arXiv subjects

R. J. Bouwens

Publications and source records attributed to R. J. Bouwens.

At least 19 recordsLinked to original sources

arXiv2025

The CatSouth Quasar Candidate Catalog for the Southern Sky and a Unified All-Sky Catalog Based on Gaia DR3

The Gaia DR3 has provided a large sample of more than 6.6 million quasar candidates with high completeness but low purity. Previous work on the CatNorth quasar candidate catalog has shown that including external multiband data and applying machine-learning methods can efficiently purify the original Gaia DR3 quasar candidate catalog and improve the redshift estimates. In this paper, we extend the Gaia DR3 quasar candidate selection to the southern hemisphere using data from SkyMappper, CatWISE, and VISTA surveys. We train an XGBoost classifier on a unified set of high-confidence stars and spectroscopically confirmed quasars and galaxies. For sources with available Gaia BP/RP spectra, spectroscopic redshifts are derived using a pre-trained convolutional neural network (RegNet). We also train an ensemble photometric redshift estimation model based on XGBoost, TabNet, and FT-Transformer, achieving an RMSE of 0.2256 and a normalized median absolute deviation of 0.0187 on the validation set. By merging CatSouth with the previously published CatNorth catalog, we construct the unified all-sky CatGlobe catalog with nearly 1.9 million sources at $G<21$, providing a comprehensive and high-purity quasar candidate sample for future spectroscopic and cosmological investigations.

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arXiv2025

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

We present the [OIII]$_{88μ\text{m}}$ spectral scan results from the ALMA Large Program REBELS (Reionization Era Bright Emission Line Survey). The generally high luminosity of [OIII]$_{88μ\text{m}}$ 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]$_{88μ\text{m}}$ luminosities of $\mathrm{\sim7.6\times10^8\ L_{\odot}}$ ($5σ$) 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]$_{88μ\text{m}}$ 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]$_{158μ\text{m}}$ ($z=7.643$) 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 ($\geq3.8σ$) 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 $3σ$. 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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arXiv2024

JWST FRESCO: a comprehensive census of H$β$+[OIII] emitters at 6.8<z<9.0 in the GOODS fields

We present the census of H$β$+[OIII] 4960,5008 Åemitters at 6.8 7$ galaxies. We report a rapid decline of the [OIII] luminosity density at $z\gtrsim 6-7$ which cannot be explained by the evolution of the cosmic star-formation rate density. Finally we find that FRESCO detects in only 2h galaxies likely accounting for $\sim 10-20\%$ of the ionising budget at $z=7-8$ (assuming an escape fraction of 10%), raising the prospect of directly detecting a significant fraction of the sources of reionisation with JWST.

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arXiv2024

Characterizing the contribution of dust-obscured star formation at $z \gtrsim$ 5 using 18 serendipitously identified [CII] emitters

We present a new method to determine the star formation rate (SFR) density of the Universe at $z \gtrsim 5$ that includes the contribution of dust-obscured star formation. For this purpose, we use a [CII] (158 $μ$m) selected sample of galaxies serendipitously identified in the fields of known $z\gtrsim 4.5$ objects to characterize the fraction of obscured SFR. The advantage of a [CII] selection is that our sample is SFR-selected, in contrast to a UV-selection that would be biased towards unobscured star formation. We obtain a sample of 23 [CII] emitters near star-forming (SF) galaxies and QSOs -- three of which we identify for the first time -- using previous literature and archival ALMA data. 18 of these serendipitously identified galaxies have sufficiently deep rest-UV data and are used to characterize the obscured fraction of the star formation in galaxies with SFRs $\gtrsim 30\ \text{M}_{\odot} \ \text{yr}^{-1}$. We find that [CII] emitters identified around SF galaxies have $\approx$63\% of their SFR obscured, while [CII] emitters around QSOs have $\approx$93\% of their SFR obscured. By forward modeling existing wide-area UV luminosity function (LF) determinations, we derive the intrinsic UV LF using our characterization of the obscured SFR. Integrating the intrinsic LF to $M_{UV}$ = $-$20 we find that the obscured SFRD contributes to $>3\%$ and $>10\%$ of the total SFRD at $z \sim 5$ and $z \sim 6$ based on our sample of companions galaxies near SFGs and QSOs, respectively. Our results suggest that dust obscuration is not negligible at $z\gtrsim 5$, further underlining the importance of far-IR observations of the $z\gtrsim 5$ Universe.

