provide total line fluxes for accurately estimating galaxy stellar masses and calibrating slit-loss corrections of NIRSpec/MSA spectra in the same field. Additionally, FRESCO results in a mosaic of F182M, F210M, and F444W imaging in the same fields to a depth of ~28.2 mag (5 $\sigma$ in 0.32" diameter apertures). Together with this publication, the v1 imaging mosaics are released as high-level science products via MAST. Here, we describe the overall survey design and the key science goals that can be addressed with FRESCO. We also highlight several, early science results, including: spectroscopic redshifts of Lyman break galaxies that were identified almost 20 years ago, the discovery of broad-line active galactic nuclei at z>4, and resolved Pa$\alpha$ maps of galaxies at z~1.4. These results demonstrate the enormous power for serendipitous discovery of NIRCam/grism observations.

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arXiv2022

Identification and properties of intense star-forming galaxies at redshifts z>10

Surveys with James Webb Space Telescope (JWST) have discovered candidate galaxies in the first 400 Myr of cosmic time. Preliminary indications have suggested these candidate galaxies may be more massive and abundant than previously thought. However, without confirmed distances, their inferred properties remain uncertain. Here we identify four galaxies located in the JWST Advanced Deep Extragalactic Survey (JADES) Near-Infrared Camera (NIRCam) imaging with photometric redshifts z~10-13. These galaxies include the first redshift z>12 systems discovered with distances spectroscopically confirmed by JWST in a companion paper. Using stellar population modelling, we find the galaxies typically contain a hundred million solar masses in stars, in stellar populations that are less than one hundred million years old. The moderate star formation rates and compact sizes suggest elevated star formation rate surface densities, a key indicator of their formation pathways. Taken together, these measurements show that the first galaxies contributing to cosmic reionisation formed rapidly and with intense internal radiation fields.

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arXiv2022

Unveiling the Nature of Infrared Bright, Optically Dark Galaxies with Early JWST Data

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.

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arXiv2022

The chemical enrichment in the early Universe as probed by JWST via direct metallicity measurements at z~8

We analyse the chemical properties of three z~8 galaxies behind the galaxy cluster SMACS J0723.3-7327, observed as part of the Early Release Observations programme of the James Webb Space Telescope (JWST). Exploiting [O III]4363 auroral line detections in NIRSpec spectra, we robustly apply the direct Te method for the very first time at such high redshift, measuring metallicities ranging from extremely metal poor (12+log(O/H)~7) to about one-third solar. We also discuss the excitation properties of these sources, and compare them with local strong-line metallicity calibrations. We find that none of the considered diagnostics match simultaneously the observed relations between metallicity and strong-line ratios for the three sources, implying that a proper re-assessment of the calibrations may be needed at these redshifts. On the mass-metallicity plane, the two galaxies at z~7.6 (log(M*/M_sun) = 8.1, 8.7) have metallicities that are consistent with the extrapolation of the mass-metallicity relation at z~2-3, while the least massive galaxy at z~8.5 (log(M*/M_sun) = 7.8) shows instead a significantly lower metallicity . The three galaxies show different level of offset relative to the Fundamental Metallicity Relation, with two of them (at z~7.6) being marginally consistent, while the z~8.5 source deviating significantly, being probably far from the smooth equilibrium between gas flows, star formation and metal enrichment in place at later epochs.

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arXiv2022

Structure and kinematics of a massive galaxy at z ~ 7

Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium can provide a unique view of the kinematics associated with the assembly of galaxies. In this paper, we analyzed the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158um line emission obtained with the Atacama Large Millimeter/submillimeter Array. These observations allowed us to compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using 3DBarolo, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the north-south direction suggesting ordered motion, as commonly found in rotating-disk galaxies. But higher-resolution observations would be needed to rule out a compact merger scenario. Our model indicates a low average velocity dispersion, $\sigma$ < 30 km s$^{-1}$. This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will be possibly perturbed at later times by accreting sources.

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arXiv2022

The ALMA REBELS Survey: The Cosmic HI Gas Mass Density in Galaxies at $z\approx 7$

The neutral atomic gas content of individual galaxies at large cosmological distances has until recently been difficult to measure due to the weakness of the hyperfine HI 21-cm transition. Here we estimate the HI gas mass of a sample of main-sequence star-forming galaxies at $z\sim 6.5 - 7.8$ surveyed for [CII]$-158\mu$m emission as part of the Reionization Era Bright Emission Line Survey (REBELS), using a recent calibration of the [CII]-to-HI conversion factor. We find that the HI gas mass excess in galaxies increases as a function of redshift, with an average of $M_{\rm HI} / M_\star \approx 10$, corresponding to HI gas mass fractions of $f_{\rm HI} = M_{\rm HI} / (M_\star + M_{\rm HI}) = 90\%$, at $z\approx 7$. Based on the [CII]-$158\mu$m luminosity function (LF) derived from the same sample of galaxies, we further place constraints on the cosmic HI gas mass density in galaxies ($\rho_{\rm HI}$) at this redshift, which we measure to be $\rho_{\rm HI} = 7.1^{+6.4}_{-3.0} \times 10^{6}\,M_{\odot}\,{\rm Mpc^{-3}}$. This estimate is substantially lower by a factor of $\approx 10$ than that inferred from an extrapolation of damped Lyman-$\alpha$ absorber (DLA) measurements, and largely depend on the exact [CII] LF adopted. However, we find this decrease in $\rho_{\rm HI}$ to be consistent with recent simulations and argue that this apparent discrepancy is likely a consequence of the DLA sightlines predominantly probing the substantial fraction of HI gas in high-$z$ galactic halos, whereas [CII] traces the HI in the ISM associated with star formation. We make predictions for this build-up of neutral gas in galaxies as a function of redshift, showing that at $z\gtrsim 5$ only $\approx 10\%$ of the cosmic HI gas content is confined in galaxies and associated with the star-forming ISM.

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arXiv2022

The ALMA REBELS Survey: Average [CII] $158\,\rm{\mu m}$ sizes of Star-Forming Galaxies from $z\sim 7$ to $z\sim 4$

We present the average [CII] $158\,\rm{\mu m}$ emission line sizes of UV-bright star-forming galaxies at $z\sim7$. Our results are derived from a stacking analysis of [CII] $158\,\rm{\mu m}$ emission lines and dust continua observed by ALMA, taking advantage of the large program Reionization Era Bright Emission Line Survey (REBELS). We find that the average [CII] emission at $z\sim7$ has an effective radius $r_e$ of $2.2\pm0.2\,\rm{kpc}$. It is $\gtrsim2\times$ larger than the dust continuum and the rest-frame UV emission, in agreement with recently reported measurements for $z\lesssim6$ galaxies. Additionally, we compared the average [CII] size with $4<z<6$ galaxies observed by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE). By analysing [CII] sizes of $4<z<6$ galaxies in two redshift bins, we find an average [CII] size of $r_{\rm e}=2.2\pm0.2\,\rm{kpc}$ and $r_{\rm e}=2.5\pm0.2\,\rm{kpc}$ for $z\sim5.5$ and $z\sim4.5$ galaxies, respectively. These measurements show that star-forming galaxies, on average, show no evolution in the size of the [CII] $158\,{\rm \mu m}$ emitting regions at redshift between $z\sim7$ and $z\sim4$. This finding suggest that the star-forming galaxies could be morphologically dominated by gas over a wide redshift range.

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