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arXiv2024

CatNorth: An Improved Gaia DR3 Quasar Candidate Catalog with Pan-STARRS1 and CatWISE

A complete and pure sample of quasars with accurate redshifts is crucial for quasar studies and cosmology. In this paper, we present CatNorth, an improved Gaia DR3 quasar candidate catalog with more than 1.5 million sources in the 3$π$ sky built with data from Gaia, Pan-STARRS1, and CatWISE2020. The XGBoost algorithm is used to reclassify the original Gaia DR3 quasar candidates as stars, galaxies, and quasars. To construct training/validation datasets for the classification, we carefully built two different master stellar samples in addition to the spectroscopic galaxy and quasar samples. An ensemble classification model is obtained by averaging two XGBoost classifiers trained with different master stellar samples. Using a probability threshold of $p_{\mathrm{QSO\_mean}}>0.95$ in our ensemble classification model and an additional cut on the logarithmic probability density of zero proper motion, we retrieved 1,545,514 reliable quasar candidates from the parent Gaia DR3 quasar candidate catalog. We provide photometric redshifts for all candidates with an ensemble regression model. For a subset of 89,100 candidates, accurate spectroscopic redshifts are estimated with the Convolutional Neural Network from the Gaia BP/RP spectra. The CatNorth catalog has a high purity of ~ 90% while maintaining high completeness, which is an ideal sample to understand the quasar population and its statistical properties. The CatNorth catalog is used as the main source of input catalog for the LAMOST phase III quasar survey, which is expected to build a highly complete sample of bright quasars with $i < 19.5$.

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arXiv2023

The ALMA Reionization Era Bright Emission Line Survey (REBELS): The molecular gas content of galaxies at z~7

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$μ$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 ($μ_{\rm mol}$) 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 $μ_{\rm mol}=2.6_{-1.4}^{4.1}$ 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$(ρ_{\rm mol}/(M_\odot {\rm Mpc}^{-3}))=6.34^{+0.34}_{-0.31}$, indicate a steady increase by an order of magnitude from $z\sim7$ to 4.

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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 $μ$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$α$ and Pa$β$ at z~1-3, HeI and [SIII] at z~2.5-4.5, H$α$ and [NII] at z~5-6.5, up to [OIII] and H$β$ 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 $σ$ 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$α$ maps of galaxies at z~1.4. These results demonstrate the enormous power for serendipitous discovery of NIRCam/grism observations.

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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 $\sim158$ $\rmμm$ 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

z~2-9 Galaxies Magnified by the Hubble Frontier Field Clusters II: Luminosity Functions and Constraints on a Faint-End Turnover

We present new determinations of the rest-UV luminosity functions (LFs) at z=2-9 to extremely low luminosities (>-14 mag) from a sample of >2500 lensed galaxies found behind the HFF clusters. For the first time, we present faint-end slope results from lensed samples that are fully consistent with blank-field results over the redshift range z=2-9, while reaching to much lower luminosities than possible from the blank-field studies. Combining the deep lensed sample with the large blank-field samples allows us to set the tight constraints on the faint-end slope alpha of the z=2-9 UV LFs and its evolution. We find a smooth flattening in alpha from -2.28+/-0.10 (z=9) to -1.53+/-0.03 (z=2) with cosmic time (d(alpha)/dz=-0.11+/-0.01), fully consistent with dark matter halo buildup. We utilize these new results to present new measurements of the evolution in the UV luminosity density rho(UV) brightward of -13 mag from z~9 to z~2. Accounting for the SFR densities to faint luminosities implied by our LF results, we find that unobscured star formation dominates the SFR density at z>~4, with obscured star formation dominant thereafter. Having shown we can quantify the faint-end slope alpha of the LF accurately with our lensed HFF samples, we also quantify the apparent curvature in the shape of the UV LF through a curvature parameter delta. The constraints on the curvature delta strongly rule out the presence of a turn-over brightward of -13.1 mag at z~3, -14.3 mag at z~6, and -15.5 mag at all other redshifts between z~9 to z~2.

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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]$-158μ$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]-$158μ$m luminosity function (LF) derived from the same sample of galaxies, we further place constraints on the cosmic HI gas mass density in galaxies ($ρ_{\rm HI}$) at this redshift, which we measure to be $ρ_{\rm HI} = 7.1^{+6.4}_{-3.0} \times 10^{6}\,M_{\odot}\,{\rm Mpc^{-3}}$. This estimate is substantially lower by a factor of $\approx 10$ than that inferred from an extrapolation of damped Lyman-$α$ absorber (DLA) measurements, and largely depend on the exact [CII] LF adopted. However, we find this decrease in $ρ_{\rm HI}$ 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

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, $σ$ < 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

z~2-9 Galaxies magnified by the Hubble Frontier Field Clusters I: Source Selection and Surface Density-Magnification Constraints from >2500 galaxies

We assemble a large comprehensive sample of 2534 z~2, 3, 4, 5, 6, 7, 8, and 9 galaxies lensed by the six clusters from the Hubble Frontier Fields (HFF) program. Making use of the availability of multiple independent magnification models for each of the HFF clusters and alternatively treating one of the models as the "truth," we show that the median magnification factors from the v4 parametric models are typically reliable to values of 30 to 50, and in one case to 100. Using the median magnification factor from the latest v4 models, we estimate the UV luminosities of the 2534 lensed z~2-9 galaxies, finding sources as faint as -12.4 mag at z~3 and -12.9 mag at z~7. We explicitly demonstrate the power of the surface density-magnification relations Sigma(z) vs. mu in the HFF clusters to constrain both distant galaxy properties and cluster lensing properties. Based on the Sigma(z) vs. mu relations, we show that the median magnification estimates from existing public models must be reliable predictors of the true magnification mu to mu<15 (95% confidence). We also use the observed Sigma(z) vs. mu relations to derive constraints on the evolution of the luminosity function faint-end slope from z~7 to z~2, showing that faint-end slope results can be consistent with blank-field studies if, and only if, the selection efficiency shows no strong dependence on the magnification factor mu. This can only be the case if very low luminosity galaxies are very small, being unresolved in deep lensing probes.

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arXiv2022

Reionization Era Bright Emission Line Survey: Selection and Characterization of Luminous Interstellar Medium Reservoirs in the z>6.5 Universe

The Reionization Era Bright Emission Line Survey (REBELS) is a cycle-7 ALMA Large Program (LP) that is identifying and performing a first characterization of many of the most luminous star-forming galaxies known in the z>6.5 universe. REBELS is providing this probe by systematically scanning 40 of the brightest UV-selected galaxies identified over a 7-deg**2 area for bright 158-micron [CII] and 88-micron [OIII] lines and dust-continuum emission. Selection of the 40 REBELS targets was done by combining our own and other photometric selections, each of which is subject to extensive vetting using three completely independent sets of photometry and template-fitting codes. Building on the observational strategy deployed in two pilot programs, we are increasing the number of massive interstellar medium (ISM) reservoirs known at z>6.5 by ~4-5x to >30. In this manuscript, we motivate the observational strategy deployed in the REBELS program and present initial results. Based on the 60.6 hours of ALMA observations taken in the first year of the program (November 2019 to January 2020), 18 highly significant >~7sigma [CII] lines have already been discovered, the bulk of which (13/18) also show >~3.3 sigma dust-continuum emission. These newly discovered lines more than triple the number of bright ISM-cooling lines known in the z>6.5 universe, such that the number of ALMA-derived redshifts at z>6.5 already rival Lya redshift discoveries. An analysis of the completeness of our search results vs. star formation rate (SFR) suggests an ~79% efficiency in scanning for [CII] when the SFR(UV+IR) is in excess of 28 M_sol/yr. These new LP results further demonstrate ALMA's efficiency as a "redshift machine", particularly in the epoch of reionization.

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arXiv2022

Spectroscopic confirmation of a gravitationally lensed Lyman break galaxy at z$_{[CII]}$ = 6.827 using NOEMA

We present the spectroscopic confirmation of the brightest known gravitationally lensed Lyman break galaxy in the Epoch of Reionisation, A1703-zD1, through the detection of [C II] at a redshift of z = 6.8269 +/- 0.0004. This source was selected behind the strong lensing cluster Abell 1703, with an intrinsic L$_{UV}$ ~ L$^*$$_{z=7}$ luminosity and a very blue Spitzer/IRAC [3.6]-[4.5] colour, implying high equivalent width line emission of [O III]+H$β$. [C II] is reliably detected at 6.1$σ$ co-spatial with the rest-frame UV counterpart, showing similar spatial extent. Correcting for the lensing magnification, the [C II] luminosity in A1703-zD1 is broadly consistent with the local L$_{[CII]}$ - SFR relation. We find a clear velocity gradient of 103 +/- 22 km/s across the source which possibly indicates rotation or an ongoing merger. We furthermore present spectral scans with no detected [C II] above 4.6$σ$ in two unlensed Lyman break galaxies in the EGS-CANDELS field at z ~ 6.6 - 6.9. This is the first time that NOEMA has been successfully used to observe [C II] in a 'normal' star-forming galaxy at z > 6, and our results demonstrate its capability to complement ALMA in confirming galaxies in the Epoch of Reionisation.

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arXiv2022

The ALMA REBELS Survey. Epoch of Reionization giants: properties of dusty galaxies at $z \approx 7$

We analyse FIR dust continuum measurements for 14 galaxies ($z\approx 7$) in the ALMA REBELS LP to derive their physical properties. Our model uses three input data: (a) the UV spectral slope, $β$, (b) the observed UV continuum flux at $1500$A, $F_{\rm UV}$, (c) the observed continuum flux at $\approx 158μ$m, $F_{158}$, and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have (28-90.5)% of their star formation obscured; the total (UV+IR) star formation rates are in the range $31.5 < {\rm SFR}/ (M_\odot {\rm yr}^{-1}) < 129.5$. The sample-averaged dust mass and temperature are $(1.3\pm 1.1)\times 10^7 M_\odot$ and $52 \pm 11$ K, respectively. In some galaxies dust is abundant (REBELS-14, $M'_d \approx 3.4 \times 10^7 M_\odot$), or hot (REBELS-18, $T'_d \approx 67$ K). The dust distribution is compact ($<0.3$ kpc for 70% of the galaxies). The dust yield per supernova is $0.1 \le y_d/M_\odot \le 3.3$, with 70% of the galaxies requiring $y_d < 0.25 M_\odot$. Three galaxies (REBELS-12, 14, 39) require $y_d > 1 M_\odot$. With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in [CII] nicely follow the local $L_{\rm CII}-$SFR relation, and are approximately located on the Kennicutt-Schmidt relation. The sample-averaged gas depletion time is of $0.11\, y_P^{-2}$ Gyr, where $y_P$ is the ratio of the gas-to-stellar distribution radius. For some systems a solution simultaneously matching the observed ($β, F_{\rm UV}, F_{158}$) values cannot be found. This occurs when the index $I_m = (F_{158}/F_{\rm UV})/(β-β_{\rm int})$, where $β_{\rm int}$ is the intrinsic UV slope, exceeds $I_m^*\approx 1120$ for a MW curve. For these objects we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX-$β$ relation.

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arXiv2022

The REBELS ALMA Survey: cosmic dust temperature evolution out to z $\sim$ 7

ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature $T_d$ remains mostly unconstrained due to the few available FIR continuum data at redshift $z>5$. This introduces large uncertainties in several properties of high-$z$ galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous [CII] 158$μ$m line and underlying dust continuum measurements, we derive $T_ d$ in the continuum and [CII] detected $z\approx 7$ galaxies in the ALMA Large Project REBELS sample. We find $39\ \mathrm{K} < T_d < 58\ \mathrm{K}$, and dust masses in the narrow range $M_d = (0.9-3.6)\times 10^7 M_{\odot}$. These results allow us to extend for the first time the reported $T_d(z)$ relation into the Epoch of Reionization. We produce a new physical model that explains the increasing $T_ d(z)$ trend with the decrease of gas depletion time, $t_{dep}=M_g/\mathrm{SFR}$, induced by the higher cosmological accretion rate at early times; this hypothesis yields $T_d \propto (1+z)^{0.4}$. The model also explains the observed $T_d$ scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, $T_d$ only depends on the gas column density (metallicity), $T_d \propto N_H^{1/6}$ ($T_d \propto Z^{-1/6}$). REBELS galaxies are on average relatively transparent, with effective gas column densities around $N_H \simeq (0.03-1)\times 10^{21} \mathrm{cm}^{-2}$. We predict that other high-$z$ galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated $T_d \gg 60$ K, are significantly obscured, low-metallicity systems. In fact $T_d$ is higher in metal-poor systems due to their smaller dust content, which for fixed $L_{ IR}$ results in warmer temperatures.

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arXiv2021

The Prevalence of Galaxy Overdensities Around UV-Luminous Lyman $\mathbfα$ Emitters in the Epoch of Reionization

Before the end of the epoch of reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly$α$ lines of $z\gtrsim6$ galaxies in the intergalactic medium. Nevertheless, Ly$α$ has been detected up to very high redshifts ($z\sim9$) for several especially UV luminous galaxies. Here, we test to what extent the galaxy's local environment might impact the Ly$α$ transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly$α$ emitters: EGS-zs8-1, EGS-zs8-2 and EGSY-z8p7 at $z_\mathrm{spec}=7.73$, 7.48, and 8.68, respectively. We combine the multi-wavelength HST imaging with Spitzer data to reliably select $z\sim7-9$ galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor $\sim 3-9\times$). Our analysis thus reveals ubiquitous overdensities around luminous Ly$α$ emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. JWST follow-up observations of the neighbouring galaxies identified here will be needed to confirm their physical association and to map out the ionized regions produced by these sources.

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